Routing in ASP.NET Core

By Ryan Nowak, Kirk Larkin, and Rick Anderson

Routing is responsible for matching incoming HTTP requests and dispatching those requests to the app's executable endpoints. Endpoints are the app's units of executable request-handling code. Endpoints are defined in the app and configured when the app starts. The endpoint matching process can extract values from the request's URL and provide those values for request processing. Using endpoint information from the app, routing is also able to generate URLs that map to endpoints.

Apps can configure routing using:

  • Controllers
  • Razor Pages
  • SignalR
  • gRPC Services
  • Endpoint-enabled middleware such as Health Checks.
  • Delegates and lambdas registered with routing.

This article covers low-level details of ASP.NET Core routing. For information on configuring routing:

Routing basics

The following code shows a basic example of routing:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

The preceding example includes a single endpoint using the MapGet method:

  • When an HTTP GET request is sent to the root URL /:
    • The request delegate executes.
    • Hello World! is written to the HTTP response.
  • If the request method is not GET or the root URL is not /, no route matches and an HTTP 404 is returned.

Routing uses a pair of middleware, registered by UseRouting and UseEndpoints:

  • UseRouting adds route matching to the middleware pipeline. This middleware looks at the set of endpoints defined in the app, and selects the best match based on the request.
  • UseEndpoints adds endpoint execution to the middleware pipeline. It runs the delegate associated with the selected endpoint.

Apps typically don't need to call UseRouting or UseEndpoints. WebApplicationBuilder configures a middleware pipeline that wraps middleware added in Program.cs with UseRouting and UseEndpoints. However, apps can change the order in which UseRouting and UseEndpoints run by calling these methods explicitly. For example, fhe following code makes an explicit call to UseRouting:

app.Use(async (context, next) =>
{
    // ...
    await next(context);
});

app.UseRouting();

app.MapGet("/", () => "Hello World!");

In the preceding code:

  • The call to app.Use registers a custom middleware that runs at the start of the pipeline.
  • The call to UseRouting configures the route matching middleware to run after the custom middleware.
  • The endpoint registered with MapGet runs at the end of the pipeline.

If the preceding example didn't include a call to UseRouting, the custom middleware would run after the route matching middleware.

Endpoints

The MapGet method is used to define an endpoint. An endpoint is something that can be:

  • Selected, by matching the URL and HTTP method.
  • Executed, by running the delegate.

Endpoints that can be matched and executed by the app are configured in UseEndpoints. For example, MapGet, MapPost, and similar methods connect request delegates to the routing system. Additional methods can be used to connect ASP.NET Core framework features to the routing system:

The following example shows routing with a more sophisticated route template:

app.MapGet("/hello/{name:alpha}", (string name) => $"Hello {name}!");

The string /hello/{name:alpha} is a route template. A route template is used to configure how the endpoint is matched. In this case, the template matches:

  • A URL like /hello/Docs
  • Any URL path that begins with /hello/ followed by a sequence of alphabetic characters. :alpha applies a route constraint that matches only alphabetic characters. Route constraints are explained later in this article.

The second segment of the URL path, {name:alpha}:

The following example shows routing with health checks and authorization:

app.UseAuthentication();
app.UseAuthorization();

app.MapHealthChecks("/healthz").RequireAuthorization();
app.MapGet("/", () => "Hello World!");

The preceding example demonstrates how:

  • The authorization middleware can be used with routing.
  • Endpoints can be used to configure authorization behavior.

The MapHealthChecks call adds a health check endpoint. Chaining RequireAuthorization on to this call attaches an authorization policy to the endpoint.

Calling UseAuthentication and UseAuthorization adds the authentication and authorization middleware. These middleware are placed between UseRouting and UseEndpoints so that they can:

  • See which endpoint was selected by UseRouting.
  • Apply an authorization policy before UseEndpoints dispatches to the endpoint.

Endpoint metadata

In the preceding example, there are two endpoints, but only the health check endpoint has an authorization policy attached. If the request matches the health check endpoint, /healthz, an authorization check is performed. This demonstrates that endpoints can have extra data attached to them. This extra data is called endpoint metadata:

  • The metadata can be processed by routing-aware middleware.
  • The metadata can be of any .NET type.

Routing concepts

The routing system builds on top of the middleware pipeline by adding the powerful endpoint concept. Endpoints represent units of the app's functionality that are distinct from each other in terms of routing, authorization, and any number of ASP.NET Core's systems.

ASP.NET Core endpoint definition

An ASP.NET Core endpoint is:

The following code shows how to retrieve and inspect the endpoint matching the current request:

app.Use(async (context, next) =>
{
    var currentEndpoint = context.GetEndpoint();

    if (currentEndpoint is null)
    {
        await next(context);
        return;
    }

    Console.WriteLine($"Endpoint: {currentEndpoint.DisplayName}");

    if (currentEndpoint is RouteEndpoint routeEndpoint)
    {
        Console.WriteLine($"  - Route Pattern: {routeEndpoint.RoutePattern}");
    }

    foreach (var endpointMetadata in currentEndpoint.Metadata)
    {
        Console.WriteLine($"  - Metadata: {endpointMetadata}");
    }

    await next(context);
});

app.MapGet("/", () => "Inspect Endpoint.");

The endpoint, if selected, can be retrieved from the HttpContext. Its properties can be inspected. Endpoint objects are immutable and cannot be modified after creation. The most common type of endpoint is a RouteEndpoint. RouteEndpoint includes information that allows it to be selected by the routing system.

In the preceding code, app.Use configures an inline middleware.

The following code shows that, depending on where app.Use is called in the pipeline, there may not be an endpoint:

app.Use(async (context, next) =>
{
    Console.WriteLine($"1. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    await next(context);
});

app.UseRouting();

app.Use(async (context, next) =>
{
    Console.WriteLine($"2. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    await next(context);
});

app.MapGet("/", (HttpContext context) =>
{
    Console.WriteLine($"3. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    return "Hello World!";
}).WithDisplayName("Hello");

app.UseEndpoints(_ => { });

app.Use(async (context, next) =>
{
    Console.WriteLine($"4. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    await next(context);
});

The preceding sample adds Console.WriteLine statements that display whether or not an endpoint has been selected. For clarity, the sample assigns a display name to the provided / endpoint.

The preceding sample also includes calls to UseRouting and UseEndpoints to control exactly when these middleware run within the pipeline.

Running this code with a URL of / displays:

1. Endpoint: (null)
2. Endpoint: Hello
3. Endpoint: Hello

Running this code with any other URL displays:

1. Endpoint: (null)
2. Endpoint: (null)
4. Endpoint: (null)

This output demonstrates that:

  • The endpoint is always null before UseRouting is called.
  • If a match is found, the endpoint is non-null between UseRouting and UseEndpoints.
  • The UseEndpoints middleware is terminal when a match is found. Terminal middleware is defined later in this article.
  • The middleware after UseEndpoints execute only when no match is found.

The UseRouting middleware uses the SetEndpoint method to attach the endpoint to the current context. It's possible to replace the UseRouting middleware with custom logic and still get the benefits of using endpoints. Endpoints are a low-level primitive like middleware, and aren't coupled to the routing implementation. Most apps don't need to replace UseRouting with custom logic.

The UseEndpoints middleware is designed to be used in tandem with the UseRouting middleware. The core logic to execute an endpoint isn't complicated. Use GetEndpoint to retrieve the endpoint, and then invoke its RequestDelegate property.

The following code demonstrates how middleware can influence or react to routing:

app.UseHttpMethodOverride();
app.UseRouting();

app.Use(async (context, next) =>
{
    if (context.GetEndpoint()?.Metadata.GetMetadata<RequiresAuditAttribute>() is not null)
    {
        Console.WriteLine($"ACCESS TO SENSITIVE DATA AT: {DateTime.UtcNow}");
    }

    await next(context);
});

app.MapGet("/", () => "Audit isn't required.");
app.MapGet("/sensitive", () => "Audit required for sensitive data.")
    .WithMetadata(new RequiresAuditAttribute());
public class RequiresAuditAttribute : Attribute { }

The preceding example demonstrates two important concepts:

  • Middleware can run before UseRouting to modify the data that routing operates upon.
  • Middleware can run between UseRouting and UseEndpoints to process the results of routing before the endpoint is executed.
    • Middleware that runs between UseRouting and UseEndpoints:
      • Usually inspects metadata to understand the endpoints.
      • Often makes security decisions, as done by UseAuthorization and UseCors.
    • The combination of middleware and metadata allows configuring policies per-endpoint.

The preceding code shows an example of a custom middleware that supports per-endpoint policies. The middleware writes an audit log of access to sensitive data to the console. The middleware can be configured to audit an endpoint with the RequiresAuditAttribute metadata. This sample demonstrates an opt-in pattern where only endpoints that are marked as sensitive are audited. It's possible to define this logic in reverse, auditing everything that isn't marked as safe, for example. The endpoint metadata system is flexible. This logic could be designed in whatever way suits the use case.

The preceding sample code is intended to demonstrate the basic concepts of endpoints. The sample is not intended for production use. A more complete version of an audit log middleware would:

  • Log to a file or database.
  • Include details such as the user, IP address, name of the sensitive endpoint, and more.

The audit policy metadata RequiresAuditAttribute is defined as an Attribute for easier use with class-based frameworks such as controllers and SignalR. When using route to code:

  • Metadata is attached with a builder API.
  • Class-based frameworks include all attributes on the corresponding method and class when creating endpoints.

The best practices for metadata types are to define them either as interfaces or attributes. Interfaces and attributes allow code reuse. The metadata system is flexible and doesn't impose any limitations.

Compare terminal middleware with routing

The following example demonstrates both terminal middleware and routing:

// Approach 1: Terminal Middleware.
app.Use(async (context, next) =>
{
    if (context.Request.Path == "/")
    {
        await context.Response.WriteAsync("Terminal Middleware.");
        return;
    }

    await next(context);
});

app.UseRouting();

// Approach 2: Routing.
app.MapGet("/Routing", () => "Routing.");

The style of middleware shown with Approach 1: is terminal middleware. It's called terminal middleware because it does a matching operation:

  • The matching operation in the preceding sample is Path == "/" for the middleware and Path == "/Routing" for routing.
  • When a match is successful, it executes some functionality and returns, rather than invoking the next middleware.

It's called terminal middleware because it terminates the search, executes some functionality, and then returns.

The following list compares terminal middleware with routing:

  • Both approaches allow terminating the processing pipeline:
    • Middleware terminates the pipeline by returning rather than invoking next.
    • Endpoints are always terminal.
  • Terminal middleware allows positioning the middleware at an arbitrary place in the pipeline:
  • Terminal middleware allows arbitrary code to determine when the middleware matches:
    • Custom route matching code can be verbose and difficult to write correctly.
    • Routing provides straightforward solutions for typical apps. Most apps don't require custom route matching code.
  • Endpoints interface with middleware such as UseAuthorization and UseCors.
    • Using a terminal middleware with UseAuthorization or UseCors requires manual interfacing with the authorization system.

An endpoint defines both:

  • A delegate to process requests.
  • A collection of arbitrary metadata. The metadata is used to implement cross-cutting concerns based on policies and configuration attached to each endpoint.

Terminal middleware can be an effective tool, but can require:

  • A significant amount of coding and testing.
  • Manual integration with other systems to achieve the desired level of flexibility.

Consider integrating with routing before writing a terminal middleware.

Existing terminal middleware that integrates with Map or MapWhen can usually be turned into a routing aware endpoint. MapHealthChecks demonstrates the pattern for router-ware:

The following code shows use of MapHealthChecks:

app.UseAuthentication();
app.UseAuthorization();

app.MapHealthChecks("/healthz").RequireAuthorization();

The preceding sample shows why returning the builder object is important. Returning the builder object allows the app developer to configure policies such as authorization for the endpoint. In this example, the health checks middleware has no direct integration with the authorization system.

The metadata system was created in response to the problems encountered by extensibility authors using terminal middleware. It's problematic for each middleware to implement its own integration with the authorization system.

URL matching

  • Is the process by which routing matches an incoming request to an endpoint.
  • Is based on data in the URL path and headers.
  • Can be extended to consider any data in the request.

When a routing middleware executes, it sets an Endpoint and route values to a request feature on the HttpContext from the current request:

Middleware running after the routing middleware can inspect the endpoint and take action. For example, an authorization middleware can interrogate the endpoint's metadata collection for an authorization policy. After all of the middleware in the request processing pipeline is executed, the selected endpoint's delegate is invoked.

The routing system in endpoint routing is responsible for all dispatching decisions. Because the middleware applies policies based on the selected endpoint, it's important that:

  • Any decision that can affect dispatching or the application of security policies is made inside the routing system.

Warning

For backwards-compatibility, when a Controller or Razor Pages endpoint delegate is executed, the properties of RouteContext.RouteData are set to appropriate values based on the request processing performed thus far.

The RouteContext type will be marked obsolete in a future release:

  • Migrate RouteData.Values to HttpRequest.RouteValues.
  • Migrate RouteData.DataTokens to retrieve IDataTokensMetadata from the endpoint metadata.

URL matching operates in a configurable set of phases. In each phase, the output is a set of matches. The set of matches can be narrowed down further by the next phase. The routing implementation does not guarantee a processing order for matching endpoints. All possible matches are processed at once. The URL matching phases occur in the following order. ASP.NET Core:

  1. Processes the URL path against the set of endpoints and their route templates, collecting all of the matches.
  2. Takes the preceding list and removes matches that fail with route constraints applied.
  3. Takes the preceding list and removes matches that fail the set of MatcherPolicy instances.
  4. Uses the EndpointSelector to make a final decision from the preceding list.

The list of endpoints is prioritized according to:

All matching endpoints are processed in each phase until the EndpointSelector is reached. The EndpointSelector is the final phase. It chooses the highest priority endpoint from the matches as the best match. If there are other matches with the same priority as the best match, an ambiguous match exception is thrown.

The route precedence is computed based on a more specific route template being given a higher priority. For example, consider the templates /hello and /{message}:

  • Both match the URL path /hello.
  • /hello is more specific and therefore higher priority.

In general, route precedence does a good job of choosing the best match for the kinds of URL schemes used in practice. Use Order only when necessary to avoid an ambiguity.

Due to the kinds of extensibility provided by routing, it isn't possible for the routing system to compute ahead of time the ambiguous routes. Consider an example such as the route templates /{message:alpha} and /{message:int}:

  • The alpha constraint matches only alphabetic characters.
  • The int constraint matches only numbers.
  • These templates have the same route precedence, but there's no single URL they both match.
  • If the routing system reported an ambiguity error at startup, it would block this valid use case.

Warning

The order of operations inside UseEndpoints doesn't influence the behavior of routing, with one exception. MapControllerRoute and MapAreaRoute automatically assign an order value to their endpoints based on the order they are invoked. This simulates long-time behavior of controllers without the routing system providing the same guarantees as older routing implementations.

Endpoint routing in ASP.NET Core:

  • Doesn't have the concept of routes.
  • Doesn't provide ordering guarantees. All endpoints are processed at once.

Route template precedence and endpoint selection order

Route template precedence is a system that assigns each route template a value based on how specific it is. Route template precedence:

  • Avoids the need to adjust the order of endpoints in common cases.
  • Attempts to match the common-sense expectations of routing behavior.

For example, consider templates /Products/List and /Products/{id}. It would be reasonable to assume that /Products/List is a better match than /Products/{id} for the URL path /Products/List. This works because the literal segment /List is considered to have better precedence than the parameter segment /{id}.

The details of how precedence works are coupled to how route templates are defined:

  • Templates with more segments are considered more specific.
  • A segment with literal text is considered more specific than a parameter segment.
  • A parameter segment with a constraint is considered more specific than one without.
  • A complex segment is considered as specific as a parameter segment with a constraint.
  • Catch-all parameters are the least specific. See catch-all in the Route templates section for important information on catch-all routes.

URL generation concepts

URL generation:

  • Is the process by which routing can create a URL path based on a set of route values.
  • Allows for a logical separation between endpoints and the URLs that access them.

Endpoint routing includes the LinkGenerator API. LinkGenerator is a singleton service available from DI. The LinkGenerator API can be used outside of the context of an executing request. Mvc.IUrlHelper and scenarios that rely on IUrlHelper, such as Tag Helpers, HTML Helpers, and Action Results, use the LinkGenerator API internally to provide link generating capabilities.

The link generator is backed by the concept of an address and address schemes. An address scheme is a way of determining the endpoints that should be considered for link generation. For example, the route name and route values scenarios many users are familiar with from controllers and Razor Pages are implemented as an address scheme.

The link generator can link to controllers and Razor Pages via the following extension methods:

Overloads of these methods accept arguments that include the HttpContext. These methods are functionally equivalent to Url.Action and Url.Page, but offer additional flexibility and options.

The GetPath* methods are most similar to Url.Action and Url.Page, in that they generate a URI containing an absolute path. The GetUri* methods always generate an absolute URI containing a scheme and host. The methods that accept an HttpContext generate a URI in the context of the executing request. The ambient route values, URL base path, scheme, and host from the executing request are used unless overridden.

LinkGenerator is called with an address. Generating a URI occurs in two steps:

  1. An address is bound to a list of endpoints that match the address.
  2. Each endpoint's RoutePattern is evaluated until a route pattern that matches the supplied values is found. The resulting output is combined with the other URI parts supplied to the link generator and returned.

The methods provided by LinkGenerator support standard link generation capabilities for any type of address. The most convenient way to use the link generator is through extension methods that perform operations for a specific address type:

Extension Method Description
GetPathByAddress Generates a URI with an absolute path based on the provided values.
GetUriByAddress Generates an absolute URI based on the provided values.

Warning

Pay attention to the following implications of calling LinkGenerator methods:

  • Use GetUri* extension methods with caution in an app configuration that doesn't validate the Host header of incoming requests. If the Host header of incoming requests isn't validated, untrusted request input can be sent back to the client in URIs in a view or page. We recommend that all production apps configure their server to validate the Host header against known valid values.

  • Use LinkGenerator with caution in middleware in combination with Map or MapWhen. Map* changes the base path of the executing request, which affects the output of link generation. All of the LinkGenerator APIs allow specifying a base path. Specify an empty base path to undo the Map* affect on link generation.

Middleware example

In the following example, a middleware uses the LinkGenerator API to create a link to an action method that lists store products. Using the link generator by injecting it into a class and calling GenerateLink is available to any class in an app:

public class ProductsMiddleware
{
    private readonly LinkGenerator _linkGenerator;

    public ProductsMiddleware(RequestDelegate next, LinkGenerator linkGenerator) =>
        _linkGenerator = linkGenerator;

    public async Task InvokeAsync(HttpContext httpContext)
    {
        httpContext.Response.ContentType = MediaTypeNames.Text.Plain;

        var productsPath = _linkGenerator.GetPathByAction("Products", "Store");

        await httpContext.Response.WriteAsync(
            $"Go to {productsPath} to see our products.");
    }
}

Route templates

Tokens within {} define route parameters that are bound if the route is matched. More than one route parameter can be defined in a route segment, but route parameters must be separated by a literal value. For example, {controller=Home}{action=Index} isn't a valid route, since there's no literal value between {controller} and {action}. Route parameters must have a name and may have additional attributes specified.

Literal text other than route parameters (for example, {id}) and the path separator / must match the text in the URL. Text matching is case-insensitive and based on the decoded representation of the URL's path. To match a literal route parameter delimiter { or }, escape the delimiter by repeating the character. For example {{ or }}.

Asterisk * or double asterisk **:

  • Can be used as a prefix to a route parameter to bind to the rest of the URI.
  • Are called a catch-all parameters. For example, blog/{**slug}:
    • Matches any URI that starts with /blog and has any value following it.
    • The value following /blog is assigned to the slug route value.

Warning

A catch-all parameter may match routes incorrectly due to a bug in routing. Apps impacted by this bug have the following characteristics:

  • A catch-all route, for example, {**slug}"
  • The catch-all route fails to match requests it should match.
  • Removing other routes makes catch-all route start working.

See GitHub bugs 18677 and 16579 for example cases that hit this bug.

An opt-in fix for this bug is contained in .NET Core 3.1.301 SDK and later. The following code sets an internal switch that fixes this bug:

public static void Main(string[] args)
{
   AppContext.SetSwitch("Microsoft.AspNetCore.Routing.UseCorrectCatchAllBehavior", 
                         true);
   CreateHostBuilder(args).Build().Run();
}
// Remaining code removed for brevity.

Catch-all parameters can also match the empty string.

The catch-all parameter escapes the appropriate characters when the route is used to generate a URL, including path separator / characters. For example, the route foo/{*path} with route values { path = "my/path" } generates foo/my%2Fpath. Note the escaped forward slash. To round-trip path separator characters, use the ** route parameter prefix. The route foo/{**path} with { path = "my/path" } generates foo/my/path.

URL patterns that attempt to capture a file name with an optional file extension have additional considerations. For example, consider the template files/{filename}.{ext?}. When values for both filename and ext exist, both values are populated. If only a value for filename exists in the URL, the route matches because the trailing . is optional. The following URLs match this route:

  • /files/myFile.txt
  • /files/myFile

Route parameters may have default values designated by specifying the default value after the parameter name separated by an equals sign (=). For example, {controller=Home} defines Home as the default value for controller. The default value is used if no value is present in the URL for the parameter. Route parameters are made optional by appending a question mark (?) to the end of the parameter name. For example, id?. The difference between optional values and default route parameters is:

  • A route parameter with a default value always produces a value.
  • An optional parameter has a value only when a value is provided by the request URL.

Route parameters may have constraints that must match the route value bound from the URL. Adding : and constraint name after the route parameter name specifies an inline constraint on a route parameter. If the constraint requires arguments, they're enclosed in parentheses (...) after the constraint name. Multiple inline constraints can be specified by appending another : and constraint name.

The constraint name and arguments are passed to the IInlineConstraintResolver service to create an instance of IRouteConstraint to use in URL processing. For example, the route template blog/{article:minlength(10)} specifies a minlength constraint with the argument 10. For more information on route constraints and a list of the constraints provided by the framework, see the Route constraints section.

Route parameters may also have parameter transformers. Parameter transformers transform a parameter's value when generating links and matching actions and pages to URLs. Like constraints, parameter transformers can be added inline to a route parameter by adding a : and transformer name after the route parameter name. For example, the route template blog/{article:slugify} specifies a slugify transformer. For more information on parameter transformers, see the Parameter transformers section.

The following table demonstrates example route templates and their behavior:

Route Template Example Matching URI The request URI…
hello /hello Only matches the single path /hello.
{Page=Home} / Matches and sets Page to Home.
{Page=Home} /Contact Matches and sets Page to Contact.
{controller}/{action}/{id?} /Products/List Maps to the Products controller and List action.
{controller}/{action}/{id?} /Products/Details/123 Maps to the Products controller and Details action withid set to 123.
{controller=Home}/{action=Index}/{id?} / Maps to the Home controller and Index method. id is ignored.
{controller=Home}/{action=Index}/{id?} /Products Maps to the Products controller and Index method. id is ignored.

Using a template is generally the simplest approach to routing. Constraints and defaults can also be specified outside the route template.

Complex segments

Complex segments are processed by matching up literal delimiters from right to left in a non-greedy way. For example, [Route("/a{b}c{d}")] is a complex segment. Complex segments work in a particular way that must be understood to use them successfully. The example in this section demonstrates why complex segments only really work well when the delimiter text doesn't appear inside the parameter values. Using a regex and then manually extracting the values is needed for more complex cases.

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

This is a summary of the steps that routing performs with the template /a{b}c{d} and the URL path /abcd. The | is used to help visualize how the algorithm works:

  • The first literal, right to left, is c. So /abcd is searched from right and finds /ab|c|d.
  • Everything to the right (d) is now matched to the route parameter {d}.
  • The next literal, right to left, is a. So /ab|c|d is searched starting where we left off, then a is found /|a|b|c|d.
  • The value to the right (b) is now matched to the route parameter {b}.
  • There is no remaining text and no remaining route template, so this is a match.

Here's an example of a negative case using the same template /a{b}c{d} and the URL path /aabcd. The | is used to help visualize how the algorithm works. This case isn't a match, which is explained by the same algorithm:

  • The first literal, right to left, is c. So /aabcd is searched from right and finds /aab|c|d.
  • Everything to the right (d) is now matched to the route parameter {d}.
  • The next literal, right to left, is a. So /aab|c|d is searched starting where we left off, then a is found /a|a|b|c|d.
  • The value to the right (b) is now matched to the route parameter {b}.
  • At this point there is remaining text a, but the algorithm has run out of route template to parse, so this is not a match.

Since the matching algorithm is non-greedy:

  • It matches the smallest amount of text possible in each step.
  • Any case where the delimiter value appears inside the parameter values results in not matching.

Regular expressions provide much more control over their matching behavior.

Greedy matching, also know as lazy matching, matches the largest possible string. Non-greedy matches the smallest possible string.

Route constraints

Route constraints execute when a match has occurred to the incoming URL and the URL path is tokenized into route values. Route constraints generally inspect the route value associated via the route template and make a true or false decision about whether the value is acceptable. Some route constraints use data outside the route value to consider whether the request can be routed. For example, the HttpMethodRouteConstraint can accept or reject a request based on its HTTP verb. Constraints are used in routing requests and link generation.

Warning

Don't use constraints for input validation. If constraints are used for input validation, invalid input results in a 404 Not Found response. Invalid input should produce a 400 Bad Request with an appropriate error message. Route constraints are used to disambiguate similar routes, not to validate the inputs for a particular route.

The following table demonstrates example route constraints and their expected behavior:

constraint Example Example Matches Notes
int {id:int} 123456789, -123456789 Matches any integer
bool {active:bool} true, FALSE Matches true or false. Case-insensitive
datetime {dob:datetime} 2016-12-31, 2016-12-31 7:32pm Matches a valid DateTime value in the invariant culture. See preceding warning.
decimal {price:decimal} 49.99, -1,000.01 Matches a valid decimal value in the invariant culture. See preceding warning.
double {weight:double} 1.234, -1,001.01e8 Matches a valid double value in the invariant culture. See preceding warning.
float {weight:float} 1.234, -1,001.01e8 Matches a valid float value in the invariant culture. See preceding warning.
guid {id:guid} CD2C1638-1638-72D5-1638-DEADBEEF1638 Matches a valid Guid value
long {ticks:long} 123456789, -123456789 Matches a valid long value
minlength(value) {username:minlength(4)} Rick String must be at least 4 characters
maxlength(value) {filename:maxlength(8)} MyFile String must be no more than 8 characters
length(length) {filename:length(12)} somefile.txt String must be exactly 12 characters long
length(min,max) {filename:length(8,16)} somefile.txt String must be at least 8 and no more than 16 characters long
min(value) {age:min(18)} 19 Integer value must be at least 18
max(value) {age:max(120)} 91 Integer value must be no more than 120
range(min,max) {age:range(18,120)} 91 Integer value must be at least 18 but no more than 120
alpha {name:alpha} Rick String must consist of one or more alphabetical characters, a-z and case-insensitive.
regex(expression) {ssn:regex(^\\d{{3}}-\\d{{2}}-\\d{{4}}$)} 123-45-6789 String must match the regular expression. See tips about defining a regular expression.
required {name:required} Rick Used to enforce that a non-parameter value is present during URL generation

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Multiple, colon delimited constraints can be applied to a single parameter. For example, the following constraint restricts a parameter to an integer value of 1 or greater:

[Route("users/{id:int:min(1)}")]
public User GetUserById(int id) { }

Warning

Route constraints that verify the URL and are converted to a CLR type always use the invariant culture. For example, conversion to the CLR type int or DateTime. These constraints assume that the URL is not localizable. The framework-provided route constraints don't modify the values stored in route values. All route values parsed from the URL are stored as strings. For example, the float constraint attempts to convert the route value to a float, but the converted value is used only to verify it can be converted to a float.

Regular expressions in constraints

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Regular expressions can be specified as inline constraints using the regex(...) route constraint. Methods in the MapControllerRoute family also accept an object literal of constraints. If that form is used, string values are interpreted as regular expressions.

The following code uses an inline regex constraint:

app.MapGet("{message:regex(^\\d{{3}}-\\d{{2}}-\\d{{4}}$)}",
    () => "Inline Regex Constraint Matched");

The following code uses an object literal to specify a regex constraint:

app.MapControllerRoute(
    name: "people",
    pattern: "people/{ssn}",
    constraints: new { ssn = "^\\d{3}-\\d{2}-\\d{4}$", },
    defaults: new { controller = "People", action = "List" });

The ASP.NET Core framework adds RegexOptions.IgnoreCase | RegexOptions.Compiled | RegexOptions.CultureInvariant to the regular expression constructor. See RegexOptions for a description of these members.

Regular expressions use delimiters and tokens similar to those used by routing and the C# language. Regular expression tokens must be escaped. To use the regular expression ^\d{3}-\d{2}-\d{4}$ in an inline constraint, use one of the following:

  • Replace \ characters provided in the string as \\ characters in the C# source file in order to escape the \ string escape character.
  • Verbatim string literals.

To escape routing parameter delimiter characters {, }, [, ], double the characters in the expression, for example, {{, }}, [[, ]]. The following table shows a regular expression and its escaped version:

Regular expression Escaped regular expression
^\d{3}-\d{2}-\d{4}$ ^\\d{{3}}-\\d{{2}}-\\d{{4}}$
^[a-z]{2}$ ^[[a-z]]{{2}}$

Regular expressions used in routing often start with the ^ character and match the starting position of the string. The expressions often end with the $ character and match the end of the string. The ^ and $ characters ensure that the regular expression matches the entire route parameter value. Without the ^ and $ characters, the regular expression matches any substring within the string, which is often undesirable. The following table provides examples and explains why they match or fail to match:

Expression String Match Comment
[a-z]{2} hello Yes Substring matches
[a-z]{2} 123abc456 Yes Substring matches
[a-z]{2} mz Yes Matches expression
[a-z]{2} MZ Yes Not case sensitive
^[a-z]{2}$ hello No See ^ and $ above
^[a-z]{2}$ 123abc456 No See ^ and $ above

For more information on regular expression syntax, see .NET Framework Regular Expressions.

To constrain a parameter to a known set of possible values, use a regular expression. For example, {action:regex(^(list|get|create)$)} only matches the action route value to list, get, or create. If passed into the constraints dictionary, the string ^(list|get|create)$ is equivalent. Constraints that are passed in the constraints dictionary that don't match one of the known constraints are also treated as regular expressions. Constraints that are passed within a template that don't match one of the known constraints are not treated as regular expressions.

Custom route constraints

Custom route constraints can be created by implementing the IRouteConstraint interface. The IRouteConstraint interface contains Match, which returns true if the constraint is satisfied and false otherwise.

Custom route constraints are rarely needed. Before implementing a custom route constraint, consider alternatives, such as model binding.

The ASP.NET Core Constraints folder provides good examples of creating a constraints. For example, GuidRouteConstraint.

To use a custom IRouteConstraint, the route constraint type must be registered with the app's ConstraintMap in the service container. A ConstraintMap is a dictionary that maps route constraint keys to IRouteConstraint implementations that validate those constraints. An app's ConstraintMap can be updated in Program.cs either as part of an AddRouting call or by configuring RouteOptions directly with builder.Services.Configure<RouteOptions>. For example:

builder.Services.AddRouting(options =>
    options.ConstraintMap.Add("noZeroes", typeof(NoZeroesRouteConstraint)));

The preceding constraint is applied in the following code:

[ApiController]
[Route("api/[controller]")]
public class NoZeroesController : ControllerBase
{
    [HttpGet("{id:noZeroes}")]
    public IActionResult Get(string id) =>
        Content(id);
}

The implementation of NoZeroesRouteConstraint prevents 0 being used in a route parameter:

public class NoZeroesRouteConstraint : IRouteConstraint
{
    private static readonly Regex _regex = new(
        @"^[1-9]*$",
        RegexOptions.CultureInvariant | RegexOptions.IgnoreCase,
        TimeSpan.FromMilliseconds(100));

    public bool Match(
        HttpContext? httpContext, IRouter? route, string routeKey,
        RouteValueDictionary values, RouteDirection routeDirection)
    {
        if (!values.TryGetValue(routeKey, out var routeValue))
        {
            return false;
        }

        var routeValueString = Convert.ToString(routeValue, CultureInfo.InvariantCulture);

        if (routeValueString is null)
        {
            return false;
        }

        return _regex.IsMatch(routeValueString);
    }
}

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

The preceding code:

  • Prevents 0 in the {id} segment of the route.
  • Is shown to provide a basic example of implementing a custom constraint. It should not be used in a production app.

The following code is a better approach to preventing an id containing a 0 from being processed:

[HttpGet("{id}")]
public IActionResult Get(string id)
{
    if (id.Contains('0'))
    {
        return StatusCode(StatusCodes.Status406NotAcceptable);
    }

    return Content(id);
}

The preceding code has the following advantages over the NoZeroesRouteConstraint approach:

  • It doesn't require a custom constraint.
  • It returns a more descriptive error when the route parameter includes 0.

Parameter transformers

Parameter transformers:

For example, a custom slugify parameter transformer in route pattern blog\{article:slugify} with Url.Action(new { article = "MyTestArticle" }) generates blog\my-test-article.

Consider the following IOutboundParameterTransformer implementation:

public class SlugifyParameterTransformer : IOutboundParameterTransformer
{
    public string? TransformOutbound(object? value)
    {
        if (value is null)
        {
            return null;
        }

        return Regex.Replace(
            value.ToString()!,
                "([a-z])([A-Z])",
            "$1-$2",
            RegexOptions.CultureInvariant,
            TimeSpan.FromMilliseconds(100))
            .ToLowerInvariant();
    }
}

To use a parameter transformer in a route pattern, configure it using ConstraintMap in Program.cs:

builder.Services.AddRouting(options =>
    options.ConstraintMap["slugify"] = typeof(SlugifyParameterTransformer));

The ASP.NET Core framework uses parameter transformers to transform the URI where an endpoint resolves. For example, parameter transformers transform the route values used to match an area, controller, action, and page:

app.MapControllerRoute(
    name: "default",
    pattern: "{controller:slugify=Home}/{action:slugify=Index}/{id?}");

With the preceding route template, the action SubscriptionManagementController.GetAll is matched with the URI /subscription-management/get-all. A parameter transformer doesn't change the route values used to generate a link. For example, Url.Action("GetAll", "SubscriptionManagement") outputs /subscription-management/get-all.

ASP.NET Core provides API conventions for using parameter transformers with generated routes:

URL generation reference

This section contains a reference for the algorithm implemented by URL generation. In practice, most complex examples of URL generation use controllers or Razor Pages. See routing in controllers for additional information.

The URL generation process begins with a call to LinkGenerator.GetPathByAddress or a similar method. The method is provided with an address, a set of route values, and optionally information about the current request from HttpContext.

The first step is to use the address to resolve a set of candidate endpoints using an IEndpointAddressScheme<TAddress> that matches the address's type.

Once the set of candidates is found by the address scheme, the endpoints are ordered and processed iteratively until a URL generation operation succeeds. URL generation does not check for ambiguities, the first result returned is the final result.

Troubleshooting URL generation with logging

The first step in troubleshooting URL generation is setting the logging level of Microsoft.AspNetCore.Routing to TRACE. LinkGenerator logs many details about its processing which can be useful to troubleshoot problems.

See URL generation reference for details on URL generation.

Addresses

Addresses are the concept in URL generation used to bind a call into the link generator to a set of candidate endpoints.

Addresses are an extensible concept that come with two implementations by default:

  • Using endpoint name (string) as the address:
    • Provides similar functionality to MVC's route name.
    • Uses the IEndpointNameMetadata metadata type.
    • Resolves the provided string against the metadata of all registered endpoints.
    • Throws an exception on startup if multiple endpoints use the same name.
    • Recommended for general-purpose use outside of controllers and Razor Pages.
  • Using route values (RouteValuesAddress) as the address:
    • Provides similar functionality to controllers and Razor Pages legacy URL generation.
    • Very complex to extend and debug.
    • Provides the implementation used by IUrlHelper, Tag Helpers, HTML Helpers, Action Results, etc.

The role of the address scheme is to make the association between the address and matching endpoints by arbitrary criteria:

  • The endpoint name scheme performs a basic dictionary lookup.
  • The route values scheme has a complex best subset of set algorithm.

Ambient values and explicit values

From the current request, routing accesses the route values of the current request HttpContext.Request.RouteValues. The values associated with the current request are referred to as the ambient values. For the purpose of clarity, the documentation refers to the route values passed in to methods as explicit values.

The following example shows ambient values and explicit values. It provides ambient values from the current request and explicit values:

public class WidgetController : ControllerBase
{
    private readonly LinkGenerator _linkGenerator;

    public WidgetController(LinkGenerator linkGenerator) =>
        _linkGenerator = linkGenerator;

    public IActionResult Index()
    {
        var indexPath = _linkGenerator.GetPathByAction(
            HttpContext, values: new { id = 17 })!;

        return Content(indexPath);
    }

    // ...

The preceding code:

The following code provides no ambient values and explicit values:

var subscribePath = _linkGenerator.GetPathByAction(
    "Subscribe", "Home", new { id = 17 })!;

The preceding method returns /Home/Subscribe/17

The following code in the WidgetController returns /Widget/Subscribe/17:

var subscribePath = _linkGenerator.GetPathByAction(
    "Subscribe", null!, new { id = 17 })!;

The following code provides the controller from ambient values in the current request and explicit values:

public class GadgetController : ControllerBase
{
    public IActionResult Index() =>
        Content(Url.Action("Edit", new { id = 17 })!);
}

In the preceding code:

  • /Gadget/Edit/17 is returned.
  • Url gets the IUrlHelper.
  • Action generates a URL with an absolute path for an action method. The URL contains the specified action name and route values.

The following code provides ambient values from the current request and explicit values:

public class IndexModel : PageModel
{
    public void OnGet()
    {
        var editUrl = Url.Page("./Edit", new { id = 17 });

        // ...
    }
}

The preceding code sets url to /Edit/17 when the Edit Razor Page contains the following page directive:

@page "{id:int}"

If the Edit page doesn't contain the "{id:int}" route template, url is /Edit?id=17.

The behavior of MVC's IUrlHelper adds a layer of complexity in addition to the rules described here:

  • IUrlHelper always provides the route values from the current request as ambient values.
  • IUrlHelper.Action always copies the current action and controller route values as explicit values unless overridden by the developer.
  • IUrlHelper.Page always copies the current page route value as an explicit value unless overridden.
  • IUrlHelper.Page always overrides the current handler route value with null as an explicit values unless overridden.

Users are often surprised by the behavioral details of ambient values, because MVC doesn't seem to follow its own rules. For historical and compatibility reasons, certain route values such as action, controller, page, and handler have their own special-case behavior.

The equivalent functionality provided by LinkGenerator.GetPathByAction and LinkGenerator.GetPathByPage duplicates these anomalies of IUrlHelper for compatibility.

URL generation process

Once the set of candidate endpoints are found, the URL generation algorithm:

  • Processes the endpoints iteratively.
  • Returns the first successful result.

The first step in this process is called route value invalidation. Route value invalidation is the process by which routing decides which route values from the ambient values should be used and which should be ignored. Each ambient value is considered and either combined with the explicit values, or ignored.

The best way to think about the role of ambient values is that they attempt to save application developers typing, in some common cases. Traditionally, the scenarios where ambient values are helpful are related to MVC:

  • When linking to another action in the same controller, the controller name doesn't need to be specified.
  • When linking to another controller in the same area, the area name doesn't need to be specified.
  • When linking to the same action method, route values don't need to be specified.
  • When linking to another part of the app, you don't want to carry over route values that have no meaning in that part of the app.

Calls to LinkGenerator or IUrlHelper that return null are usually caused by not understanding route value invalidation. Troubleshoot route value invalidation by explicitly specifying more of the route values to see if that solves the problem.

Route value invalidation works on the assumption that the app's URL scheme is hierarchical, with a hierarchy formed from left-to-right. Consider the basic controller route template {controller}/{action}/{id?} to get an intuitive sense of how this works in practice. A change to a value invalidates all of the route values that appear to the right. This reflects the assumption about hierarchy. If the app has an ambient value for id, and the operation specifies a different value for the controller:

  • id won't be reused because {controller} is to the left of {id?}.

Some examples demonstrating this principle:

  • If the explicit values contain a value for id, the ambient value for id is ignored. The ambient values for controller and action can be used.
  • If the explicit values contain a value for action, any ambient value for action is ignored. The ambient values for controller can be used. If the explicit value for action is different from the ambient value for action, the id value won't be used. If the explicit value for action is the same as the ambient value for action, the id value can be used.
  • If the explicit values contain a value for controller, any ambient value for controller is ignored. If the explicit value for controller is different from the ambient value for controller, the action and id values won't be used. If the explicit value for controller is the same as the ambient value for controller, the action and id values can be used.

This process is further complicated by the existence of attribute routes and dedicated conventional routes. Controller conventional routes such as {controller}/{action}/{id?} specify a hierarchy using route parameters. For dedicated conventional routes and attribute routes to controllers and Razor Pages:

  • There is a hierarchy of route values.
  • They don't appear in the template.

For these cases, URL generation defines the required values concept. Endpoints created by controllers and Razor Pages have required values specified that allow route value invalidation to work.

The route value invalidation algorithm in detail:

  • The required value names are combined with the route parameters, then processed from left-to-right.
  • For each parameter, the ambient value and explicit value are compared:
    • If the ambient value and explicit value are the same, the process continues.
    • If the ambient value is present and the explicit value isn't, the ambient value is used when generating the URL.
    • If the ambient value isn't present and the explicit value is, reject the ambient value and all subsequent ambient values.
    • If the ambient value and the explicit value are present, and the two values are different, reject the ambient value and all subsequent ambient values.

At this point, the URL generation operation is ready to evaluate route constraints. The set of accepted values is combined with the parameter default values, which is provided to constraints. If the constraints all pass, the operation continues.

Next, the accepted values can be used to expand the route template. The route template is processed:

  • From left-to-right.
  • Each parameter has its accepted value substituted.
  • With the following special cases:
    • If the accepted values is missing a value and the parameter has a default value, the default value is used.
    • If the accepted values is missing a value and the parameter is optional, processing continues.
    • If any route parameter to the right of a missing optional parameter has a value, the operation fails.
    • Contiguous default-valued parameters and optional parameters are collapsed where possible.

Values explicitly provided that don't match a segment of the route are added to the query string. The following table shows the result when using the route template {controller}/{action}/{id?}.

Ambient Values Explicit Values Result
controller = "Home" action = "About" /Home/About
controller = "Home" controller = "Order", action = "About" /Order/About
controller = "Home", color = "Red" action = "About" /Home/About
controller = "Home" action = "About", color = "Red" /Home/About?color=Red

Problems with route value invalidation

The following code shows an example of a URL generation scheme that's not supported by routing:

app.MapControllerRoute(
    "default",
    "{culture}/{controller=Home}/{action=Index}/{id?}");

app.MapControllerRoute(
    "blog",
    "{culture}/{**slug}",
    new { controller = "Blog", action = "ReadPost" });

In the preceding code, the culture route parameter is used for localization. The desire is to have the culture parameter always accepted as an ambient value. However, the culture parameter is not accepted as an ambient value because of the way required values work:

  • In the "default" route template, the culture route parameter is to the left of controller, so changes to controller won't invalidate culture.
  • In the "blog" route template, the culture route parameter is considered to be to the right of controller, which appears in the required values.

Configure endpoint metadata

The following links provide information on how to configure endpoint metadata:

Host matching in routes with RequireHost

RequireHost applies a constraint to the route which requires the specified host. The RequireHost or [Host] parameter can be a:

  • Host: www.domain.com, matches www.domain.com with any port.
  • Host with wildcard: *.domain.com, matches www.domain.com, subdomain.domain.com, or www.subdomain.domain.com on any port.
  • Port: *:5000, matches port 5000 with any host.
  • Host and port: www.domain.com:5000 or *.domain.com:5000, matches host and port.

Multiple parameters can be specified using RequireHost or [Host]. The constraint matches hosts valid for any of the parameters. For example, [Host("domain.com", "*.domain.com")] matches domain.com, www.domain.com, and subdomain.domain.com.

The following code uses RequireHost to require the specified host on the route:

app.MapGet("/", () => "Contoso").RequireHost("contoso.com");
app.MapGet("/", () => "AdventureWorks").RequireHost("adventure-works.com");

app.MapHealthChecks("/healthz").RequireHost("*:8080");

The following code uses the [Host] attribute on the controller to require any of the specified hosts:

[Host("contoso.com", "adventure-works.com")]
public class ProductsController : Controller
{
    public IActionResult Index() =>
        View();

    [Host("example.com")]
    public IActionResult Example() =>
        View();
}

When the [Host] attribute is applied to both the controller and action method:

  • The attribute on the action is used.
  • The controller attribute is ignored.

Performance guidance for routing

When an app has performance problems, routing is often suspected as the problem. The reason routing is suspected is that frameworks like controllers and Razor Pages report the amount of time spent inside the framework in their logging messages. When there's a significant difference between the time reported by controllers and the total time of the request:

  • Developers eliminate their app code as the source of the problem.
  • It's common to assume routing is the cause.

Routing is performance tested using thousands of endpoints. It's unlikely that a typical app will encounter a performance problem just by being too large. The most common root cause of slow routing performance is usually a badly-behaving custom middleware.

This following code sample demonstrates a basic technique for narrowing down the source of delay:

var logger = app.Services.GetRequiredService<ILogger<Program>>();

app.Use(async (context, next) =>
{
    var stopwatch = Stopwatch.StartNew();
    await next(context);
    stopwatch.Stop();

    logger.LogInformation("Time 1: {ElapsedMilliseconds}ms", stopwatch.ElapsedMilliseconds);
});

app.UseRouting();

app.Use(async (context, next) =>
{
    var stopwatch = Stopwatch.StartNew();
    await next(context);
    stopwatch.Stop();

    logger.LogInformation("Time 2: {ElapsedMilliseconds}ms", stopwatch.ElapsedMilliseconds);
});

app.UseAuthorization();

app.Use(async (context, next) =>
{
    var stopwatch = Stopwatch.StartNew();
    await next(context);
    stopwatch.Stop();

    logger.LogInformation("Time 3: {ElapsedMilliseconds}ms", stopwatch.ElapsedMilliseconds);
});

app.MapGet("/", () => "Timing Test.");

To time routing:

  • Interleave each middleware with a copy of the timing middleware shown in the preceding code.
  • Add a unique identifier to correlate the timing data with the code.

This is a basic way to narrow down the delay when it's significant, for example, more than 10ms. Subtracting Time 2 from Time 1 reports the time spent inside the UseRouting middleware.

The following code uses a more compact approach to the preceding timing code:

public sealed class AutoStopwatch : IDisposable
{
    private readonly ILogger _logger;
    private readonly string _message;
    private readonly Stopwatch _stopwatch;
    private bool _disposed;

    public AutoStopwatch(ILogger logger, string message) =>
        (_logger, _message, _stopwatch) = (logger, message, Stopwatch.StartNew());

    public void Dispose()
    {
        if (_disposed)
        {
            return;
        }

        _logger.LogInformation("{Message}: {ElapsedMilliseconds}ms",
            _message, _stopwatch.ElapsedMilliseconds);

        _disposed = true;
    }
}
var logger = app.Services.GetRequiredService<ILogger<Program>>();
var timerCount = 0;

app.Use(async (context, next) =>
{
    using (new AutoStopwatch(logger, $"Time {++timerCount}"))
    {
        await next(context);
    }
});

app.UseRouting();

app.Use(async (context, next) =>
{
    using (new AutoStopwatch(logger, $"Time {++timerCount}"))
    {
        await next(context);
    }
});

app.UseAuthorization();

app.Use(async (context, next) =>
{
    using (new AutoStopwatch(logger, $"Time {++timerCount}"))
    {
        await next(context);
    }
});

app.MapGet("/", () => "Timing Test.");

Potentially expensive routing features

The following list provides some insight into routing features that are relatively expensive compared with basic route templates:

  • Regular expressions: It's possible to write regular expressions that are complex, or have long running time with a small amount of input.
  • Complex segments ({x}-{y}-{z}):
    • Are significantly more expensive than parsing a regular URL path segment.
    • Result in many more substrings being allocated.
  • Synchronous data access: Many complex apps have database access as part of their routing. Use extensibility points such as MatcherPolicy and EndpointSelectorContext, which are asynchronous.

Guidance for large route tables

By default ASP.NET Core uses a routing algorithm that trades memory for CPU time. This has the nice effect that route matching time is dependent only on the length of the path to match and not the number of routes. However, this approach can be potentially problematic in some cases, when the app has a large number of routes (in the thousands) and there is a high amount of variable prefixes in the routes. For example, if the routes have parameters in early segments of the route, like {parameter}/some/literal.

It is unlikely for an app to run into a situation where this is a problem unless:

  • There are a high number of routes in the app using this pattern.
  • There is a large number of routes in in the app.

How to determine if an app is running into the large route table problem

  • There are two symptoms to look for:
    • The app is slow to start on the first request.
      • Note that this is required but not sufficient. There are many other non-route problems than can cause slow app startup. Check for the condition below to accurately determine the app is running into this situation.
    • The app consumes a lot of memory during startup and a memory dump shows a large number of Microsoft.AspNetCore.Routing.Matching.DfaNode instances.

How to address this issue

There are several techniques and optimizations can be applied to routes that will largely improve this scenario:

  • Apply route constraints to your parameters, for example {parameter:int}, {parameter:guid}, {parameter:regex(\\d+)}, etc. where possible.
    • This allows the routing algorithm to internally optimize the structures used for matching and drastically reduce the memory used.
    • In the vast majority of cases this will suffice to get back to an acceptable behavior.
  • Change the routes to move parameters to later segments in the template.
    • This reduces the number of possible "paths" to match an endpoint given a path.
  • Use a dynamic route and perform the mapping to a controller/page dynamically.
    • This can be achieved using MapDynamicControllerRoute and MapDynamicPageRoute.

Guidance for library authors

This section contains guidance for library authors building on top of routing. These details are intended to ensure that app developers have a good experience using libraries and frameworks that extend routing.

Define endpoints

To create a framework that uses routing for URL matching, start by defining a user experience that builds on top of UseEndpoints.

DO build on top of IEndpointRouteBuilder. This allows users to compose your framework with other ASP.NET Core features without confusion. Every ASP.NET Core template includes routing. Assume routing is present and familiar for users.

// Your framework
app.MapMyFramework(...);

app.MapHealthChecks("/healthz");

DO return a sealed concrete type from a call to MapMyFramework(...) that implements IEndpointConventionBuilder. Most framework Map... methods follow this pattern. The IEndpointConventionBuilder interface:

  • Allows for metadata to be composed.
  • Is targeted by a variety of extension methods.

Declaring your own type allows you to add your own framework-specific functionality to the builder. It's ok to wrap a framework-declared builder and forward calls to it.

// Your framework
app.MapMyFramework(...)
    .RequireAuthorization()
    .WithMyFrameworkFeature(awesome: true);

app.MapHealthChecks("/healthz");

CONSIDER writing your own EndpointDataSource. EndpointDataSource is the low-level primitive for declaring and updating a collection of endpoints. EndpointDataSource is a powerful API used by controllers and Razor Pages.

The routing tests have a basic example of a non-updating data source.

DO NOT attempt to register an EndpointDataSource by default. Require users to register your framework in UseEndpoints. The philosophy of routing is that nothing is included by default, and that UseEndpoints is the place to register endpoints.

Creating routing-integrated middleware

CONSIDER defining metadata types as an interface.

DO make it possible to use metadata types as an attribute on classes and methods.

public interface ICoolMetadata
{
    bool IsCool { get; }
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class CoolMetadataAttribute : Attribute, ICoolMetadata
{
    public bool IsCool => true;
}

Frameworks like controllers and Razor Pages support applying metadata attributes to types and methods. If you declare metadata types:

  • Make them accessible as attributes.
  • Most users are familiar with applying attributes.

Declaring a metadata type as an interface adds another layer of flexibility:

  • Interfaces are composable.
  • Developers can declare their own types that combine multiple policies.

DO make it possible to override metadata, as shown in the following example:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class SuppressCoolMetadataAttribute : Attribute, ICoolMetadata
{
    public bool IsCool => false;
}

[CoolMetadata]
public class MyController : Controller
{
    public void MyCool() { }

    [SuppressCoolMetadata]
    public void Uncool() { }
}

The best way to follow these guidelines is to avoid defining marker metadata:

  • Don't just look for the presence of a metadata type.
  • Define a property on the metadata and check the property.

The metadata collection is ordered and supports overriding by priority. In the case of controllers, metadata on the action method is most specific.

DO make middleware useful with and without routing:

app.UseAuthorization(new AuthorizationPolicy() { ... });

// Your framework
app.MapMyFramework(...).RequireAuthorization();

As an example of this guideline, consider the UseAuthorization middleware. The authorization middleware allows you to pass in a fallback policy. The fallback policy, if specified, applies to both:

  • Endpoints without a specified policy.
  • Requests that don't match an endpoint.

This makes the authorization middleware useful outside of the context of routing. The authorization middleware can be used for traditional middleware programming.

Debug diagnostics

For detailed routing diagnostic output, set Logging:LogLevel:Microsoft to Debug. In the development environment, set the log level in appsettings.Development.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Debug",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  }
}

Additional resources

Routing is responsible for matching incoming HTTP requests and dispatching those requests to the app's executable endpoints. Endpoints are the app's units of executable request-handling code. Endpoints are defined in the app and configured when the app starts. The endpoint matching process can extract values from the request's URL and provide those values for request processing. Using endpoint information from the app, routing is also able to generate URLs that map to endpoints.

Apps can configure routing using:

  • Controllers
  • Razor Pages
  • SignalR
  • gRPC Services
  • Endpoint-enabled middleware such as Health Checks.
  • Delegates and lambdas registered with routing.

This document covers low-level details of ASP.NET Core routing. For information on configuring routing:

The endpoint routing system described in this document applies to ASP.NET Core 3.0 and later. For information on the previous routing system based on IRouter, select the ASP.NET Core 2.1 version using one of the following approaches:

View or download sample code (how to download)

The download samples for this document are enabled by a specific Startup class. To run a specific sample, modify Program.cs to call the desired Startup class.

Routing basics

All ASP.NET Core templates include routing in the generated code. Routing is registered in the middleware pipeline in Startup.Configure.

The following code shows a basic example of routing:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
    });
}

Routing uses a pair of middleware, registered by UseRouting and UseEndpoints:

  • UseRouting adds route matching to the middleware pipeline. This middleware looks at the set of endpoints defined in the app, and selects the best match based on the request.
  • UseEndpoints adds endpoint execution to the middleware pipeline. It runs the delegate associated with the selected endpoint.

The preceding example includes a single route to code endpoint using the MapGet method:

  • When an HTTP GET request is sent to the root URL /:
    • The request delegate shown executes.
    • Hello World! is written to the HTTP response. By default, the root URL / is https://localhost:5001/.
  • If the request method is not GET or the root URL is not /, no route matches and an HTTP 404 is returned.

Endpoint

The MapGet method is used to define an endpoint. An endpoint is something that can be:

  • Selected, by matching the URL and HTTP method.
  • Executed, by running the delegate.

Endpoints that can be matched and executed by the app are configured in UseEndpoints. For example, MapGet, MapPost, and similar methods connect request delegates to the routing system. Additional methods can be used to connect ASP.NET Core framework features to the routing system:

The following example shows routing with a more sophisticated route template:

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/hello/{name:alpha}", async context =>
    {
        var name = context.Request.RouteValues["name"];
        await context.Response.WriteAsync($"Hello {name}!");
    });
});

The string /hello/{name:alpha} is a route template. It is used to configure how the endpoint is matched. In this case, the template matches:

  • A URL like /hello/Ryan
  • Any URL path that begins with /hello/ followed by a sequence of alphabetic characters. :alpha applies a route constraint that matches only alphabetic characters. Route constraints are explained later in this document.

The second segment of the URL path, {name:alpha}:

The endpoint routing system described in this document is new as of ASP.NET Core 3.0. However, all versions of ASP.NET Core support the same set of route template features and route constraints.

The following example shows routing with health checks and authorization:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    // Matches request to an endpoint.
    app.UseRouting();

    // Endpoint aware middleware. 
    // Middleware can use metadata from the matched endpoint.
    app.UseAuthentication();
    app.UseAuthorization();

    // Execute the matched endpoint.
    app.UseEndpoints(endpoints =>
    {
        // Configure the Health Check endpoint and require an authorized user.
        endpoints.MapHealthChecks("/healthz").RequireAuthorization();

        // Configure another endpoint, no authorization requirements.
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
    });
}

If you would like to see code comments translated to languages other than English, let us know in this GitHub discussion issue.

The preceding example demonstrates how:

  • The authorization middleware can be used with routing.
  • Endpoints can be used to configure authorization behavior.

The MapHealthChecks call adds a health check endpoint. Chaining RequireAuthorization on to this call attaches an authorization policy to the endpoint.

Calling UseAuthentication and UseAuthorization adds the authentication and authorization middleware. These middleware are placed between UseRouting and UseEndpoints so that they can:

  • See which endpoint was selected by UseRouting.
  • Apply an authorization policy before UseEndpoints dispatches to the endpoint.

Endpoint metadata

In the preceding example, there are two endpoints, but only the health check endpoint has an authorization policy attached. If the request matches the health check endpoint, /healthz, an authorization check is performed. This demonstrates that endpoints can have extra data attached to them. This extra data is called endpoint metadata:

  • The metadata can be processed by routing-aware middleware.
  • The metadata can be of any .NET type.

Routing concepts

The routing system builds on top of the middleware pipeline by adding the powerful endpoint concept. Endpoints represent units of the app's functionality that are distinct from each other in terms of routing, authorization, and any number of ASP.NET Core's systems.

ASP.NET Core endpoint definition

An ASP.NET Core endpoint is:

The following code shows how to retrieve and inspect the endpoint matching the current request:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.UseRouting();

    app.Use(next => context =>
    {
        var endpoint = context.GetEndpoint();
        if (endpoint is null)
        {
            return Task.CompletedTask;
        }
        
        Console.WriteLine($"Endpoint: {endpoint.DisplayName}");

        if (endpoint is RouteEndpoint routeEndpoint)
        {
            Console.WriteLine("Endpoint has route pattern: " +
                routeEndpoint.RoutePattern.RawText);
        }

        foreach (var metadata in endpoint.Metadata)
        {
            Console.WriteLine($"Endpoint has metadata: {metadata}");
        }

        return Task.CompletedTask;
    });

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
    });
}

The endpoint, if selected, can be retrieved from the HttpContext. Its properties can be inspected. Endpoint objects are immutable and cannot be modified after creation. The most common type of endpoint is a RouteEndpoint. RouteEndpoint includes information that allows it to be selected by the routing system.

In the preceding code, app.Use configures an in-line middleware.

The following code shows that, depending on where app.Use is called in the pipeline, there may not be an endpoint:

// Location 1: before routing runs, endpoint is always null here
app.Use(next => context =>
{
    Console.WriteLine($"1. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    return next(context);
});

app.UseRouting();

// Location 2: after routing runs, endpoint will be non-null if routing found a match
app.Use(next => context =>
{
    Console.WriteLine($"2. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    return next(context);
});

app.UseEndpoints(endpoints =>
{
    // Location 3: runs when this endpoint matches
    endpoints.MapGet("/", context =>
    {
        Console.WriteLine(
            $"3. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
        return Task.CompletedTask;
    }).WithDisplayName("Hello");
});

// Location 4: runs after UseEndpoints - will only run if there was no match
app.Use(next => context =>
{
    Console.WriteLine($"4. Endpoint: {context.GetEndpoint()?.DisplayName ?? "(null)"}");
    return next(context);
});

This preceding sample adds Console.WriteLine statements that display whether or not an endpoint has been selected. For clarity, the sample assigns a display name to the provided / endpoint.

Running this code with a URL of / displays:

1. Endpoint: (null)
2. Endpoint: Hello
3. Endpoint: Hello

Running this code with any other URL displays:

1. Endpoint: (null)
2. Endpoint: (null)
4. Endpoint: (null)

This output demonstrates that:

  • The endpoint is always null before UseRouting is called.
  • If a match is found, the endpoint is non-null between UseRouting and UseEndpoints.
  • The UseEndpoints middleware is terminal when a match is found. Terminal middleware is defined later in this document.
  • The middleware after UseEndpoints execute only when no match is found.

The UseRouting middleware uses the SetEndpoint method to attach the endpoint to the current context. It's possible to replace the UseRouting middleware with custom logic and still get the benefits of using endpoints. Endpoints are a low-level primitive like middleware, and aren't coupled to the routing implementation. Most apps don't need to replace UseRouting with custom logic.

The UseEndpoints middleware is designed to be used in tandem with the UseRouting middleware. The core logic to execute an endpoint isn't complicated. Use GetEndpoint to retrieve the endpoint, and then invoke its RequestDelegate property.

The following code demonstrates how middleware can influence or react to routing:

public class IntegratedMiddlewareStartup
{ 
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        // Location 1: Before routing runs. Can influence request before routing runs.
        app.UseHttpMethodOverride();

        app.UseRouting();

        // Location 2: After routing runs. Middleware can match based on metadata.
        app.Use(next => context =>
        {
            var endpoint = context.GetEndpoint();
            if (endpoint?.Metadata.GetMetadata<AuditPolicyAttribute>()?.NeedsAudit
                                                                            == true)
            {
                Console.WriteLine($"ACCESS TO SENSITIVE DATA AT: {DateTime.UtcNow}");
            }

            return next(context);
        });

        app.UseEndpoints(endpoints =>
        {         
            endpoints.MapGet("/", async context =>
            {
                await context.Response.WriteAsync("Hello world!");
            });

            // Using metadata to configure the audit policy.
            endpoints.MapGet("/sensitive", async context =>
            {
                await context.Response.WriteAsync("sensitive data");
            })
            .WithMetadata(new AuditPolicyAttribute(needsAudit: true));
        });

    } 
}

public class AuditPolicyAttribute : Attribute
{
    public AuditPolicyAttribute(bool needsAudit)
    {
        NeedsAudit = needsAudit;
    }

    public bool NeedsAudit { get; }
}

The preceding example demonstrates two important concepts:

  • Middleware can run before UseRouting to modify the data that routing operates upon.
  • Middleware can run between UseRouting and UseEndpoints to process the results of routing before the endpoint is executed.
    • Middleware that runs between UseRouting and UseEndpoints:
      • Usually inspects metadata to understand the endpoints.
      • Often makes security decisions, as done by UseAuthorization and UseCors.
    • The combination of middleware and metadata allows configuring policies per-endpoint.

The preceding code shows an example of a custom middleware that supports per-endpoint policies. The middleware writes an audit log of access to sensitive data to the console. The middleware can be configured to audit an endpoint with the AuditPolicyAttribute metadata. This sample demonstrates an opt-in pattern where only endpoints that are marked as sensitive are audited. It's possible to define this logic in reverse, auditing everything that isn't marked as safe, for example. The endpoint metadata system is flexible. This logic could be designed in whatever way suits the use case.

The preceding sample code is intended to demonstrate the basic concepts of endpoints. The sample is not intended for production use. A more complete version of an audit log middleware would:

  • Log to a file or database.
  • Include details such as the user, IP address, name of the sensitive endpoint, and more.

The audit policy metadata AuditPolicyAttribute is defined as an Attribute for easier use with class-based frameworks such as controllers and SignalR. When using route to code:

  • Metadata is attached with a builder API.
  • Class-based frameworks include all attributes on the corresponding method and class when creating endpoints.

The best practices for metadata types are to define them either as interfaces or attributes. Interfaces and attributes allow code reuse. The metadata system is flexible and doesn't impose any limitations.

Comparing a terminal middleware and routing

The following code sample contrasts using middleware with using routing:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    // Approach 1: Writing a terminal middleware.
    app.Use(next => async context =>
    {
        if (context.Request.Path == "/")
        {
            await context.Response.WriteAsync("Hello terminal middleware!");
            return;
        }

        await next(context);
    });

    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        // Approach 2: Using routing.
        endpoints.MapGet("/Movie", async context =>
        {
            await context.Response.WriteAsync("Hello routing!");
        });
    });
}

The style of middleware shown with Approach 1: is terminal middleware. It's called terminal middleware because it does a matching operation:

  • The matching operation in the preceding sample is Path == "/" for the middleware and Path == "/Movie" for routing.
  • When a match is successful, it executes some functionality and returns, rather than invoking the next middleware.

It's called terminal middleware because it terminates the search, executes some functionality, and then returns.

Comparing a terminal middleware and routing:

  • Both approaches allow terminating the processing pipeline:
    • Middleware terminates the pipeline by returning rather than invoking next.
    • Endpoints are always terminal.
  • Terminal middleware allows positioning the middleware at an arbitrary place in the pipeline:
  • Terminal middleware allows arbitrary code to determine when the middleware matches:
    • Custom route matching code can be verbose and difficult to write correctly.
    • Routing provides straightforward solutions for typical apps. Most apps don't require custom route matching code.
  • Endpoints interface with middleware such as UseAuthorization and UseCors.
    • Using a terminal middleware with UseAuthorization or UseCors requires manual interfacing with the authorization system.

An endpoint defines both:

  • A delegate to process requests.
  • A collection of arbitrary metadata. The metadata is used to implement cross-cutting concerns based on policies and configuration attached to each endpoint.

Terminal middleware can be an effective tool, but can require:

  • A significant amount of coding and testing.
  • Manual integration with other systems to achieve the desired level of flexibility.

Consider integrating with routing before writing a terminal middleware.

Existing terminal middleware that integrates with Map or MapWhen can usually be turned into a routing aware endpoint. MapHealthChecks demonstrates the pattern for router-ware:

The following code shows use of MapHealthChecks:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    // Matches request to an endpoint.
    app.UseRouting();

    // Endpoint aware middleware. 
    // Middleware can use metadata from the matched endpoint.
    app.UseAuthentication();
    app.UseAuthorization();

    // Execute the matched endpoint.
    app.UseEndpoints(endpoints =>
    {
        // Configure the Health Check endpoint and require an authorized user.
        endpoints.MapHealthChecks("/healthz").RequireAuthorization();

        // Configure another endpoint, no authorization requirements.
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
    });
}

The preceding sample shows why returning the builder object is important. Returning the builder object allows the app developer to configure policies such as authorization for the endpoint. In this example, the health checks middleware has no direct integration with the authorization system.

The metadata system was created in response to the problems encountered by extensibility authors using terminal middleware. It's problematic for each middleware to implement its own integration with the authorization system.

URL matching

  • Is the process by which routing matches an incoming request to an endpoint.
  • Is based on data in the URL path and headers.
  • Can be extended to consider any data in the request.

When a routing middleware executes, it sets an Endpoint and route values to a request feature on the HttpContext from the current request:

Middleware running after the routing middleware can inspect the endpoint and take action. For example, an authorization middleware can interrogate the endpoint's metadata collection for an authorization policy. After all of the middleware in the request processing pipeline is executed, the selected endpoint's delegate is invoked.

The routing system in endpoint routing is responsible for all dispatching decisions. Because the middleware applies policies based on the selected endpoint, it's important that:

  • Any decision that can affect dispatching or the application of security policies is made inside the routing system.

Warning

For backwards-compatibility, when a Controller or Razor Pages endpoint delegate is executed, the properties of RouteContext.RouteData are set to appropriate values based on the request processing performed thus far.

The RouteContext type will be marked obsolete in a future release:

  • Migrate RouteData.Values to HttpRequest.RouteValues.
  • Migrate RouteData.DataTokens to retrieve IDataTokensMetadata from the endpoint metadata.

URL matching operates in a configurable set of phases. In each phase, the output is a set of matches. The set of matches can be narrowed down further by the next phase. The routing implementation does not guarantee a processing order for matching endpoints. All possible matches are processed at once. The URL matching phases occur in the following order. ASP.NET Core:

  1. Processes the URL path against the set of endpoints and their route templates, collecting all of the matches.
  2. Takes the preceding list and removes matches that fail with route constraints applied.
  3. Takes the preceding list and removes matches that fail the set of MatcherPolicy instances.
  4. Uses the EndpointSelector to make a final decision from the preceding list.

The list of endpoints is prioritized according to:

All matching endpoints are processed in each phase until the EndpointSelector is reached. The EndpointSelector is the final phase. It chooses the highest priority endpoint from the matches as the best match. If there are other matches with the same priority as the best match, an ambiguous match exception is thrown.

The route precedence is computed based on a more specific route template being given a higher priority. For example, consider the templates /hello and /{message}:

  • Both match the URL path /hello.
  • /hello is more specific and therefore higher priority.

In general, route precedence does a good job of choosing the best match for the kinds of URL schemes used in practice. Use Order only when necessary to avoid an ambiguity.

Due to the kinds of extensibility provided by routing, it isn't possible for the routing system to compute ahead of time the ambiguous routes. Consider an example such as the route templates /{message:alpha} and /{message:int}:

  • The alpha constraint matches only alphabetic characters.
  • The int constraint matches only numbers.
  • These templates have the same route precedence, but there's no single URL they both match.
  • If the routing system reported an ambiguity error at startup, it would block this valid use case.

Warning

The order of operations inside UseEndpoints doesn't influence the behavior of routing, with one exception. MapControllerRoute and MapAreaRoute automatically assign an order value to their endpoints based on the order they are invoked. This simulates long-time behavior of controllers without the routing system providing the same guarantees as older routing implementations.

In the legacy implementation of routing, it's possible to implement routing extensibility that has a dependency on the order in which routes are processed. Endpoint routing in ASP.NET Core 3.0 and later:

  • Doesn't have a concept of routes.
  • Doesn't provide ordering guarantees. All endpoints are processed at once.

Route template precedence and endpoint selection order

Route template precedence is a system that assigns each route template a value based on how specific it is. Route template precedence:

  • Avoids the need to adjust the order of endpoints in common cases.
  • Attempts to match the common-sense expectations of routing behavior.

For example, consider templates /Products/List and /Products/{id}. It would be reasonable to assume that /Products/List is a better match than /Products/{id} for the URL path /Products/List. This works because the literal segment /List is considered to have better precedence than the parameter segment /{id}.

The details of how precedence works are coupled to how route templates are defined:

  • Templates with more segments are considered more specific.
  • A segment with literal text is considered more specific than a parameter segment.
  • A parameter segment with a constraint is considered more specific than one without.
  • A complex segment is considered as specific as a parameter segment with a constraint.
  • Catch-all parameters are the least specific. See catch-all in the Route template reference for important information on catch-all routes.

See the source code on GitHub for a reference of exact values.

URL generation concepts

URL generation:

  • Is the process by which routing can create a URL path based on a set of route values.
  • Allows for a logical separation between endpoints and the URLs that access them.

Endpoint routing includes the LinkGenerator API. LinkGenerator is a singleton service available from DI. The LinkGenerator API can be used outside of the context of an executing request. Mvc.IUrlHelper and scenarios that rely on IUrlHelper, such as Tag Helpers, HTML Helpers, and Action Results, use the LinkGenerator API internally to provide link generating capabilities.

The link generator is backed by the concept of an address and address schemes. An address scheme is a way of determining the endpoints that should be considered for link generation. For example, the route name and route values scenarios many users are familiar with from controllers and Razor Pages are implemented as an address scheme.

The link generator can link to controllers and Razor Pages via the following extension methods:

Overloads of these methods accept arguments that include the HttpContext. These methods are functionally equivalent to Url.Action and Url.Page, but offer additional flexibility and options.

The GetPath* methods are most similar to Url.Action and Url.Page, in that they generate a URI containing an absolute path. The GetUri* methods always generate an absolute URI containing a scheme and host. The methods that accept an HttpContext generate a URI in the context of the executing request. The ambient route values, URL base path, scheme, and host from the executing request are used unless overridden.

LinkGenerator is called with an address. Generating a URI occurs in two steps:

  1. An address is bound to a list of endpoints that match the address.
  2. Each endpoint's RoutePattern is evaluated until a route pattern that matches the supplied values is found. The resulting output is combined with the other URI parts supplied to the link generator and returned.

The methods provided by LinkGenerator support standard link generation capabilities for any type of address. The most convenient way to use the link generator is through extension methods that perform operations for a specific address type:

Extension Method Description
GetPathByAddress Generates a URI with an absolute path based on the provided values.
GetUriByAddress Generates an absolute URI based on the provided values.

Warning

Pay attention to the following implications of calling LinkGenerator methods:

  • Use GetUri* extension methods with caution in an app configuration that doesn't validate the Host header of incoming requests. If the Host header of incoming requests isn't validated, untrusted request input can be sent back to the client in URIs in a view or page. We recommend that all production apps configure their server to validate the Host header against known valid values.

  • Use LinkGenerator with caution in middleware in combination with Map or MapWhen. Map* changes the base path of the executing request, which affects the output of link generation. All of the LinkGenerator APIs allow specifying a base path. Specify an empty base path to undo the Map* affect on link generation.

Middleware example

In the following example, a middleware uses the LinkGenerator API to create a link to an action method that lists store products. Using the link generator by injecting it into a class and calling GenerateLink is available to any class in an app:

public class ProductsLinkMiddleware
{
    private readonly LinkGenerator _linkGenerator;

    public ProductsLinkMiddleware(RequestDelegate next, LinkGenerator linkGenerator)
    {
        _linkGenerator = linkGenerator;
    }

    public async Task InvokeAsync(HttpContext httpContext)
    {
        var url = _linkGenerator.GetPathByAction("ListProducts", "Store");

        httpContext.Response.ContentType = "text/plain";

        await httpContext.Response.WriteAsync($"Go to {url} to see our products.");
    }
}

Route template reference

Tokens within {} define route parameters that are bound if the route is matched. More than one route parameter can be defined in a route segment, but route parameters must be separated by a literal value. For example, {controller=Home}{action=Index} isn't a valid route, since there's no literal value between {controller} and {action}. Route parameters must have a name and may have additional attributes specified.

Literal text other than route parameters (for example, {id}) and the path separator / must match the text in the URL. Text matching is case-insensitive and based on the decoded representation of the URL's path. To match a literal route parameter delimiter { or }, escape the delimiter by repeating the character. For example {{ or }}.

Asterisk * or double asterisk **:

  • Can be used as a prefix to a route parameter to bind to the rest of the URI.
  • Are called a catch-all parameters. For example, blog/{**slug}:
    • Matches any URI that starts with /blog and has any value following it.
    • The value following /blog is assigned to the slug route value.

Warning

A catch-all parameter may match routes incorrectly due to a bug in routing. Apps impacted by this bug have the following characteristics:

  • A catch-all route, for example, {**slug}"
  • The catch-all route fails to match requests it should match.
  • Removing other routes makes catch-all route start working.

See GitHub bugs 18677 and 16579 for example cases that hit this bug.

An opt-in fix for this bug is contained in .NET Core 3.1.301 SDK and later. The following code sets an internal switch that fixes this bug:

public static void Main(string[] args)
{
   AppContext.SetSwitch("Microsoft.AspNetCore.Routing.UseCorrectCatchAllBehavior", 
                         true);
   CreateHostBuilder(args).Build().Run();
}
// Remaining code removed for brevity.

Catch-all parameters can also match the empty string.

The catch-all parameter escapes the appropriate characters when the route is used to generate a URL, including path separator / characters. For example, the route foo/{*path} with route values { path = "my/path" } generates foo/my%2Fpath. Note the escaped forward slash. To round-trip path separator characters, use the ** route parameter prefix. The route foo/{**path} with { path = "my/path" } generates foo/my/path.

URL patterns that attempt to capture a file name with an optional file extension have additional considerations. For example, consider the template files/{filename}.{ext?}. When values for both filename and ext exist, both values are populated. If only a value for filename exists in the URL, the route matches because the trailing . is optional. The following URLs match this route:

  • /files/myFile.txt
  • /files/myFile

Route parameters may have default values designated by specifying the default value after the parameter name separated by an equals sign (=). For example, {controller=Home} defines Home as the default value for controller. The default value is used if no value is present in the URL for the parameter. Route parameters are made optional by appending a question mark (?) to the end of the parameter name. For example, id?. The difference between optional values and default route parameters is:

  • A route parameter with a default value always produces a value.
  • An optional parameter has a value only when a value is provided by the request URL.

Route parameters may have constraints that must match the route value bound from the URL. Adding : and constraint name after the route parameter name specifies an inline constraint on a route parameter. If the constraint requires arguments, they're enclosed in parentheses (...) after the constraint name. Multiple inline constraints can be specified by appending another : and constraint name.

The constraint name and arguments are passed to the IInlineConstraintResolver service to create an instance of IRouteConstraint to use in URL processing. For example, the route template blog/{article:minlength(10)} specifies a minlength constraint with the argument 10. For more information on route constraints and a list of the constraints provided by the framework, see the Route constraint reference section.

Route parameters may also have parameter transformers. Parameter transformers transform a parameter's value when generating links and matching actions and pages to URLs. Like constraints, parameter transformers can be added inline to a route parameter by adding a : and transformer name after the route parameter name. For example, the route template blog/{article:slugify} specifies a slugify transformer. For more information on parameter transformers, see the Parameter transformer reference section.

The following table demonstrates example route templates and their behavior:

Route Template Example Matching URI The request URI…
hello /hello Only matches the single path /hello.
{Page=Home} / Matches and sets Page to Home.
{Page=Home} /Contact Matches and sets Page to Contact.
{controller}/{action}/{id?} /Products/List Maps to the Products controller and List action.
{controller}/{action}/{id?} /Products/Details/123 Maps to the Products controller and Details action withid set to 123.
{controller=Home}/{action=Index}/{id?} / Maps to the Home controller and Index method. id is ignored.
{controller=Home}/{action=Index}/{id?} /Products Maps to the Products controller and Index method. id is ignored.

Using a template is generally the simplest approach to routing. Constraints and defaults can also be specified outside the route template.

Complex segments

Complex segments are processed by matching up literal delimiters from right to left in a non-greedy way. For example, [Route("/a{b}c{d}")] is a complex segment. Complex segments work in a particular way that must be understood to use them successfully. The example in this section demonstrates why complex segments only really work well when the delimiter text doesn't appear inside the parameter values. Using a regex and then manually extracting the values is needed for more complex cases.

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

This is a summary of the steps that routing performs with the template /a{b}c{d} and the URL path /abcd. The | is used to help visualize how the algorithm works:

  • The first literal, right to left, is c. So /abcd is searched from right and finds /ab|c|d.
  • Everything to the right (d) is now matched to the route parameter {d}.
  • The next literal, right to left, is a. So /ab|c|d is searched starting where we left off, then a is found /|a|b|c|d.
  • The value to the right (b) is now matched to the route parameter {b}.
  • There is no remaining text and no remaining route template, so this is a match.

Here's an example of a negative case using the same template /a{b}c{d} and the URL path /aabcd. The | is used to help visualize how the algorithm works. This case isn't a match, which is explained by the same algorithm:

  • The first literal, right to left, is c. So /aabcd is searched from right and finds /aab|c|d.
  • Everything to the right (d) is now matched to the route parameter {d}.
  • The next literal, right to left, is a. So /aab|c|d is searched starting where we left off, then a is found /a|a|b|c|d.
  • The value to the right (b) is now matched to the route parameter {b}.
  • At this point there is remaining text a, but the algorithm has run out of route template to parse, so this is not a match.

Since the matching algorithm is non-greedy:

  • It matches the smallest amount of text possible in each step.
  • Any case where the delimiter value appears inside the parameter values results in not matching.

Regular expressions provide much more control over their matching behavior.

Greedy matching, also know as lazy matching, matches the largest possible string. Non-greedy matches the smallest possible string.

Route constraint reference

Route constraints execute when a match has occurred to the incoming URL and the URL path is tokenized into route values. Route constraints generally inspect the route value associated via the route template and make a true or false decision about whether the value is acceptable. Some route constraints use data outside the route value to consider whether the request can be routed. For example, the HttpMethodRouteConstraint can accept or reject a request based on its HTTP verb. Constraints are used in routing requests and link generation.

Warning

Don't use constraints for input validation. If constraints are used for input validation, invalid input results in a 404 Not Found response. Invalid input should produce a 400 Bad Request with an appropriate error message. Route constraints are used to disambiguate similar routes, not to validate the inputs for a particular route.

The following table demonstrates example route constraints and their expected behavior:

constraint Example Example Matches Notes
int {id:int} 123456789, -123456789 Matches any integer
bool {active:bool} true, FALSE Matches true or false. Case-insensitive
datetime {dob:datetime} 2016-12-31, 2016-12-31 7:32pm Matches a valid DateTime value in the invariant culture. See preceding warning.
decimal {price:decimal} 49.99, -1,000.01 Matches a valid decimal value in the invariant culture. See preceding warning.
double {weight:double} 1.234, -1,001.01e8 Matches a valid double value in the invariant culture. See preceding warning.
float {weight:float} 1.234, -1,001.01e8 Matches a valid float value in the invariant culture. See preceding warning.
guid {id:guid} CD2C1638-1638-72D5-1638-DEADBEEF1638 Matches a valid Guid value
long {ticks:long} 123456789, -123456789 Matches a valid long value
minlength(value) {username:minlength(4)} Rick String must be at least 4 characters
maxlength(value) {filename:maxlength(8)} MyFile String must be no more than 8 characters
length(length) {filename:length(12)} somefile.txt String must be exactly 12 characters long
length(min,max) {filename:length(8,16)} somefile.txt String must be at least 8 and no more than 16 characters long
min(value) {age:min(18)} 19 Integer value must be at least 18
max(value) {age:max(120)} 91 Integer value must be no more than 120
range(min,max) {age:range(18,120)} 91 Integer value must be at least 18 but no more than 120
alpha {name:alpha} Rick String must consist of one or more alphabetical characters, a-z and case-insensitive.
regex(expression) {ssn:regex(^\\d{{3}}-\\d{{2}}-\\d{{4}}$)} 123-45-6789 String must match the regular expression. See tips about defining a regular expression.
required {name:required} Rick Used to enforce that a non-parameter value is present during URL generation

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Multiple, colon delimited constraints can be applied to a single parameter. For example, the following constraint restricts a parameter to an integer value of 1 or greater:

[Route("users/{id:int:min(1)}")]
public User GetUserById(int id) { }

Warning

Route constraints that verify the URL and are converted to a CLR type always use the invariant culture. For example, conversion to the CLR type int or DateTime. These constraints assume that the URL is not localizable. The framework-provided route constraints don't modify the values stored in route values. All route values parsed from the URL are stored as strings. For example, the float constraint attempts to convert the route value to a float, but the converted value is used only to verify it can be converted to a float.

Regular expressions in constraints

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

Regular expressions can be specified as inline constraints using the regex(...) route constraint. Methods in the MapControllerRoute family also accept an object literal of constraints. If that form is used, string values are interpreted as regular expressions.

The following code uses an inline regex constraint:

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("{message:regex(^\\d{{3}}-\\d{{2}}-\\d{{4}}$)}",
        context => 
        {
            return context.Response.WriteAsync("inline-constraint match");
        });
 });

The following code uses an object literal to specify a regex constraint:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(
        name: "people",
        pattern: "People/{ssn}",
        constraints: new { ssn = "^\\d{3}-\\d{2}-\\d{4}$", },
        defaults: new { controller = "People", action = "List", });
});

The ASP.NET Core framework adds RegexOptions.IgnoreCase | RegexOptions.Compiled | RegexOptions.CultureInvariant to the regular expression constructor. See RegexOptions for a description of these members.

Regular expressions use delimiters and tokens similar to those used by routing and the C# language. Regular expression tokens must be escaped. To use the regular expression ^\d{3}-\d{2}-\d{4}$ in an inline constraint, use one of the following:

  • Replace \ characters provided in the string as \\ characters in the C# source file in order to escape the \ string escape character.
  • Verbatim string literals.

To escape routing parameter delimiter characters {, }, [, ], double the characters in the expression, for example, {{, }}, [[, ]]. The following table shows a regular expression and its escaped version:

Regular expression Escaped regular expression
^\d{3}-\d{2}-\d{4}$ ^\\d{{3}}-\\d{{2}}-\\d{{4}}$
^[a-z]{2}$ ^[[a-z]]{{2}}$

Regular expressions used in routing often start with the ^ character and match the starting position of the string. The expressions often end with the $ character and match the end of the string. The ^ and $ characters ensure that the regular expression matches the entire route parameter value. Without the ^ and $ characters, the regular expression matches any substring within the string, which is often undesirable. The following table provides examples and explains why they match or fail to match:

Expression String Match Comment
[a-z]{2} hello Yes Substring matches
[a-z]{2} 123abc456 Yes Substring matches
[a-z]{2} mz Yes Matches expression
[a-z]{2} MZ Yes Not case sensitive
^[a-z]{2}$ hello No See ^ and $ above
^[a-z]{2}$ 123abc456 No See ^ and $ above

For more information on regular expression syntax, see .NET Framework Regular Expressions.

To constrain a parameter to a known set of possible values, use a regular expression. For example, {action:regex(^(list|get|create)$)} only matches the action route value to list, get, or create. If passed into the constraints dictionary, the string ^(list|get|create)$ is equivalent. Constraints that are passed in the constraints dictionary that don't match one of the known constraints are also treated as regular expressions. Constraints that are passed within a template that don't match one of the known constraints are not treated as regular expressions.

Custom route constraints

Custom route constraints can be created by implementing the IRouteConstraint interface. The IRouteConstraint interface contains Match, which returns true if the constraint is satisfied and false otherwise.

Custom route constraints are rarely needed. Before implementing a custom route constraint, consider alternatives, such as model binding.

The ASP.NET Core Constraints folder provides good examples of creating a constraints. For example, GuidRouteConstraint.

To use a custom IRouteConstraint, the route constraint type must be registered with the app's ConstraintMap in the service container. A ConstraintMap is a dictionary that maps route constraint keys to IRouteConstraint implementations that validate those constraints. An app's ConstraintMap can be updated in Startup.ConfigureServices either as part of a services.AddRouting call or by configuring RouteOptions directly with services.Configure<RouteOptions>. For example:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    services.AddRouting(options =>
    {
        options.ConstraintMap.Add("customName", typeof(MyCustomConstraint));
    });
}

The preceding constraint is applied in the following code:

[Route("api/[controller]")]
[ApiController]
public class TestController : ControllerBase
{
    // GET /api/test/3
    [HttpGet("{id:customName}")]
    public IActionResult Get(string id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    // GET /api/test/my/3
    [HttpGet("my/{id:customName}")]
    public IActionResult Get(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

MyDisplayRouteInfo is provided by the Rick.Docs.Samples.RouteInfo NuGet package and displays route information.

The implementation of MyCustomConstraint prevents 0 being applied to a route parameter:

class MyCustomConstraint : IRouteConstraint
{
    private Regex _regex;

    public MyCustomConstraint()
    {
        _regex = new Regex(@"^[1-9]*$",
                            RegexOptions.CultureInvariant | RegexOptions.IgnoreCase,
                            TimeSpan.FromMilliseconds(100));
    }
    public bool Match(HttpContext httpContext, IRouter route, string routeKey,
                      RouteValueDictionary values, RouteDirection routeDirection)
    {
        if (values.TryGetValue(routeKey, out object value))
        {
            var parameterValueString = Convert.ToString(value,
                                                        CultureInfo.InvariantCulture);
            if (parameterValueString == null)
            {
                return false;
            }

            return _regex.IsMatch(parameterValueString);
        }

        return false;
    }
}

Warning

When using System.Text.RegularExpressions to process untrusted input, pass a timeout. A malicious user can provide input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout.

The preceding code:

  • Prevents 0 in the {id} segment of the route.
  • Is shown to provide a basic example of implementing a custom constraint. It should not be used in a production app.

The following code is a better approach to preventing an id containing a 0 from being processed:

[HttpGet("{id}")]
public IActionResult Get(string id)
{
    if (id.Contains('0'))
    {
        return StatusCode(StatusCodes.Status406NotAcceptable);
    }

    return ControllerContext.MyDisplayRouteInfo(id);
}

The preceding code has the following advantages over the MyCustomConstraint approach:

  • It doesn't require a custom constraint.
  • It returns a more descriptive error when the route parameter includes 0.

Parameter transformer reference

Parameter transformers:

For example, a custom slugify parameter transformer in route pattern blog\{article:slugify} with Url.Action(new { article = "MyTestArticle" }) generates blog\my-test-article.

Consider the following IOutboundParameterTransformer implementation:

public class SlugifyParameterTransformer : IOutboundParameterTransformer
{
    public string TransformOutbound(object value)
    {
        if (value == null) { return null; }

        return Regex.Replace(value.ToString(), 
                             "([a-z])([A-Z])",
                             "$1-$2",
                             RegexOptions.CultureInvariant,
                             TimeSpan.FromMilliseconds(100)).ToLowerInvariant();
    }
}

To use a parameter transformer in a route pattern, configure it using ConstraintMap in Startup.ConfigureServices:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    services.AddRouting(options =>
    {
        options.ConstraintMap["slugify"] = typeof(SlugifyParameterTransformer);
    });
}

The ASP.NET Core framework uses parameter transformers to transform the URI where an endpoint resolves. For example, parameter transformers transform the route values used to match an area, controller, action, and page.

routes.MapControllerRoute(
    name: "default",
    template: "{controller:slugify=Home}/{action:slugify=Index}/{id?}");

With the preceding route template, the action SubscriptionManagementController.GetAll is matched with the URI /subscription-management/get-all. A parameter transformer doesn't change the route values used to generate a link. For example, Url.Action("GetAll", "SubscriptionManagement") outputs /subscription-management/get-all.

ASP.NET Core provides API conventions for using parameter transformers with generated routes:

URL generation reference

This section contains a reference for the algorithm implemented by URL generation. In practice, most complex examples of URL generation use controllers or Razor Pages. See routing in controllers for additional information.

The URL generation process begins with a call to LinkGenerator.GetPathByAddress or a similar method. The method is provided with an address, a set of route values, and optionally information about the current request from HttpContext.

The first step is to use the address to resolve a set of candidate endpoints using an IEndpointAddressScheme<TAddress> that matches the address's type.

Once the set of candidates is found by the address scheme, the endpoints are ordered and processed iteratively until a URL generation operation succeeds. URL generation does not check for ambiguities, the first result returned is the final result.

Troubleshooting URL generation with logging

The first step in troubleshooting URL generation is setting the logging level of Microsoft.AspNetCore.Routing to TRACE. LinkGenerator logs many details about its processing which can be useful to troubleshoot problems.

See URL generation reference for details on URL generation.

Addresses

Addresses are the concept in URL generation used to bind a call into the link generator to a set of candidate endpoints.

Addresses are an extensible concept that come with two implementations by default:

  • Using endpoint name (string) as the address:
    • Provides similar functionality to MVC's route name.
    • Uses the IEndpointNameMetadata metadata type.
    • Resolves the provided string against the metadata of all registered endpoints.
    • Throws an exception on startup if multiple endpoints use the same name.
    • Recommended for general-purpose use outside of controllers and Razor Pages.
  • Using route values (RouteValuesAddress) as the address:
    • Provides similar functionality to controllers and Razor Pages legacy URL generation.
    • Very complex to extend and debug.
    • Provides the implementation used by IUrlHelper, Tag Helpers, HTML Helpers, Action Results, etc.

The role of the address scheme is to make the association between the address and matching endpoints by arbitrary criteria:

  • The endpoint name scheme performs a basic dictionary lookup.
  • The route values scheme has a complex best subset of set algorithm.

Ambient values and explicit values

From the current request, routing accesses the route values of the current request HttpContext.Request.RouteValues. The values associated with the current request are referred to as the ambient values. For the purpose of clarity, the documentation refers to the route values passed in to methods as explicit values.

The following example shows ambient values and explicit values. It provides ambient values from the current request and explicit values: { id = 17, }:

public class WidgetController : Controller
{
    private readonly LinkGenerator _linkGenerator;

    public WidgetController(LinkGenerator linkGenerator)
    {
        _linkGenerator = linkGenerator;
    }

    public IActionResult Index()
    {
        var url = _linkGenerator.GetPathByAction(HttpContext,
                                                 null, null,
                                                 new { id = 17, });
        return Content(url);
    }

The preceding code:

The following code provides no ambient values and explicit values: { controller = "Home", action = "Subscribe", id = 17, }:

public IActionResult Index2()
{
    var url = _linkGenerator.GetPathByAction("Subscribe", "Home",
                                             new { id = 17, });
    return Content(url);
}

The preceding method returns /Home/Subscribe/17

The following code in the WidgetController returns /Widget/Subscribe/17:

var url = _linkGenerator.GetPathByAction("Subscribe", null,
                                         new { id = 17, });

The following code provides the controller from ambient values in the current request and explicit values: { action = "Edit", id = 17, }:

public class GadgetController : Controller
{
    public IActionResult Index()
    {
        var url = Url.Action("Edit", new { id = 17, });
        return Content(url);
    }

In the preceding code:

  • /Gadget/Edit/17 is returned.
  • Url gets the IUrlHelper.
  • Action generates a URL with an absolute path for an action method. The URL contains the specified action name and route values.

The following code provides ambient values from the current request and explicit values: { page = "./Edit, id = 17, }:

public class IndexModel : PageModel
{
    public void OnGet()
    {
        var url = Url.Page("./Edit", new { id = 17, });
        ViewData["URL"] = url;
    }
}

The preceding code sets url to /Edit/17 when the Edit Razor Page contains the following page directive:

@page "{id:int}"

If the Edit page doesn't contain the "{id:int}" route template, url is /Edit?id=17.

The behavior of MVC's IUrlHelper adds a layer of complexity in addition to the rules described here:

  • IUrlHelper always provides the route values from the current request as ambient values.
  • IUrlHelper.Action always copies the current action and controller route values as explicit values unless overridden by the developer.
  • IUrlHelper.Page always copies the current page route value as an explicit value unless overridden.
  • IUrlHelper.Page always overrides the current handler route value with null as an explicit values unless overridden.

Users are often surprised by the behavioral details of ambient values, because MVC doesn't seem to follow its own rules. For historical and compatibility reasons, certain route values such as action, controller, page, and handler have their own special-case behavior.

The equivalent functionality provided by LinkGenerator.GetPathByAction and LinkGenerator.GetPathByPage duplicates these anomalies of IUrlHelper for compatibility.

URL generation process

Once the set of candidate endpoints are found, the URL generation algorithm:

  • Processes the endpoints iteratively.
  • Returns the first successful result.

The first step in this process is called route value invalidation. Route value invalidation is the process by which routing decides which route values from the ambient values should be used and which should be ignored. Each ambient value is considered and either combined with the explicit values, or ignored.

The best way to think about the role of ambient values is that they attempt to save application developers typing, in some common cases. Traditionally, the scenarios where ambient values are helpful are related to MVC:

  • When linking to another action in the same controller, the controller name doesn't need to be specified.
  • When linking to another controller in the same area, the area name doesn't need to be specified.
  • When linking to the same action method, route values don't need to be specified.
  • When linking to another part of the app, you don't want to carry over route values that have no meaning in that part of the app.

Calls to LinkGenerator or IUrlHelper that return null are usually caused by not understanding route value invalidation. Troubleshoot route value invalidation by explicitly specifying more of the route values to see if that solves the problem.

Route value invalidation works on the assumption that the app's URL scheme is hierarchical, with a hierarchy formed from left-to-right. Consider the basic controller route template {controller}/{action}/{id?} to get an intuitive sense of how this works in practice. A change to a value invalidates all of the route values that appear to the right. This reflects the assumption about hierarchy. If the app has an ambient value for id, and the operation specifies a different value for the controller:

  • id won't be reused because {controller} is to the left of {id?}.

Some examples demonstrating this principle:

  • If the explicit values contain a value for id, the ambient value for id is ignored. The ambient values for controller and action can be used.
  • If the explicit values contain a value for action, any ambient value for action is ignored. The ambient values for controller can be used. If the explicit value for action is different from the ambient value for action, the id value won't be used. If the explicit value for action is the same as the ambient value for action, the id value can be used.
  • If the explicit values contain a value for controller, any ambient value for controller is ignored. If the explicit value for controller is different from the ambient value for controller, the action and id values won't be used. If the explicit value for controller is the same as the ambient value for controller, the action and id values can be used.

This process is further complicated by the existence of attribute routes and dedicated conventional routes. Controller conventional routes such as {controller}/{action}/{id?} specify a hierarchy using route parameters. For dedicated conventional routes and attribute routes to controllers and Razor Pages:

  • There is a hierarchy of route values.
  • They don't appear in the template.

For these cases, URL generation defines the required values concept. Endpoints created by controllers and Razor Pages have required values specified that allow route value invalidation to work.

The route value invalidation algorithm in detail:

  • The required value names are combined with the route parameters, then processed from left-to-right.
  • For each parameter, the ambient value and explicit value are compared:
    • If the ambient value and explicit value are the same, the process continues.
    • If the ambient value is present and the explicit value isn't, the ambient value is used when generating the URL.
    • If the ambient value isn't present and the explicit value is, reject the ambient value and all subsequent ambient values.
    • If the ambient value and the explicit value are present, and the two values are different, reject the ambient value and all subsequent ambient values.

At this point, the URL generation operation is ready to evaluate route constraints. The set of accepted values is combined with the parameter default values, which is provided to constraints. If the constraints all pass, the operation continues.

Next, the accepted values can be used to expand the route template. The route template is processed:

  • From left-to-right.
  • Each parameter has its accepted value substituted.
  • With the following special cases:
    • If the accepted values is missing a value and the parameter has a default value, the default value is used.
    • If the accepted values is missing a value and the parameter is optional, processing continues.
    • If any route parameter to the right of a missing optional parameter has a value, the operation fails.
    • Contiguous default-valued parameters and optional parameters are collapsed where possible.

Values explicitly provided that don't match a segment of the route are added to the query string. The following table shows the result when using the route template {controller}/{action}/{id?}.

Ambient Values Explicit Values Result
controller = "Home" action = "About" /Home/About
controller = "Home" controller = "Order", action = "About" /Order/About
controller = "Home", color = "Red" action = "About" /Home/About
controller = "Home" action = "About", color = "Red" /Home/About?color=Red

Problems with route value invalidation

As of ASP.NET Core 3.0, some URL generation schemes used in earlier ASP.NET Core versions don't work well with URL generation. The ASP.NET Core team plans to add features to address these needs in a future release. For now the best solution is to use legacy routing.

The following code shows an example of a URL generation scheme that's not supported by routing.

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute("default", 
                                     "{culture}/{controller=Home}/{action=Index}/{id?}");
    endpoints.MapControllerRoute("blog", "{culture}/{**slug}", 
                                      new { controller = "Blog", action = "ReadPost", });
});

In the preceding code, the culture route parameter is used for localization. The desire is to have the culture parameter always accepted as an ambient value. However, the culture parameter is not accepted as an ambient value because of the way required values work:

  • In the "default" route template, the culture route parameter is to the left of controller, so changes to controller won't invalidate culture.
  • In the "blog" route template, the culture route parameter is considered to be to the right of controller, which appears in the required values.

Configuring endpoint metadata

The following links provide information on configuring endpoint metadata:

Host matching in routes with RequireHost

RequireHost applies a constraint to the route which requires the specified host. The RequireHost or [Host] parameter can be:

  • Host: www.domain.com, matches www.domain.com with any port.
  • Host with wildcard: *.domain.com, matches www.domain.com, subdomain.domain.com, or www.subdomain.domain.com on any port.
  • Port: *:5000, matches port 5000 with any host.
  • Host and port: www.domain.com:5000 or *.domain.com:5000, matches host and port.

Multiple parameters can be specified using RequireHost or [Host]. The constraint matches hosts valid for any of the parameters. For example, [Host("domain.com", "*.domain.com")] matches domain.com, www.domain.com, and subdomain.domain.com.

The following code uses RequireHost to require the specified host on the route:

public void Configure(IApplicationBuilder app)
{
    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapGet("/", context => context.Response.WriteAsync("Hi Contoso!"))
            .RequireHost("contoso.com");
        endpoints.MapGet("/", context => context.Response.WriteAsync("AdventureWorks!"))
            .RequireHost("adventure-works.com");
        endpoints.MapHealthChecks("/healthz").RequireHost("*:8080");
    });
}

The following code uses the [Host] attribute on the controller to require any of the specified hosts:

[Host("contoso.com", "adventure-works.com")]
public class ProductController : Controller
{
    public IActionResult Index()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [Host("example.com:8080")]
    public IActionResult Privacy()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

When the [Host] attribute is applied to both the controller and action method:

  • The attribute on the action is used.
  • The controller attribute is ignored.

Performance guidance for routing

Most of routing was updated in ASP.NET Core 3.0 to increase performance.

When an app has performance problems, routing is often suspected as the problem. The reason routing is suspected is that frameworks like controllers and Razor Pages report the amount of time spent inside the framework in their logging messages. When there's a significant difference between the time reported by controllers and the total time of the request:

  • Developers eliminate their app code as the source of the problem.
  • It's common to assume routing is the cause.

Routing is performance tested using thousands of endpoints. It's unlikely that a typical app will encounter a performance problem just by being too large. The most common root cause of slow routing performance is usually a badly-behaving custom middleware.

This following code sample demonstrates a basic technique for narrowing down the source of delay:

public void Configure(IApplicationBuilder app, ILogger<Startup> logger)
{
    app.Use(next => async context =>
    {
        var sw = Stopwatch.StartNew();
        await next(context);
        sw.Stop();

        logger.LogInformation("Time 1: {ElapsedMilliseconds}ms", sw.ElapsedMilliseconds);
    });

    app.UseRouting();

    app.Use(next => async context =>
    {
        var sw = Stopwatch.StartNew();
        await next(context);
        sw.Stop();

        logger.LogInformation("Time 2: {ElapsedMilliseconds}ms", sw.ElapsedMilliseconds);
    });

    app.UseAuthorization();

    app.Use(next => async context =>
    {
        var sw = Stopwatch.StartNew();
        await next(context);
        sw.Stop();

        logger.LogInformation("Time 3: {ElapsedMilliseconds}ms", sw.ElapsedMilliseconds);
    });

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Timing test.");
        });
    });
}

To time routing:

  • Interleave each middleware with a copy of the timing middleware shown in the preceding code.
  • Add a unique identifier to correlate the timing data with the code.

This is a basic way to narrow down the delay when it's significant, for example, more than 10ms. Subtracting Time 2 from Time 1 reports the time spent inside the UseRouting middleware.

The following code uses a more compact approach to the preceding timing code:

public sealed class MyStopwatch : IDisposable
{
    ILogger<Startup> _logger;
    string _message;
    Stopwatch _sw;

    public MyStopwatch(ILogger<Startup> logger, string message)
    {
        _logger = logger;
        _message = message;
        _sw = Stopwatch.StartNew();
    }

    private bool disposed = false;


    public void Dispose()
    {
        if (!disposed)
        {
            _logger.LogInformation("{Message }: {ElapsedMilliseconds}ms",
                                    _message, _sw.ElapsedMilliseconds);

            disposed = true;
        }
    }
}
public void Configure(IApplicationBuilder app, ILogger<Startup> logger)
{
    int count = 0;
    app.Use(next => async context =>
    {
        using (new MyStopwatch(logger, $"Time {++count}"))
        {
            await next(context);
        }

    });

    app.UseRouting();

    app.Use(next => async context =>
    {
        using (new MyStopwatch(logger, $"Time {++count}"))
        {
            await next(context);
        }
    });

    app.UseAuthorization();

    app.Use(next => async context =>
    {
        using (new MyStopwatch(logger, $"Time {++count}"))
        {
            await next(context);
        }
    });

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapGet("/", async context =>
        {
            await context.Response.WriteAsync("Timing test.");
        });
    });
}

Potentially expensive routing features

The following list provides some insight into routing features that are relatively expensive compared with basic route templates:

  • Regular expressions: It's possible to write regular expressions that are complex, or have long running time with a small amount of input.
  • Complex segments ({x}-{y}-{z}):
    • Are significantly more expensive than parsing a regular URL path segment.
    • Result in many more substrings being allocated.
    • The complex segment logic was not updated in ASP.NET Core 3.0 routing performance update.
  • Synchronous data access: Many complex apps have database access as part of their routing. ASP.NET Core 2.2 and earlier routing might not provide the right extensibility points to support database access routing. For example, IRouteConstraint, and IActionConstraint are synchronous. Extensibility points such as MatcherPolicy and EndpointSelectorContext are asynchronous.

Guidance for library authors

This section contains guidance for library authors building on top of routing. These details are intended to ensure that app developers have a good experience using libraries and frameworks that extend routing.

Define endpoints

To create a framework that uses routing for URL matching, start by defining a user experience that builds on top of UseEndpoints.

DO build on top of IEndpointRouteBuilder. This allows users to compose your framework with other ASP.NET Core features without confusion. Every ASP.NET Core template includes routing. Assume routing is present and familiar for users.

app.UseEndpoints(endpoints =>
{
    // Your framework
    endpoints.MapMyFramework(...);

    endpoints.MapHealthChecks("/healthz");
});

DO return a sealed concrete type from a call to MapMyFramework(...) that implements IEndpointConventionBuilder. Most framework Map... methods follow this pattern. The IEndpointConventionBuilder interface:

  • Allows composability of metadata.
  • Is targeted by a variety of extension methods.

Declaring your own type allows you to add your own framework-specific functionality to the builder. It's ok to wrap a framework-declared builder and forward calls to it.

app.UseEndpoints(endpoints =>
{
    // Your framework
    endpoints.MapMyFramework(...).RequireAuthorization()
                                 .WithMyFrameworkFeature(awesome: true);

    endpoints.MapHealthChecks("/healthz");
});

CONSIDER writing your own EndpointDataSource. EndpointDataSource is the low-level primitive for declaring and updating a collection of endpoints. EndpointDataSource is a powerful API used by controllers and Razor Pages.

The routing tests have a basic example of a non-updating data source.

DO NOT attempt to register an EndpointDataSource by default. Require users to register your framework in UseEndpoints. The philosophy of routing is that nothing is included by default, and that UseEndpoints is the place to register endpoints.

Creating routing-integrated middleware

CONSIDER defining metadata types as an interface.

DO make it possible to use metadata types as an attribute on classes and methods.

public interface ICoolMetadata
{
    bool IsCool { get; }
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class CoolMetadataAttribute : Attribute, ICoolMetadata
{
    public bool IsCool => true;
}

Frameworks like controllers and Razor Pages support applying metadata attributes to types and methods. If you declare metadata types:

  • Make them accessible as attributes.
  • Most users are familiar with applying attributes.

Declaring a metadata type as an interface adds another layer of flexibility:

  • Interfaces are composable.
  • Developers can declare their own types that combine multiple policies.

DO make it possible to override metadata, as shown in the following example:

public interface ICoolMetadata
{
    bool IsCool { get; }
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class CoolMetadataAttribute : Attribute, ICoolMetadata
{
    public bool IsCool => true;
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class SuppressCoolMetadataAttribute : Attribute, ICoolMetadata
{
    public bool IsCool => false;
}

[CoolMetadata]
public class MyController : Controller
{
    public void MyCool() { }

    [SuppressCoolMetadata]
    public void Uncool() { }
}

The best way to follow these guidelines is to avoid defining marker metadata:

  • Don't just look for the presence of a metadata type.
  • Define a property on the metadata and check the property.

The metadata collection is ordered and supports overriding by priority. In the case of controllers, metadata on the action method is most specific.

DO make middleware useful with and without routing.

app.UseRouting();

app.UseAuthorization(new AuthorizationPolicy() { ... });

app.UseEndpoints(endpoints =>
{
    // Your framework
    endpoints.MapMyFramework(...).RequireAuthorization();
});

As an example of this guideline, consider the UseAuthorization middleware. The authorization middleware allows you to pass in a fallback policy. The fallback policy, if specified, applies to both:

  • Endpoints without a specified policy.
  • Requests that don't match an endpoint.

This makes the authorization middleware useful outside of the context of routing. The authorization middleware can be used for traditional middleware programming.

Debug diagnostics

For detailed routing diagnostic output, set Logging:LogLevel:Microsoft to Debug. In the development environment, set the log level in appsettings.Development.json:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Debug",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  }
}