Routing in ASP.NET Core

By Ryan Nowak, Steve Smith, and Rick Anderson

For the 1.1 version of this topic, download Routing in ASP.NET Core (version 1.1, PDF).

Routing is responsible for mapping request URIs to endpoint selectors and dispatching incoming requests to endpoints. Routes are defined in the app and configured when the app starts. A route can optionally extract values from the URL contained in the request, and these values can then be used for request processing. Using route information from the app, routing is also able to generate URLs that map to endpoint selectors.

To use the latest routing scenarios in ASP.NET Core 2.2, specify the compatibility version to the MVC services registration in Startup.ConfigureServices:

services.AddMvc()
    .SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

The EnableEndpointRouting option determines if routing should internally use endpoint-based logic or the IRouter-based logic of ASP.NET Core 2.1 or earlier. When the compatibility version is set to 2.2 or later, the default value is true. Set the value to false to use the prior routing logic:

// Use the routing logic of ASP.NET Core 2.1 or earlier:
services.AddMvc(options => options.EnableEndpointRouting = false)
    .SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

For more information on IRouter-based routing, see the ASP.NET Core 2.1 version of this topic.

Routing is responsible for mapping request URIs to route handlers and dispatching an incoming requests. Routes are defined in the app and configured when the app starts. A route can optionally extract values from the URL contained in the request, and these values can then be used for request processing. Using configured routes from the app, routing is able to generate URLs that map to route handlers.

To use the latest routing scenarios in ASP.NET Core 2.1, specify the compatibility version to the MVC services registration in Startup.ConfigureServices:

services.AddMvc()
    .SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

Important

This document covers low-level ASP.NET Core routing. For information on ASP.NET Core MVC routing, see Routing to controller actions in ASP.NET Core. For information on routing conventions in Razor Pages, see Razor Pages route and app conventions in ASP.NET Core.

View or download sample code (how to download)

Routing basics

Most apps should choose a basic and descriptive routing scheme so that URLs are readable and meaningful. The default conventional route {controller=Home}/{action=Index}/{id?}:

  • Supports a basic and descriptive routing scheme.
  • Is a useful starting point for UI-based apps.

Developers commonly add additional terse routes to high-traffic areas of an app in specialized situations (for example, blog and ecommerce endpoints) using attribute routing or dedicated conventional routes.

Web APIs should use attribute routing to model the app's functionality as a set of resources where operations are represented by HTTP verbs. This means that many operations (for example, GET, POST) on the same logical resource will use the same URL. Attribute routing provides a level of control that's needed to carefully design an API's public endpoint layout.

Razor Pages apps use default conventional routing to serve named resources in the Pages folder of an app. Additional conventions are available that allow you to customize Razor Pages routing behavior. For more information, see Introduction to Razor Pages in ASP.NET Core and Razor Pages route and app conventions in ASP.NET Core.

URL generation support allows the app to be developed without hard-coding URLs to link the app together. This support allows for starting with a basic routing configuration and modifying the routes after the app's resource layout is determined.

Routing uses endpoints (Endpoint) to represent logical endpoints in an app.

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

The routing system has the following characteristics:

  • Route template syntax is used to define routes with tokenized route parameters.

  • Conventional-style and attribute-style endpoint configuration is permitted.

  • IRouteConstraint is used to determine whether a URL parameter contains a valid value for a given endpoint constraint.

  • App models, such as MVC/Razor Pages, register all of their endpoints, which have a predictable implementation of routing scenarios.

  • The routing implementation makes routing decisions wherever desired in the middleware pipeline.

  • Middleware that appears after a Routing Middleware can inspect the result of the Routing Middleware's endpoint decision for a given request URI.

  • It's possible to enumerate all of the endpoints in the app anywhere in the middleware pipeline.

  • An app can use routing to generate URLs (for example, for redirection or links) based on endpoint information and thus avoid hard-coded URLs, which helps maintainability.

  • URL generation is based on addresses, which support arbitrary extensibility:

    • The Link Generator API (LinkGenerator) can be resolved anywhere using dependency injection (DI) to generate URLs.
    • Where the Link Generator API isn't available via DI, IUrlHelper offers methods to build URLs.

Note

With the release of endpoint routing in ASP.NET Core 2.2, endpoint linking is limited to MVC/Razor Pages actions and pages. The expansions of endpoint-linking capabilities is planned for future releases.

Routing uses routes (implementations of IRouter) to:

  • Map incoming requests to route handlers.
  • Generate the URLs used in responses.

By default, an app has a single collection of routes. When a request arrives, the routes in the collection are processed in the order that they exist in the collection. The framework attempts to match an incoming request URL to a route in the collection by calling the RouteAsync method on each route in the collection. A response can use routing to generate URLs (for example, for redirection or links) based on route information and thus avoid hard-coded URLs, which helps maintainability.

The routing system has the following characteristics:

  • Route template syntax is used to define routes with tokenized route parameters.
  • Conventional-style and attribute-style endpoint configuration is permitted.
  • IRouteConstraint is used to determine whether a URL parameter contains a valid value for a given endpoint constraint.
  • App models, such as MVC/Razor Pages, register all of their routes, which have a predictable implementation of routing scenarios.
  • A response can use routing to generate URLs (for example, for redirection or links) based on route information and thus avoid hard-coded URLs, which helps maintainability.
  • URL generation is based on routes, which support arbitrary extensibility. IUrlHelper offers methods to build URLs.

Routing is connected to the middleware pipeline by the RouterMiddleware class. ASP.NET Core MVC adds routing to the middleware pipeline as part of its configuration and handles routing in MVC and Razor Pages apps. To learn how to use routing as a standalone component, see the Use Routing Middleware section.

URL matching

URL matching is the process by which routing dispatches an incoming request to an endpoint. This process is based on data in the URL path but can be extended to consider any data in the request. The ability to dispatch requests to separate handlers is key to scaling the size and complexity of an app.

The routing system in endpoint routing is responsible for all dispatching decisions. Since 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.

When the endpoint delegate is executed, the properties of RouteContext.RouteData are set to appropriate values based on the request processing performed thus far.

URL matching is the process by which routing dispatches an incoming request to a handler. This process is based on data in the URL path but can be extended to consider any data in the request. The ability to dispatch requests to separate handlers is key to scaling the size and complexity of an app.

Incoming requests enter the RouterMiddleware, which calls the RouteAsync method on each route in sequence. The IRouter instance chooses whether to handle the request by setting the RouteContext.Handler to a non-null RequestDelegate. If a route sets a handler for the request, route processing stops, and the handler is invoked to process the request. If no route handler is found to process the request, the middleware hands the request off to the next middleware in the request pipeline.

The primary input to RouteAsync is the RouteContext.HttpContext associated with the current request. The RouteContext.Handler and RouteContext.RouteData are outputs set after a route is matched.

A match that calls RouteAsync also sets the properties of the RouteContext.RouteData to appropriate values based on the request processing performed thus far.

RouteData.Values is a dictionary of route values produced from the route. These values are usually determined by tokenizing the URL and can be used to accept user input or to make further dispatching decisions inside the app.

RouteData.DataTokens is a property bag of additional data related to the matched route. DataTokens are provided to support associating state data with each route so that the app can make decisions based on which route matched. These values are developer-defined and do not affect the behavior of routing in any way. Additionally, values stashed in RouteData.DataTokens can be of any type, in contrast to RouteData.Values, which must be convertible to and from strings.

RouteData.Routers is a list of the routes that took part in successfully matching the request. Routes can be nested inside of one another. The Routers property reflects the path through the logical tree of routes that resulted in a match. Generally, the first item in Routers is the route collection and should be used for URL generation. The last item in Routers is the route handler that matched.

URL generation

URL generation is the process by which routing can create a URL path based on a set of route values. This allows for a logical separation between your endpoints and the URLs that access them.

Endpoint routing includes the Link Generator API (LinkGenerator). LinkGenerator is a singleton service that can be retrieved from DI. The API can be used outside of the context of an executing request. MVC's IUrlHelper and scenarios that rely on IUrlHelper, such as Tag Helpers, HTML Helpers, and Action Results, use the link generator 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 MVC/Razor Pages are implemented as an address scheme.

The link generator can link to MVC/Razor Pages actions and pages via the following extension methods:

  • GetPathByAction
  • GetUriByAction
  • GetPathByPage
  • GetUriByPage

An overload of these methods accepts 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/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. Always specify an empty base path to undo Map*'s affect on link generation.

Differences from earlier versions of routing

A few differences exist between endpoint routing in ASP.NET Core 2.2 or later and earlier versions of routing in ASP.NET Core:

  • The endpoint routing system doesn't support IRouter-based extensibility, including inheriting from Route.

  • Endpoint routing doesn't support WebApiCompatShim. Use the 2.1 compatibility version (.SetCompatibilityVersion(CompatibilityVersion.Version_2_1)) to continue using the compatibility shim.

  • Endpoint Routing has different behavior for the casing of generated URIs when using conventional routes.

    Consider the following default route template:

    app.UseMvc(routes =>
    {
        routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");
    });
    

    Suppose you generate a link to an action using the following route:

    var link = Url.Action("ReadPost", "blog", new { id = 17, });
    

    With IRouter-based routing, this code generates a URI of /blog/ReadPost/17, which respects the casing of the provided route value. Endpoint routing in ASP.NET Core 2.2 or later produces /Blog/ReadPost/17 ("Blog" is capitalized). Endpoint routing provides the IOutboundParameterTransformer interface that can be used to customize this behavior globally or to apply different conventions for mapping URLs.

    For more information, see the Parameter transformer reference section.

  • Link Generation used by MVC/Razor Pages with conventional routes behaves differently when attempting to link to an controller/action or page that doesn't exist.

    Consider the following default route template:

    app.UseMvc(routes =>
    {
        routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");
    });
    

    Suppose you generate a link to an action using the default template with the following:

    var link = Url.Action("ReadPost", "Blog", new { id = 17, });
    

    With IRouter-based routing, the result is always /Blog/ReadPost/17, even if the BlogController doesn't exist or doesn't have a ReadPost action method. As expected, endpoint routing in ASP.NET Core 2.2 or later produces /Blog/ReadPost/17 if the action method exists. However, endpoint routing produces an empty string if the action doesn't exist. Conceptually, endpoint routing doesn't assume that the endpoint exists if the action doesn't exist.

  • The link generation ambient value invalidation algorithm behaves differently when used with endpoint routing.

    Ambient value invalidation is the algorithm that decides which route values from the currently executing request (the ambient values) can be used in link generation operations. Conventional routing always invalidated extra route values when linking to a different action. Attribute routing didn't have this behavior prior to the release of ASP.NET Core 2.2. In earlier versions of ASP.NET Core, links to another action that use the same route parameter names resulted in link generation errors. In ASP.NET Core 2.2 or later, both forms of routing invalidate values when linking to another action.

    Consider the following example in ASP.NET Core 2.1 or earlier. When linking to another action (or another page), route values can be reused in undesirable ways.

    In /Pages/Store/Product.cshtml:

    @page "{id}"
    @Url.Page("/Login")
    

    In /Pages/Login.cshtml:

    @page "{id?}"
    

    If the URI is /Store/Product/18 in ASP.NET Core 2.1 or earlier, the link generated on the Store/Info page by @Url.Page("/Login") is /Login/18. The id value of 18 is reused, even though the link destination is different part of the app entirely. The id route value in the context of the /Login page is probably a user ID value, not a store product ID value.

    In endpoint routing with ASP.NET Core 2.2 or later, the result is /Login. Ambient values aren't reused when the linked destination is a different action or page.

  • Round-tripping route parameter syntax: Forward slashes aren't encoded when using a double-asterisk (**) catch-all parameter syntax.

    During link generation, the routing system encodes the value captured in a double-asterisk (**) catch-all parameter (for example, {**myparametername}) except the forward slashes. The double-asterisk catch-all is supported with IRouter-based routing in ASP.NET Core 2.2 or later.

    The single asterisk catch-all parameter syntax in prior versions of ASP.NET Core ({*myparametername}) remains supported, and forward slashes are encoded.

    Route Link generated with
    Url.Action(new { category = "admin/products" })
    /search/{*page} /search/admin%2Fproducts (the forward slash is encoded)
    /search/{**page} /search/admin/products

Middleware example

In the following example, a middleware uses the LinkGenerator API to create 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.GenerateLink(new { controller = "Store",
                                                    action = "ListProducts" });

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

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

URL generation is the process by which routing can create a URL path based on a set of route values. This allows for a logical separation between route handlers and the URLs that access them.

URL generation follows a similar iterative process, but it starts with user or framework code calling into the GetVirtualPath method of the route collection. Each route has its GetVirtualPath method called in sequence until a non-null VirtualPathData is returned.

The primary inputs to GetVirtualPath are:

Routes primarily use the route values provided by Values and AmbientValues to decide whether it's possible to generate a URL and what values to include. The AmbientValues are the set of route values that were produced from matching the current request. In contrast, Values are the route values that specify how to generate the desired URL for the current operation. The HttpContext is provided in case a route should obtain services or additional data associated with the current context.

Tip

Think of VirtualPathContext.Values as a set of overrides for the VirtualPathContext.AmbientValues. URL generation attempts to reuse route values from the current request to generate URLs for links using the same route or route values.

The output of GetVirtualPath is a VirtualPathData. VirtualPathData is a parallel of RouteData. VirtualPathData contains the VirtualPath for the output URL and some additional properties that should be set by the route.

The VirtualPathData.VirtualPath property contains the virtual path produced by the route. Depending on your needs, you may need to process the path further. If you want to render the generated URL in HTML, prepend the base path of the app.

The VirtualPathData.Router is a reference to the route that successfully generated the URL.

The VirtualPathData.DataTokens properties is a dictionary of additional data related to the route that generated the URL. This is the parallel of RouteData.DataTokens.

Create routes

Routing provides the Route class as the standard implementation of IRouter. Route uses the route template syntax to define patterns to match against the URL path when RouteAsync is called. Route uses the same route template to generate a URL when GetVirtualPath is called.

Most apps create routes by calling MapRoute or one of the similar extension methods defined on IRouteBuilder. Any of the IRouteBuilder extension methods create an instance of Route and add it to the route collection.

MapRoute doesn't accept a route handler parameter. MapRoute only adds routes that are handled by the DefaultHandler. To learn more about routing in MVC, see Routing to controller actions in ASP.NET Core.

MapRoute doesn't accept a route handler parameter. MapRoute only adds routes that are handled by the DefaultHandler. The default handler is an IRouter, and the handler might not handle the request. For example, ASP.NET Core MVC is typically configured as a default handler that only handles requests that match an available controller and action. To learn more about routing in MVC, see Routing to controller actions in ASP.NET Core.

The following code example is an example of a MapRoute call used by a typical ASP.NET Core MVC route definition:

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

This template matches a URL path and extracts the route values. For example, the path /Products/Details/17 generates the following route values: { controller = Products, action = Details, id = 17 }.

Route values are determined by splitting the URL path into segments and matching each segment with the route parameter name in the route template. Route parameters are named. The parameters defined by enclosing the parameter name in braces { ... }.

The preceding template could also match the URL path / and produce the values { controller = Home, action = Index }. This occurs because the {controller} and {action} route parameters have default values and the id route parameter is optional. An equals sign (=) followed by a value after the route parameter name defines a default value for the parameter. A question mark (?) after the route parameter name defines an optional parameter.

Route parameters with a default value always produce a route value when the route matches. Optional parameters don't produce a route value if there was no corresponding URL path segment. See the Route template reference section for a thorough description of route template scenarios and syntax.

In the following example, the route parameter definition {id:int} defines a route constraint for the id route parameter:

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

This template matches a URL path like /Products/Details/17 but not /Products/Details/Apples. Route constraints implement IRouteConstraint and inspect route values to verify them. In this example, the route value id must be convertible to an integer. See route-constraint-reference for an explanation of route constraints provided by the framework.

Additional overloads of MapRoute accept values for constraints, dataTokens, and defaults. The typical usage of these parameters is to pass an anonymously typed object, where the property names of the anonymous type match route parameter names.

The following MapRoute examples create equivalent routes:

routes.MapRoute(
    name: "default_route",
    template: "{controller}/{action}/{id?}",
    defaults: new { controller = "Home", action = "Index" });

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

Tip

The inline syntax for defining constraints and defaults can be convenient for simple routes. However, there are scenarios, such as data tokens, that aren't supported by inline syntax.

The following example demonstrates a few additional scenarios:

routes.MapRoute(
    name: "blog",
    template: "Blog/{**article}",
    defaults: new { controller = "Blog", action = "ReadArticle" });

The preceding template matches a URL path like /Blog/All-About-Routing/Introduction and extracts the values { controller = Blog, action = ReadArticle, article = All-About-Routing/Introduction }. The default route values for controller and action are produced by the route even though there are no corresponding route parameters in the template. Default values can be specified in the route template. The article route parameter is defined as a catch-all by the appearance of an double asterisk (**) before the route parameter name. Catch-all route parameters capture the remainder of the URL path and can also match the empty string.

routes.MapRoute(
    name: "blog",
    template: "Blog/{*article}",
    defaults: new { controller = "Blog", action = "ReadArticle" });

The preceding template matches a URL path like /Blog/All-About-Routing/Introduction and extracts the values { controller = Blog, action = ReadArticle, article = All-About-Routing/Introduction }. The default route values for controller and action are produced by the route even though there are no corresponding route parameters in the template. Default values can be specified in the route template. The article route parameter is defined as a catch-all by the appearance of an asterisk (*) before the route parameter name. Catch-all route parameters capture the remainder of the URL path and can also match the empty string.

The following example adds route constraints and data tokens:

routes.MapRoute(
    name: "us_english_products",
    template: "en-US/Products/{id}",
    defaults: new { controller = "Products", action = "Details" },
    constraints: new { id = new IntRouteConstraint() },
    dataTokens: new { locale = "en-US" });

The preceding template matches a URL path like /en-US/Products/5 and extracts the values { controller = Products, action = Details, id = 5 } and the data tokens { locale = en-US }.

Locals Windows tokens

Route class URL generation

The Route class can also perform URL generation by combining a set of route values with its route template. This is logically the reverse process of matching the URL path.

Tip

To better understand URL generation, imagine what URL you want to generate and then think about how a route template would match that URL. What values would be produced? This is the rough equivalent of how URL generation works in the Route class.

The following example uses a general ASP.NET Core MVC default route:

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

With the route values { controller = Products, action = List }, the URL /Products/List is generated. The route values are substituted for the corresponding route parameters to form the URL path. Since id is an optional route parameter, the URL is successfully generated without a value for id.

With the route values { controller = Home, action = Index }, the URL / is generated. The provided route values match the default values, and the segments corresponding to the default values are safely omitted.

Both URLs generated round-trip with the following route definition (/Home/Index and /) produce the same route values that were used to generate the URL.

Note

An app using ASP.NET Core MVC should use UrlHelper to generate URLs instead of calling into routing directly.

For more information on URL generation, see the Url generation reference section.

Use Routing Middleware

Reference the Microsoft.AspNetCore.App metapackage in the app's project file.

Add routing to the service container in Startup.ConfigureServices:

public void ConfigureServices(IServiceCollection services)
{
    services.AddRouting();
}

Routes must be configured in the Startup.Configure method. The sample app uses the following APIs:

var trackPackageRouteHandler = new RouteHandler(context =>
{
    var routeValues = context.GetRouteData().Values;
    return context.Response.WriteAsync(
        $"Hello! Route values: {string.Join(", ", routeValues)}");
});

var routeBuilder = new RouteBuilder(app, trackPackageRouteHandler);

routeBuilder.MapRoute(
    "Track Package Route",
    "package/{operation:regex(^track|create|detonate$)}/{id:int}");

routeBuilder.MapGet("hello/{name}", context =>
{
    var name = context.GetRouteValue("name");
    // The route handler when HTTP GET "hello/<anything>" matches
    // To match HTTP GET "hello/<anything>/<anything>, 
    // use routeBuilder.MapGet("hello/{*name}"
    return context.Response.WriteAsync($"Hi, {name}!");
});

var routes = routeBuilder.Build();
app.UseRouter(routes);

The following table shows the responses with the given URIs.

URI Response
/package/create/3 Hello! Route values: [operation, create], [id, 3]
/package/track/-3 Hello! Route values: [operation, track], [id, -3]
/package/track/-3/ Hello! Route values: [operation, track], [id, -3]
/package/track/ The request falls through, no match.
GET /hello/Joe Hi, Joe!
POST /hello/Joe The request falls through, matches HTTP GET only.
GET /hello/Joe/Smith The request falls through, no match.

If you're configuring a single route, call UseRouter passing in an IRouter instance. You won't need to use RouteBuilder.

The framework provides a set of extension methods for creating routes (RequestDelegateRouteBuilderExtensions):

  • MapDelete
  • MapGet
  • MapMiddlewareDelete
  • MapMiddlewareGet
  • MapMiddlewarePost
  • MapMiddlewarePut
  • MapMiddlewareRoute
  • MapMiddlewareVerb
  • MapPost
  • MapPut
  • MapRoute
  • MapVerb

Some of listed methods, such as MapGet, require a RequestDelegate. The RequestDelegate is used as the route handler when the route matches. Other methods in this family allow configuring a middleware pipeline for use as the route handler. If the Map* method doesn't accept a handler, such as MapRoute, it uses the DefaultHandler.

The Map[Verb] methods use constraints to limit the route to the HTTP Verb in the method name. For example, see MapGet and MapVerb.

Route template reference

Tokens within curly braces ({ ... }) define route parameters that are bound if the route is matched. You can define more than one route parameter in a route segment, but they 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}. These 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 URLs path. To match a literal route parameter delimiter ({ or }), escape the delimiter by repeating the character ({{ or }}).

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 period (.) is optional. The following URLs match this route:

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

You can use an asterisk (*) or double asterisk (**) as a prefix to a route parameter to bind to the rest of the URI. These are called a catch-all parameters. For example, blog/{**slug} matches any URI that starts with /blog and has any value following it, which is assigned to the slug route value. 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.

You can use the asterisk (*) as a prefix to a route parameter to bind to the rest of the URI. This is called a catch-all parameter. For example, blog/{*slug} matches any URI that starts with /blog and has any value following it, which is assigned to the slug route value. 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.

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, as in id?. The difference between optional values and default route parameters is that 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 a colon (:) 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 colon (:) 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, which 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 colon (:) 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 (id set to 123).
{controller=Home}/{action=Index}/{id?} / Maps to the Home 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.

Tip

Enable Logging to see how the built-in routing implementations, such as Route, match requests.

Reserved routing names

The following keywords are reserved names and can't be used as route names or parameters:

  • action
  • area
  • controller
  • handler
  • page

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 yes/no decision about whether or not 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 instead of 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 warning)
decimal {price:decimal} 49.99, -1,000.01 Matches a valid decimal value (in the invariant culture - see warning)
double {weight:double} 1.234, -1,001.01e8 Matches a valid double value (in the invariant culture - see warning)
float {weight:float} 1.234, -1,001.01e8 Matches a valid float value (in the invariant culture - see warning)
guid {id:guid} CD2C1638-1638-72D5-1638-DEADBEEF1638, {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)} Richard 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, 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

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 (such as int or DateTime) always use the invariant culture. These constraints assume that the URL is non-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

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 routing, the expression must have the \ (single backslash) characters provided in the string as \\ (double backslash) characters in the C# source file in order to escape the \ string escape character (unless using verbatim string literals). To escape routing parameter delimiter characters ({, }, [, ]), double the characters in the expression ({{, }, [[, ]]). The following table shows a regular expression and the 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 caret (^) character and match starting position of the string. The expressions often end with the dollar sign ($) character and match end of the string. The ^ and $ characters ensure that the regular expression match the entire route parameter value. Without the ^ and $ characters, the regular expression match 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 (not inline within a template) that don't match one of the known constraints are also treated as regular expressions.

Parameter transformer reference

Parameter transformers:

  • Execute when generating a link for a Route.
  • Implement Microsoft.AspNetCore.Routing.IOutboundParameterTransformer.
  • Are configured using ConstraintMap.
  • Take the parameter's route value and transform it to a new string value.
  • Result in using the transformed value in the generated link.

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

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

services.AddRouting(options =>
{
    // Replace the type and the name used to refer to it with your own
    // IOutboundParameterTransformer implementation
    options.ConstraintMap["slugify"] = typeof(SlugifyParameterTransformer);
});

Parameter transformers are used by the framework to transform the URI where an endpoint resolves. For example, ASP.NET Core MVC uses parameter transformers to transform the route value used to match an area, controller, action, and page.

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

With the preceding route, 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 a parameter transformers with generated routes:

  • ASP.NET Core MVC has the Microsoft.AspNetCore.Mvc.ApplicationModels.RouteTokenTransformerConvention API convention. This convention applies a specified parameter transformer to all attribute routes in the app. The parameter transformer transforms attribute route tokens as they are replaced. For more information, see Use a parameter transformer to customize token replacement.
  • Razor Pages has the Microsoft.AspNetCore.Mvc.ApplicationModels.PageRouteTransformerConvention API convention. This convention applies a specified parameter transformer to all automatically discovered Razor Pages. The parameter transformer transforms the folder and file name segments of Razor Pages routes. For more information, see Use a parameter transformer to customize page routes.

URL generation reference

The following example shows how to generate a link to a route given a dictionary of route values and a RouteCollection.

app.Run(async (context) =>
{
    var dictionary = new RouteValueDictionary
    {
        { "operation", "create" },
        { "id", 123}
    };

    var vpc = new VirtualPathContext(context, null, dictionary, 
        "Track Package Route");
    var path = routes.GetVirtualPath(vpc).VirtualPath;

    context.Response.ContentType = "text/html";
    await context.Response.WriteAsync("Menu<hr/>");
    await context.Response.WriteAsync(
        $"<a href='{path}'>Create Package 123</a><br/>");
});

The VirtualPath generated at the end of the preceding sample is /package/create/123. The dictionary supplies the operation and id route values of the "Track Package Route" template, package/{operation}/{id}. For details, see the sample code in the Use Routing Middleware section or the sample app.

The second parameter to the VirtualPathContext constructor is a collection of ambient values. Ambient values are convenient to use because they limit the number of values a developer must specify within a request context. The current route values of the current request are considered ambient values for link generation. In an ASP.NET Core MVC app's About action of the HomeController, you don't need to specify the controller route value to link to the Index action—the ambient value of Home is used.

Ambient values that don't match a parameter are ignored. Ambient values are also ignored when an explicitly provided value overrides the ambient value. Matching occurs from left to right in the URL.

Values explicitly provided but 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

If a route has a default value that doesn't correspond to a parameter and that value is explicitly provided, it must match the default value:

routes.MapRoute("blog_route", "blog/{*slug}",
    defaults: new { controller = "Blog", action = "ReadPost" });

Link generation only generates a link for this route when the matching values for controller and action are provided.