Routing to controller actions in ASP.NET Core

By Ryan Nowak, Kirk Larkin, and Rick Anderson

ASP.NET Core controllers use the Routing middleware to match the URLs of incoming requests and map them to actions. Routes templates:

  • Are defined in startup code or attributes.
  • Describe how URL paths are matched to actions.
  • Are used to generate URLs for links. The generated links are typically returned in responses.

Actions are either conventionally-routed or attribute-routed. Placing a route on the controller or action makes it attribute-routed. See Mixed routing for more information.

This document:

  • Explains the interactions between MVC and routing:
    • How typical MVC apps make use of routing features.
    • Covers both:
    • See Routing for advanced routing details.
  • Refers to the default routing system added in ASP.NET Core 3.0, called endpoint routing. It's possible to use controllers with the previous version of routing for compatibility purposes. See the 2.2-3.0 migration guide for instructions. Refer to the 2.2 version of this document for reference material on the legacy routing system.

Set up conventional route

Startup.Configure typically has code similar to the following when using conventional routing:

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

Inside the call to UseEndpoints, MapControllerRoute is used to create a single route. The single route is named default route. Most apps with controllers and views use a route template similar to the default route. REST APIs should use attribute routing.

The route template "{controller=Home}/{action=Index}/{id?}":

  • Matches a URL path like /Products/Details/5

  • Extracts the route values { controller = Products, action = Details, id = 5 } by tokenizing the path. The extraction of route values results in a match if the app has a controller named ProductsController and a Details action:

    public class ProductsController : Controller
    {
        public IActionResult Details(int id)
        {
            return ControllerContext.MyDisplayRouteInfo(id);
        }
    }
    

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

  • /Products/Details/5 model binds the value of id = 5 to set the id parameter to 5. See Model Binding for more details.

  • {controller=Home} defines Home as the default controller.

  • {action=Index} defines Index as the default action.

  • The ? character in {id?} defines id as optional.

  • Default and optional route parameters don't need to be present in the URL path for a match. See Route Template Reference for a detailed description of route template syntax.

  • Matches the URL path /.

  • Produces the route values { controller = Home, action = Index }.

The values for controller and action make use of the default values. id doesn't produce a value since there's no corresponding segment in the URL path. / only matches if there exists a HomeController and Index action:

public class HomeController : Controller
{
  public IActionResult Index() { ... }
}

Using the preceding controller definition and route template, the HomeController.Index action is run for the following URL paths:

  • /Home/Index/17
  • /Home/Index
  • /Home
  • /

The URL path / uses the route template default Home controllers and Index action. The URL path /Home uses the route template default Index action.

The convenience method MapDefaultControllerRoute:

endpoints.MapDefaultControllerRoute();

Replaces:

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

Important

Routing is configured using the UseRouting and UseEndpoints middleware. To use controllers:

Conventional routing

Conventional routing is used with controllers and views. The default route:

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

is an example of a conventional routing. It's called conventional routing because it establishes a convention for URL paths:

  • The first path segment, {controller=Home}, maps to the controller name.
  • The second segment, {action=Index}, maps to the action name.
  • The third segment, {id?} is used for an optional id. The ? in {id?} makes it optional. id is used to map to a model entity.

Using this default route, the URL path:

  • /Products/List maps to the ProductsController.List action.
  • /Blog/Article/17 maps to BlogController.Article and typically model binds the id parameter to 17.

This mapping:

  • Is based on the controller and action names only.
  • Isn't based on namespaces, source file locations, or method parameters.

Using conventional routing with the default route allows creating the app without having to come up with a new URL pattern for each action. For an app with CRUD style actions, having consistency for the URLs across controllers:

  • Helps simplify the code.
  • Makes the UI more predictable.

Warning

The id in the preceding code is defined as optional by the route template. Actions can execute without the optional ID provided as part of the URL. Generally, whenid is omitted from the URL:

  • id is set to 0 by model binding.
  • No entity is found in the database matching id == 0.

Attribute routing provides fine-grained control to make the ID required for some actions and not for others. By convention, the documentation includes optional parameters like id when they're likely to appear in correct usage.

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.
  • Is the only route template needed for many web UI apps. For larger web UI apps, another route using Areas if frequently all that's needed.

MapControllerRoute and MapAreaRoute :

  • Automatically assign an order value to their endpoints based on the order they are invoked.

Endpoint routing in ASP.NET Core 3.0 and later:

  • Doesn't have a concept of routes.
  • Doesn't provide ordering guarantees for the execution of extensibility, all endpoints are processed at once.

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

Attribute routing is explained later in this document.

Multiple conventional routes

Multiple conventional routes can be added inside UseEndpoints by adding more calls to MapControllerRoute and MapAreaControllerRoute. Doing so allows defining multiple conventions, or to adding conventional routes that are dedicated to a specific action, such as:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(name: "blog",
                pattern: "blog/{*article}",
                defaults: new { controller = "Blog", action = "Article" });
    endpoints.MapControllerRoute(name: "default",
                pattern: "{controller=Home}/{action=Index}/{id?}");
});

The blog route in the preceding code is a dedicated conventional route. It's called a dedicated conventional route because:

Because controller and action don't appear in the route template "blog/{*article}" as parameters:

  • They can only have the default values { controller = "Blog", action = "Article" }.
  • This route always maps to the action BlogController.Article.

/Blog, /Blog/Article, and /Blog/{any-string} are the only URL paths that match the blog route.

The preceding example:

  • blog route has a higher priority for matches than the default route because it is added first.
  • Is an example of Slug style routing where it's typical to have an article name as part of the URL.

Warning

In ASP.NET Core 3.0 and later, routing doesn't:

  • Define a concept called a route. UseRouting adds route matching to the middleware pipeline. The UseRouting middleware looks at the set of endpoints defined in the app, and selects the best endpoint match based on the request.
  • Provide guarantees about the execution order of extensibility like IRouteConstraint or IActionConstraint.

See Routing for reference material on routing.

Conventional routing order

Conventional routing only matches a combination of action and controller that are defined by the app. This is intended to simplify cases where conventional routes overlap. Adding routes using MapControllerRoute, MapDefaultControllerRoute, and MapAreaControllerRoute automatically assign an order value to their endpoints based on the order they are invoked. Matches from a route that appears earlier have a higher priority. Conventional routing is order-dependent. In general, routes with areas should be placed earlier as they're more specific than routes without an area. Dedicated conventional routes with catch-all route parameters like {*article} can make a route too greedy, meaning that it matches URLs that you intended to be matched by other routes. Put the greedy routes later in the route table to prevent greedy matches.

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.

Resolving ambiguous actions

When two endpoints match through routing, routing must do one of the following:

  • Choose the best candidate.
  • Throw an exception.

For example:

    public class Products33Controller : Controller
    {
        public IActionResult Edit(int id)
        {
            return ControllerContext.MyDisplayRouteInfo(id);
        }

        [HttpPost]
        public IActionResult Edit(int id, Product product)
        {
            return ControllerContext.MyDisplayRouteInfo(id, product.name);
        }
    }
}

The preceding controller defines two actions that match:

  • The URL path /Products33/Edit/17
  • Route data { controller = Products33, action = Edit, id = 17 }.

This is a typical pattern for MVC controllers:

  • Edit(int) displays a form to edit a product.
  • Edit(int, Product) processes the posted form.

To resolve the correct route:

  • Edit(int, Product) is selected when the request is an HTTP POST.
  • Edit(int) is selected when the HTTP verb is anything else. Edit(int) is generally called via GET.

The HttpPostAttribute, [HttpPost], is provided to routing so that it can choose based on the HTTP method of the request. The HttpPostAttribute makes Edit(int, Product) a better match than Edit(int).

It's important to understand the role of attributes like HttpPostAttribute. Similar attributes are defined for other HTTP verbs. In conventional routing, it's common for actions to use the same action name when they're part of a show form, submit form workflow. For example, see Examine the two Edit action methods.

If routing can't choose a best candidate, an AmbiguousMatchException is thrown, listing the multiple matched endpoints.

Conventional route names

The strings "blog" and "default" in the following examples are conventional route names:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(name: "blog",
                pattern: "blog/{*article}",
                defaults: new { controller = "Blog", action = "Article" });
    endpoints.MapControllerRoute(name: "default",
                pattern: "{controller=Home}/{action=Index}/{id?}");
});

The route names give the route a logical name. The named route can be used for URL generation. Using a named route simplifies URL creation when the ordering of routes could make URL generation complicated. Route names must be unique application wide.

Route names:

  • Have no impact on URL matching or handling of requests.
  • Are used only for URL generation.

The route name concept is represented in routing as IEndpointNameMetadata. The terms route name and endpoint name:

  • Are interchangeable.
  • Which one is used in documentation and code depends on the API being described.

Attribute routing for REST APIs

REST APIs should use attribute routing to model the app's functionality as a set of resources where operations are represented by HTTP verbs.

Attribute routing uses a set of attributes to map actions directly to route templates. The following StartUp.Configure code is typical for a REST API and is used in the next sample:

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

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

    app.UseHttpsRedirection();

    app.UseRouting();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });
}

In the preceding code, MapControllers is called inside UseEndpoints to map attribute routed controllers.

In the following example:

  • The preceding Configure method is used.
  • HomeController matches a set of URLs similar to what the default conventional route {controller=Home}/{action=Index}/{id?} matches.
public class HomeController : Controller
{
    [Route("")]
    [Route("Home")]
    [Route("Home/Index")]
    [Route("Home/Index/{id?}")]
    public IActionResult Index(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    [Route("Home/About")]
    [Route("Home/About/{id?}")]
    public IActionResult About(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

The HomeController.Index action is run for any of the URL paths /, /Home, /Home/Index, or /Home/Index/3.

This example highlights a key programming difference between attribute routing and conventional routing. Attribute routing requires more input to specify a route. The conventional default route handles routes more succinctly. However, attribute routing allows and requires precise control of which route templates apply to each action.

With attribute routing, the controller and action names play no part in which action is matched, unless token replacement is used. The following example matches the same URLs as the previous example:

public class MyDemoController : Controller
{
    [Route("")]
    [Route("Home")]
    [Route("Home/Index")]
    [Route("Home/Index/{id?}")]
    public IActionResult MyIndex(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    [Route("Home/About")]
    [Route("Home/About/{id?}")]
    public IActionResult MyAbout(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

The following code uses token replacement for action and controller:

public class HomeController : Controller
{
    [Route("")]
    [Route("Home")]
    [Route("[controller]/[action]")]
    public IActionResult Index()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [Route("[controller]/[action]")]
    public IActionResult About()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

The following code applies [Route("[controller]/[action]")] to the controller:

[Route("[controller]/[action]")]
public class HomeController : Controller
{
    [Route("~/")]
    [Route("/Home")]
    [Route("~/Home/Index")]
    public IActionResult Index()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    public IActionResult About()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

In the preceding code, the Index method templates must prepend / or ~/ to the route templates. Route templates applied to an action that begin with / or ~/ don't get combined with route templates applied to the controller.

See Route template precedence for information on route template selection.

Reserved routing names

The following keywords are reserved route parameter names when using Controllers or Razor Pages:

  • action
  • area
  • controller
  • handler
  • page

Using page as a route parameter with attribute routing is a common error. Doing that results in inconsistent and confusing behavior with URL generation.

public class MyDemo2Controller : Controller
{
    [Route("/articles/{page}")]
    public IActionResult ListArticles(int page)
    {
        return ControllerContext.MyDisplayRouteInfo(page);
    }
}

The special parameter names are used by the URL generation to determine if a URL generation operation refers to a Razor Page or to a Controller.

HTTP verb templates

ASP.NET Core has the following HTTP verb templates:

Route templates

ASP.NET Core has the following route templates:

Attribute routing with Http verb attributes

Consider the following controller:

[Route("api/[controller]")]
[ApiController]
public class Test2Controller : ControllerBase
{
    [HttpGet]   // GET /api/test2
    public IActionResult ListProducts()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [HttpGet("{id}")]   // GET /api/test2/xyz
    public IActionResult GetProduct(string id)
    {
       return ControllerContext.MyDisplayRouteInfo(id);
    }

    [HttpGet("int/{id:int}")] // GET /api/test2/int/3
    public IActionResult GetIntProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    [HttpGet("int2/{id}")]  // GET /api/test2/int2/3
    public IActionResult GetInt2Product(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

In the preceding code:

  • Each action contains the [HttpGet] attribute, which constrains matching to HTTP GET requests only.
  • The GetProduct action includes the "{id}" template, therefore id is appended to the "api/[controller]" template on the controller. The methods template is "api/[controller]/"{id}"". Therefore this action only matches GET requests of for the form /api/test2/xyz,/api/test2/123,/api/test2/{any string}, etc.
    [HttpGet("{id}")]   // GET /api/test2/xyz
    public IActionResult GetProduct(string id)
    {
       return ControllerContext.MyDisplayRouteInfo(id);
    }
    
  • The GetIntProduct action contains the "int/{id:int}") template. The :int portion of the template constrains the id route values to strings that can be converted to an integer. A GET request to /api/test2/int/abc:
    • Doesn't match this action.
    • Returns a 404 Not Found error.
      [HttpGet("int/{id:int}")] // GET /api/test2/int/3
      public IActionResult GetIntProduct(int id)
      {
          return ControllerContext.MyDisplayRouteInfo(id);
      }
      
  • The GetInt2Product action contains {id} in the template, but doesn't constrain id to values that can be converted to an integer. A GET request to /api/test2/int2/abc:
    • Matches this route.
    • Model binding fails to convert abc to an integer. The id parameter of the method is integer.
    • Returns a 400 Bad Request because model binding failed to convertabc to an integer.
      [HttpGet("int2/{id}")]  // GET /api/test2/int2/3
      public IActionResult GetInt2Product(int id)
      {
          return ControllerContext.MyDisplayRouteInfo(id);
      }
      

Attribute routing can use HttpMethodAttribute attributes such as HttpPostAttribute, HttpPutAttribute, and HttpDeleteAttribute. All of the HTTP verb attributes accept a route template. The following example shows two actions that match the same route template:

[ApiController]
public class MyProductsController : ControllerBase
{
    [HttpGet("/products3")]
    public IActionResult ListProducts()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [HttpPost("/products3")]
    public IActionResult CreateProduct(MyProduct myProduct)
    {
        return ControllerContext.MyDisplayRouteInfo(myProduct.Name);
    }
}

Using the URL path /products3:

  • The MyProductsController.ListProducts action runs when the HTTP verb is GET.
  • The MyProductsController.CreateProduct action runs when the HTTP verb is POST.

When building a REST API, it's rare that you'll need to use [Route(...)] on an action method because the action accepts all HTTP methods. It's better to use the more specific HTTP verb attribute to be precise about what your API supports. Clients of REST APIs are expected to know what paths and HTTP verbs map to specific logical operations.

REST 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 and POST on the same logical resource use the same URL. Attribute routing provides a level of control that's needed to carefully design an API's public endpoint layout.

Since an attribute route applies to a specific action, it's easy to make parameters required as part of the route template definition. In the following example, id is required as part of the URL path:

[ApiController]
public class Products2ApiController : ControllerBase
{
    [HttpGet("/products2/{id}", Name = "Products_List")]
    public IActionResult GetProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

The Products2ApiController.GetProduct(int) action:

  • Is run with URL path like /products2/3
  • Isn't run with the URL path /products2.

The [Consumes] attribute allows an action to limit the supported request content types. For more information, see Define supported request content types with the Consumes attribute.

See Routing for a full description of route templates and related options.

For more information on [ApiController], see ApiController attribute.

Route name

The following code defines a route name of Products_List:

[ApiController]
public class Products2ApiController : ControllerBase
{
    [HttpGet("/products2/{id}", Name = "Products_List")]
    public IActionResult GetProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

Route names can be used to generate a URL based on a specific route. Route names:

  • Have no impact on the URL matching behavior of routing.
  • Are only used for URL generation.

Route names must be unique application-wide.

Contrast the preceding code with the conventional default route, which defines the id parameter as optional ({id?}). The ability to precisely specify APIs has advantages, such as allowing /products and /products/5 to be dispatched to different actions.

Combining attribute routes

To make attribute routing less repetitive, route attributes on the controller are combined with route attributes on the individual actions. Any route templates defined on the controller are prepended to route templates on the actions. Placing a route attribute on the controller makes all actions in the controller use attribute routing.

[ApiController]
[Route("products")]
public class ProductsApiController : ControllerBase
{
    [HttpGet]
    public IActionResult ListProducts()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [HttpGet("{id}")]
    public IActionResult GetProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

In the preceding example:

  • The URL path /products can match ProductsApi.ListProducts
  • The URL path /products/5 can match ProductsApi.GetProduct(int).

Both of these actions only match HTTP GET because they're marked with the [HttpGet] attribute.

Route templates applied to an action that begin with / or ~/ don't get combined with route templates applied to the controller. The following example matches a set of URL paths similar to the default route.

[Route("Home")]
public class HomeController : Controller
{
    [Route("")]
    [Route("Index")]
    [Route("/")]
    public IActionResult Index()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [Route("About")]
    public IActionResult About()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

The following table explains the [Route] attributes in the preceding code:

Attribute Combines with [Route("Home")] Defines route template
[Route("")] Yes "Home"
[Route("Index")] Yes "Home/Index"
[Route("/")] No ""
[Route("About")] Yes "Home/About"

Attribute route order

Routing builds a tree and matches all endpoints simultaneously:

  • The route entries behave as if placed in an ideal ordering.
  • The most specific routes have a chance to execute before the more general routes.

For example, an attribute route like blog/search/{topic} is more specific than an attribute route like blog/{*article}. The blog/search/{topic} route has higher priority, by default, because it's more specific. Using conventional routing, the developer is responsible for placing routes in the desired order.

Attribute routes can configure an order using the Order property. All of the framework provided route attributes include Order . Routes are processed according to an ascending sort of the Order property. The default order is 0. Setting a route using Order = -1 runs before routes that don't set an order. Setting a route using Order = 1 runs after default route ordering.

Avoid depending on Order. If an app's URL-space requires explicit order values to route correctly, then it's likely confusing to clients as well. In general, attribute routing selects the correct route with URL matching. If the default order used for URL generation isn't working, using a route name as an override is usually simpler than applying the Order property.

Consider the following two controllers which both define the route matching /home:

public class HomeController : Controller
{
    [Route("")]
    [Route("Home")]
    [Route("Home/Index")]
    [Route("Home/Index/{id?}")]
    public IActionResult Index(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    [Route("Home/About")]
    [Route("Home/About/{id?}")]
    public IActionResult About(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}
public class MyDemoController : Controller
{
    [Route("")]
    [Route("Home")]
    [Route("Home/Index")]
    [Route("Home/Index/{id?}")]
    public IActionResult MyIndex(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }

    [Route("Home/About")]
    [Route("Home/About/{id?}")]
    public IActionResult MyAbout(int? id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

Requesting /home with the preceding code throws an exception similar to the following:

AmbiguousMatchException: The request matched multiple endpoints. Matches:

 WebMvcRouting.Controllers.HomeController.Index
 WebMvcRouting.Controllers.MyDemoController.MyIndex

Adding Order to one of the route attributes resolves the ambiguity:

[Route("")]
[Route("Home", Order = 2)]
[Route("Home/MyIndex")]
public IActionResult MyIndex()
{
    return ControllerContext.MyDisplayRouteInfo();
}

With the preceding code, /home runs the HomeController.Index endpoint. To get to the MyDemoController.MyIndex, request /home/MyIndex. Note:

  • The preceding code is an example or poor routing design. It was used to illustrate the Order property.
  • The Order property only resolves the ambiguity, that template cannot be matched. It would be better to remove the [Route("Home")] template.

See Razor Pages route and app conventions: Route order for information on route order with Razor Pages.

In some cases, an HTTP 500 error is returned with ambiguous routes. Use logging to see which endpoints caused the AmbiguousMatchException.

Token replacement in route templates [controller], [action], [area]

For convenience, attribute routes support token replacement for reserved route parameters by enclosing a token in one of the following:

  • Square brackets: []
  • Curly braces: {}

The tokens [action], [area], and [controller] are replaced with the values of the action name, area name, and controller name from the action where the route is defined:

[Route("[controller]/[action]")]
public class Products0Controller : Controller
{
    [HttpGet]
    public IActionResult List()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }


    [HttpGet("{id}")]
    public IActionResult Edit(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

In the preceding code:

[HttpGet]
public IActionResult List()
{
    return ControllerContext.MyDisplayRouteInfo();
}
  • Matches /Products0/List
[HttpGet("{id}")]
public IActionResult Edit(int id)
{
    return ControllerContext.MyDisplayRouteInfo(id);
}
  • Matches /Products0/Edit/{id}

Token replacement occurs as the last step of building the attribute routes. The preceding example behaves the same as the following code:

public class Products20Controller : Controller
{
    [HttpGet("[controller]/[action]")]  // Matches '/Products20/List'
    public IActionResult List()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [HttpGet("[controller]/[action]/{id}")]   // Matches '/Products20/Edit/{id}'
    public IActionResult Edit(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

If you are reading this in a language other than English, let us know in this GitHub discussion issue if you'd like to see the code comments in your native language.

Attribute routes can also be combined with inheritance. This is powerful combined with token replacement. Token replacement also applies to route names defined by attribute routes. [Route("[controller]/[action]", Name="[controller]_[action]")]generates a unique route name for each action:

[ApiController]
[Route("api/[controller]/[action]", Name = "[controller]_[action]")]
public abstract class MyBase2Controller : ControllerBase
{
}

public class Products11Controller : MyBase2Controller
{
    [HttpGet]                      // /api/products11/
    public IActionResult List()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

    [HttpGet("{id}")]             //    /api/products11/edit/3
    public IActionResult Edit(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

Token replacement also applies to route names defined by attribute routes. [Route("[controller]/[action]", Name="[controller]_[action]")] generates a unique route name for each action.

To match the literal token replacement delimiter [ or ], escape it by repeating the character ([[ or ]]).

Use a parameter transformer to customize token replacement

Token replacement can be customized using a parameter transformer. A parameter transformer implements IOutboundParameterTransformer and transforms the value of parameters. For example, a custom SlugifyParameterTransformer parameter transformer changes the SubscriptionManagement route value to subscription-management:

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();
    }
}

The RouteTokenTransformerConvention is an application model convention that:

  • Applies a parameter transformer to all attribute routes in an application.
  • Customizes the attribute route token values as they are replaced.
public class SubscriptionManagementController : Controller
{
    [HttpGet("[controller]/[action]")]
    public IActionResult ListAll()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

The preceding ListAll method matches /subscription-management/list-all.

The RouteTokenTransformerConvention is registered as an option in ConfigureServices.

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews(options =>
    {
        options.Conventions.Add(new RouteTokenTransformerConvention(
                                     new SlugifyParameterTransformer()));
    });
}

See MDN web docs on Slug for the definition of Slug.

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 attribute routes

Attribute routing supports defining multiple routes that reach the same action. The most common usage of this is to mimic the behavior of the default conventional route as shown in the following example:

[Route("[controller]")]
public class Products13Controller : Controller
{
    [Route("")]     // Matches 'Products13'
    [Route("Index")] // Matches 'Products13/Index'
    public IActionResult Index()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

Putting multiple route attributes on the controller means that each one combines with each of the route attributes on the action methods:

[Route("Store")]
[Route("[controller]")]
public class Products6Controller : Controller
{
    [HttpPost("Buy")]       // Matches 'Products6/Buy' and 'Store/Buy'
    [HttpPost("Checkout")]  // Matches 'Products6/Checkout' and 'Store/Checkout'
    public IActionResult Buy()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

All the HTTP verb route constraints implement IActionConstraint.

When multiple route attributes that implement IActionConstraint are placed on an action:

  • Each action constraint combines with the route template applied to the controller.
[Route("api/[controller]")]
public class Products7Controller : ControllerBase
{
    [HttpPut("Buy")]        // Matches PUT 'api/Products7/Buy'
    [HttpPost("Checkout")]  // Matches POST 'api/Products7/Checkout'
    public IActionResult Buy()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

Using multiple routes on actions might seem useful and powerful, it's better to keep your app's URL space basic and well defined. Use multiple routes on actions only where needed, for example, to support existing clients.

Specifying attribute route optional parameters, default values, and constraints

Attribute routes support the same inline syntax as conventional routes to specify optional parameters, default values, and constraints.

public class Products14Controller : Controller
{
    [HttpPost("product14/{id:int}")]
    public IActionResult ShowProduct(int id)
    {
        return ControllerContext.MyDisplayRouteInfo(id);
    }
}

In the preceding code, [HttpPost("product/{id:int}")] applies a route constraint. The ProductsController.ShowProduct action is matched only by URL paths like /product/3. The route template portion {id:int} constrains that segment to only integers.

See Route Template Reference for a detailed description of route template syntax.

Custom route attributes using IRouteTemplateProvider

All of the route attributes implement IRouteTemplateProvider. The ASP.NET Core runtime:

  • Looks for attributes on controller classes and action methods when the app starts.
  • Uses the attributes that implement IRouteTemplateProvider to build the initial set of routes.

Implement IRouteTemplateProvider to define custom route attributes. Each IRouteTemplateProvider allows you to define a single route with a custom route template, order, and name:

public class MyApiControllerAttribute : Attribute, IRouteTemplateProvider
{
    public string Template => "api/[controller]";
    public int? Order => 2;
    public string Name { get; set; }
}

[MyApiController]
[ApiController]
public class MyTestApiController : ControllerBase
{
    // GET /api/MyTestApi
    [HttpGet]
    public IActionResult Get()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

The preceding Get method returns Order = 2, Template = api/MyTestApi.

Use application model to customize attribute routes

The application model:

  • Is an object model created at startup.
  • Contains all of the metadata used by ASP.NET Core to route and execute the actions in an app.

The application model includes all of the data gathered from route attributes. The data from route attributes is provided by the IRouteTemplateProvider implementation. Conventions:

  • Can be written to modify the application model to customize how routing behaves.
  • Are read at app startup.

This section shows a basic example of customizing routing using application model. The following code makes routes roughly line up with the folder structure of the project.

public class NamespaceRoutingConvention : Attribute, IControllerModelConvention
{
    private readonly string _baseNamespace;

    public NamespaceRoutingConvention(string baseNamespace)
    {
        _baseNamespace = baseNamespace;
    }

    public void Apply(ControllerModel controller)
    {
        var hasRouteAttributes = controller.Selectors.Any(selector =>
                                                selector.AttributeRouteModel != null);
        if (hasRouteAttributes)
        {
            return;
        }

        var namespc = controller.ControllerType.Namespace;
        if (namespc == null)
            return;
        var template = new StringBuilder();
        template.Append(namespc, _baseNamespace.Length + 1,
                        namespc.Length - _baseNamespace.Length - 1);
        template.Replace('.', '/');
        template.Append("/[controller]/[action]/{id?}");

        foreach (var selector in controller.Selectors)
        {
            selector.AttributeRouteModel = new AttributeRouteModel()
            {
                Template = template.ToString()
            };
        }
    }
}

The following code prevents the namespace convention from being applied to controllers that are attribute routed:

public void Apply(ControllerModel controller)
{
    var hasRouteAttributes = controller.Selectors.Any(selector =>
                                            selector.AttributeRouteModel != null);
    if (hasRouteAttributes)
    {
        return;
    }

For example, the following controller doesn't use NamespaceRoutingConvention:

[Route("[controller]/[action]/{id?}")]
public class ManagersController : Controller
{
    // /managers/index
    public IActionResult Index()
    {
        var template = ControllerContext.ActionDescriptor.AttributeRouteInfo?.Template;
        return Content($"Index- template:{template}");
    }

    public IActionResult List(int? id)
    {
        var path = Request.Path.Value;
        return Content($"List- Path:{path}");
    }
}

The NamespaceRoutingConvention.Apply method:

  • Does nothing if the controller is attribute routed.
  • Sets the controllers template based on the namespace, with the base namespace removed.

The NamespaceRoutingConvention can be applied in Startup.ConfigureServices:

namespace My.Application
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews(options =>
            {
                options.Conventions.Add(
                    new NamespaceRoutingConvention(typeof(Startup).Namespace));
            });
        }
        // Remaining code ommitted for brevity.

For example, consider the following controller:

using Microsoft.AspNetCore.Mvc;

namespace My.Application.Admin.Controllers
{
    public class UsersController : Controller
    {
        // GET /admin/controllers/users/index
        public IActionResult Index()
        {
            var fullname = typeof(UsersController).FullName;
            var template = 
                ControllerContext.ActionDescriptor.AttributeRouteInfo?.Template;
            var path = Request.Path.Value;

            return Content($"Path: {path} fullname: {fullname}  template:{template}");
        }

        public IActionResult List(int? id)
        {
            var path = Request.Path.Value;
            return Content($"Path: {path} ID:{id}");
        }
    }
}

In the preceding code:

  • The base namespace is My.Application.
  • The full name of the preceding controller is My.Application.Admin.Controllers.UsersController.
  • The NamespaceRoutingConvention sets the controllers template to Admin/Controllers/Users/[action]/{id?.

The NamespaceRoutingConvention can also be applied as an attribute on a controller:

[NamespaceRoutingConvention("My.Application")]
public class TestController : Controller
{
    // /admin/controllers/test/index
    public IActionResult Index()
    {
        var template = ControllerContext.ActionDescriptor.AttributeRouteInfo?.Template;
        var actionname = ControllerContext.ActionDescriptor.ActionName;
        return Content($"Action- {actionname} template:{template}");
    }

    public IActionResult List(int? id)
    {
        var path = Request.Path.Value;
        return Content($"List- Path:{path}");
    }
}

Mixed routing: Attribute routing vs conventional routing

ASP.NET Core apps can mix the use of conventional routing and attribute routing. It's typical to use conventional routes for controllers serving HTML pages for browsers, and attribute routing for controllers serving REST APIs.

Actions are either conventionally routed or attribute routed. Placing a route on the controller or the action makes it attribute routed. Actions that define attribute routes cannot be reached through the conventional routes and vice-versa. Any route attribute on the controller makes all actions in the controller attribute routed.

Attribute routing and conventional routing use the same routing engine.

URL Generation and ambient values

Apps can use routing URL generation features to generate URL links to actions. Generating URLs eliminates hardcoding URLs, making code more robust and maintainable. This section focuses on the URL generation features provided by MVC and only cover basics of how URL generation works. See Routing for a detailed description of URL generation.

The IUrlHelper interface is the underlying element of infrastructure between MVC and routing for URL generation. An instance of IUrlHelper is available through the Url property in controllers, views, and view components.

In the following example, the IUrlHelper interface is used through the Controller.Url property to generate a URL to another action.

public class UrlGenerationController : Controller
{
    public IActionResult Source()
    {
        // Generates /UrlGeneration/Destination
        var url = Url.Action("Destination");
        return ControllerContext.MyDisplayRouteInfo("", $" URL = {url}");
    }

    public IActionResult Destination()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }
}

If the app is using the default conventional route, the value of the url variable is the URL path string /UrlGeneration/Destination. This URL path is created by routing by combining:

  • The route values from the current request, which are called ambient values.
  • The values passed to Url.Action and substituting those values into the route template:
ambient values: { controller = "UrlGeneration", action = "Source" }
values passed to Url.Action: { controller = "UrlGeneration", action = "Destination" }
route template: {controller}/{action}/{id?}

result: /UrlGeneration/Destination

Each route parameter in the route template has its value substituted by matching names with the values and ambient values. A route parameter that doesn't have a value can:

  • Use a default value if it has one.
  • Be skipped if it's optional. For example, the id from the route template {controller}/{action}/{id?}.

URL generation fails if any required route parameter doesn't have a corresponding value. If URL generation fails for a route, the next route is tried until all routes have been tried or a match is found.

The preceding example of Url.Action assumes conventional routing. URL generation works similarly with attribute routing, though the concepts are different. With conventional routing:

  • The route values are used to expand a template.
  • The route values for controller and action usually appear in that template. This works because the URLs matched by routing adhere to a convention.

The following example uses attribute routing:

public class UrlGenerationAttrController : Controller
{
    [HttpGet("custom")]
    public IActionResult Source()
    {
        var url = Url.Action("Destination");
        return ControllerContext.MyDisplayRouteInfo("", $" URL = {url}");
    }

    [HttpGet("custom/url/to/destination")]
    public IActionResult Destination()
    {
       return ControllerContext.MyDisplayRouteInfo();
    }
}

The Source action in the preceding code generates custom/url/to/destination.

LinkGenerator was added in ASP.NET Core 3.0 as an alternative to IUrlHelper. LinkGenerator offers similar but more flexible functionality. Each method on IUrlHelper has a corresponding family of methods on LinkGenerator as well.

Generating URLs by action name

Url.Action, LinkGenerator.GetPathByAction, and all related overloads all are designed to generate the target endpoint by specifying a controller name and action name.

When using Url.Action, the current route values for controller and action are provided by the runtime:

  • The value of controller and action are part of both ambient values and values. The method Url.Action always uses the current values of action and controller and generates a URL path that routes to the current action.

Routing attempts to use the values in ambient values to fill in information that wasn't provided when generating a URL. Consider a route like {a}/{b}/{c}/{d} with ambient values { a = Alice, b = Bob, c = Carol, d = David }:

  • Routing has enough information to generate a URL without any additional values.
  • Routing has enough information because all route parameters have a value.

If the value { d = Donovan } is added:

  • The value { d = David } is ignored.
  • The generated URL path is Alice/Bob/Carol/Donovan.

Warning: URL paths are hierarchical. In the preceding example, if the value { c = Cheryl } is added:

  • Both of the values { c = Carol, d = David } are ignored.
  • There is no longer a value for d and URL generation fails.
  • The desired values of c and d must be specified to generate a URL.

You might expect to hit this problem with the default route {controller}/{action}/{id?}. This problem is rare in practice because Url.Action always explicitly specifies a controller and action value.

Several overloads of Url.Action take a route values object to provide values for route parameters other than controller and action. The route values object is frequently used with id. For example, Url.Action("Buy", "Products", new { id = 17 }). The route values object:

  • By convention is usually an object of anonymous type.
  • Can be an IDictionary<> or a POCO).

Any additional route values that don't match route parameters are put in the query string.

public IActionResult Index()
{
    var url = Url.Action("Buy", "Products", new { id = 17, color = "red" });
    return Content(url);
}

The preceding code generates /Products/Buy/17?color=red.

The following code generates an absolute URL:

public IActionResult Index2()
{
    var url = Url.Action("Buy", "Products", new { id = 17 }, protocol: Request.Scheme);
    // Returns https://localhost:5001/Products/Buy/17
    return Content(url);
}

To create an absolute URL, use one of the following:

  • An overload that accepts a protocol. For example, the preceding code.
  • LinkGenerator.GetUriByAction, which generates absolute URIs by default.

Generate URLs by route

The preceding code demonstrated generating a URL by passing in the controller and action name. IUrlHelper also provides the Url.RouteUrl family of methods. These methods are similar to Url.Action, but they don't copy the current values of action and controller to the route values. The most common usage of Url.RouteUrl:

  • Specifies a route name to generate the URL.
  • Generally doesn't specify a controller or action name.
public class UrlGeneration2Controller : Controller
{
    [HttpGet("")]
    public IActionResult Source()
    {
        var url = Url.RouteUrl("Destination_Route");
        return ControllerContext.MyDisplayRouteInfo("", $" URL = {url}");
    }

    [HttpGet("custom/url/to/destination2", Name = "Destination_Route")]
    public IActionResult Destination()
    {
        return ControllerContext.MyDisplayRouteInfo();
    }

The following Razor file generates an HTML link to the Destination_Route:

<h1>Test Links</h1>

<ul>
    <li><a href="@Url.RouteUrl("Destination_Route")">Test Destination_Route</a></li>
</ul>

Generate URLs in HTML and Razor

IHtmlHelper provides the HtmlHelper methods Html.BeginForm and Html.ActionLink to generate <form> and <a> elements respectively. These methods use the Url.Action method to generate a URL and they accept similar arguments. The Url.RouteUrl companions for HtmlHelper are Html.BeginRouteForm and Html.RouteLink which have similar functionality.

TagHelpers generate URLs through the form TagHelper and the <a> TagHelper. Both of these use IUrlHelper for their implementation. See Tag Helpers in forms for more information.

Inside views, the IUrlHelper is available through the Url property for any ad-hoc URL generation not covered by the above.

URL generation in Action Results

The preceding examples showed using IUrlHelper in a controller. The most common usage in a controller is to generate a URL as part of an action result.

The ControllerBase and Controller base classes provide convenience methods for action results that reference another action. One typical usage is to redirect after accepting user input:

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Edit(int id, Customer customer)
{
    if (ModelState.IsValid)
    {
        // Update DB with new details.
        ViewData["Message"] = $"Successful edit of customer {id}";
        return RedirectToAction("Index");
    }
    return View(customer);
}

The action results factory methods such as RedirectToAction and CreatedAtAction follow a similar pattern to the methods on IUrlHelper.

Special case for dedicated conventional routes

Conventional routing can use a special kind of route definition called a dedicated conventional route. In the following example, the route named blog is a dedicated conventional route:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(name: "blog",
                pattern: "blog/{*article}",
                defaults: new { controller = "Blog", action = "Article" });
    endpoints.MapControllerRoute(name: "default",
                pattern: "{controller=Home}/{action=Index}/{id?}");
});

Using the preceding route definitions, Url.Action("Index", "Home") generates the URL path / using the default route, but why? You might guess the route values { controller = Home, action = Index } would be enough to generate a URL using blog, and the result would be /blog?action=Index&controller=Home.

Dedicated conventional routes rely on a special behavior of default values that don't have a corresponding route parameter that prevents the route from being too greedy with URL generation. In this case the default values are { controller = Blog, action = Article }, and neither controller nor action appears as a route parameter. When routing performs URL generation, the values provided must match the default values. URL generation using blog fails because the values { controller = Home, action = Index } don't match { controller = Blog, action = Article }. Routing then falls back to try default, which succeeds.

Areas

Areas are an MVC feature used to organize related functionality into a group as a separate:

  • Routing namespace for controller actions.
  • Folder structure for views.

Using areas allows an app to have multiple controllers with the same name, as long as they have different areas. Using areas creates a hierarchy for the purpose of routing by adding another route parameter, area to controller and action. This section discusses how routing interacts with areas. See Areas for details about how areas are used with views.

The following example configures MVC to use the default conventional route and an area route for an area named Blog:

app.UseEndpoints(endpoints =>
{
    endpoints.MapAreaControllerRoute("blog_route", "Blog",
        "Manage/{controller}/{action}/{id?}");
    endpoints.MapControllerRoute("default_route", "{controller}/{action}/{id?}");
});

In the preceding code, MapAreaControllerRoute is called to create the "blog_route". The second parameter, "Blog", is the area name.

When matching a URL path like /Manage/Users/AddUser, the "blog_route" route generates the route values { area = Blog, controller = Users, action = AddUser }. The area route value is produced by a default value for area. The route created by MapAreaControllerRoute is equivalent to the following:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute("blog_route", "Manage/{controller}/{action}/{id?}",
        defaults: new { area = "Blog" }, constraints: new { area = "Blog" });
    endpoints.MapControllerRoute("default_route", "{controller}/{action}/{id?}");
});

MapAreaControllerRoute creates a route using both a default value and constraint for area using the provided area name, in this case Blog. The default value ensures that the route always produces { area = Blog, ... }, the constraint requires the value { area = Blog, ... } for URL generation.

Conventional routing is order-dependent. In general, routes with areas should be placed earlier as they're more specific than routes without an area.

Using the preceding example, the route values { area = Blog, controller = Users, action = AddUser } match the following action:

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace1
{
    [Area("Blog")]
    public class UsersController : Controller
    {
        // GET /manage/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}

The [Area] attribute is what denotes a controller as part of an area. This controller is in the Blog area. Controllers without an [Area] attribute are not members of any area, and do not match when the area route value is provided by routing. In the following example, only the first controller listed can match the route values { area = Blog, controller = Users, action = AddUser }.

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace1
{
    [Area("Blog")]
    public class UsersController : Controller
    {
        // GET /manage/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace2
{
    // Matches { area = Zebra, controller = Users, action = AddUser }
    [Area("Zebra")]
    public class UsersController : Controller
    {
        // GET /zebra/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace3
{
    // Matches { area = string.Empty, controller = Users, action = AddUser }
    // Matches { area = null, controller = Users, action = AddUser }
    // Matches { controller = Users, action = AddUser }
    public class UsersController : Controller
    {
        // GET /users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }
    }
}

The namespace of each controller is shown here for completeness. If the preceding controllers uses the same namespace, a compiler error would be generated. Class namespaces have no effect on MVC's routing.

The first two controllers are members of areas, and only match when their respective area name is provided by the area route value. The third controller isn't a member of any area, and can only match when no value for area is provided by routing.

In terms of matching no value, the absence of the area value is the same as if the value for area were null or the empty string.

When executing an action inside an area, the route value for area is available as an ambient value for routing to use for URL generation. This means that by default areas act sticky for URL generation as demonstrated by the following sample.

app.UseEndpoints(endpoints =>
{
    endpoints.MapAreaControllerRoute(name: "duck_route", 
                                     areaName: "Duck",
                                     pattern: "Manage/{controller}/{action}/{id?}");
    endpoints.MapControllerRoute(name: "default",
                                 pattern: "Manage/{controller=Home}/{action=Index}/{id?}");
});
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace4
{
    [Area("Duck")]
    public class UsersController : Controller
    {
        // GET /Manage/users/GenerateURLInArea
        public IActionResult GenerateURLInArea()
        {
            // Uses the 'ambient' value of area.
            var url = Url.Action("Index", "Home");
            // Returns /Manage/Home/Index
            return Content(url);
        }

        // GET /Manage/users/GenerateURLOutsideOfArea
        public IActionResult GenerateURLOutsideOfArea()
        {
            // Uses the empty value for area.
            var url = Url.Action("Index", "Home", new { area = "" });
            // Returns /Manage
            return Content(url);
        }
    }
}

The following code generates a URL to /Zebra/Users/AddUser:

public class HomeController : Controller
{
    public IActionResult About()
    {
        var url = Url.Action("AddUser", "Users", new { Area = "Zebra" });
        return Content($"URL: {url}");
    }

Action definition

Public methods on a controller, except those with the NonAction attribute, are actions.

Sample code

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"
    }
  }
}

ASP.NET Core MVC uses the Routing middleware to match the URLs of incoming requests and map them to actions. Routes are defined in startup code or attributes. Routes describe how URL paths should be matched to actions. Routes are also used to generate URLs (for links) sent out in responses.

Actions are either conventionally routed or attribute routed. Placing a route on the controller or the action makes it attribute routed. See Mixed routing for more information.

This document will explain the interactions between MVC and routing, and how typical MVC apps make use of routing features. See Routing for details on advanced routing.

Setting up Routing Middleware

In your Configure method you may see code similar to:

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

Inside the call to UseMvc, MapRoute is used to create a single route, which we'll refer to as the default route. Most MVC apps will use a route with a template similar to the default route.

The route template "{controller=Home}/{action=Index}/{id?}" can match a URL path like /Products/Details/5 and will extract the route values { controller = Products, action = Details, id = 5 } by tokenizing the path. MVC will attempt to locate a controller named ProductsController and run the action Details:

public class ProductsController : Controller
{
   public IActionResult Details(int id) { ... }
}

Note that in this example, model binding would use the value of id = 5 to set the id parameter to 5 when invoking this action. See the Model Binding for more details.

Using the default route:

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

The route template:

  • {controller=Home} defines Home as the default controller

  • {action=Index} defines Index as the default action

  • {id?} defines id as optional

Default and optional route parameters don't need to be present in the URL path for a match. See Route Template Reference for a detailed description of route template syntax.

"{controller=Home}/{action=Index}/{id?}" can match the URL path / and will produce the route values { controller = Home, action = Index }. The values for controller and action make use of the default values, id doesn't produce a value since there's no corresponding segment in the URL path. MVC would use these route values to select the HomeController and Index action:

public class HomeController : Controller
{
  public IActionResult Index() { ... }
}

Using this controller definition and route template, the HomeController.Index action would be executed for any of the following URL paths:

  • /Home/Index/17

  • /Home/Index

  • /Home

  • /

The convenience method UseMvcWithDefaultRoute:

app.UseMvcWithDefaultRoute();

Can be used to replace:

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

UseMvc and UseMvcWithDefaultRoute add an instance of RouterMiddleware to the middleware pipeline. MVC doesn't interact directly with middleware, and uses routing to handle requests. MVC is connected to the routes through an instance of MvcRouteHandler. The code inside of UseMvc is similar to the following:

var routes = new RouteBuilder(app);

// Add connection to MVC, will be hooked up by calls to MapRoute.
routes.DefaultHandler = new MvcRouteHandler(...);

// Execute callback to register routes.
// routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");

// Create route collection and add the middleware.
app.UseRouter(routes.Build());

UseMvc doesn't directly define any routes, it adds a placeholder to the route collection for the attribute route. The overload UseMvc(Action<IRouteBuilder>) lets you add your own routes and also supports attribute routing. UseMvc and all of its variations add a placeholder for the attribute route - attribute routing is always available regardless of how you configure UseMvc. UseMvcWithDefaultRoute defines a default route and supports attribute routing. The Attribute Routing section includes more details on attribute routing.

Conventional routing

The default route:

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

The preceding code is an example of a conventional routing. This style is called conventional routing because it establishes a convention for URL paths:

  • The first path segment maps to the controller name.
  • The second maps to the action name.
  • The third segment is used for an optional id. id maps to a model entity.

Using this default route, the URL path /Products/List maps to the ProductsController.List action, and /Blog/Article/17 maps to BlogController.Article. This mapping is based on the controller and action names only and isn't based on namespaces, source file locations, or method parameters.

Tip

Using conventional routing with the default route allows you to build the application quickly without having to come up with a new URL pattern for each action you define. For an application with CRUD style actions, having consistency for the URLs across your controllers can help simplify your code and make your UI more predictable.

Warning

The id is defined as optional by the route template, meaning that your actions can execute without the ID provided as part of the URL. Usually what will happen if id is omitted from the URL is that it will be set to 0 by model binding, and as a result no entity will be found in the database matching id == 0. Attribute routing can give you fine-grained control to make the ID required for some actions and not for others. By convention the documentation will include optional parameters like id when they're likely to appear in correct usage.

Multiple routes

You can add multiple routes inside UseMvc by adding more calls to MapRoute. Doing so allows you to define multiple conventions, or to add conventional routes that are dedicated to a specific action, such as:

app.UseMvc(routes =>
{
   routes.MapRoute("blog", "blog/{*article}",
            defaults: new { controller = "Blog", action = "Article" });
   routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");
});

The blog route here is a dedicated conventional route, meaning that it uses the conventional routing system, but is dedicated to a specific action. Since controller and action don't appear in the route template as parameters, they can only have the default values, and thus this route will always map to the action BlogController.Article.

Routes in the route collection are ordered, and will be processed in the order they're added. So in this example, the blog route will be tried before the default route.

Note

Dedicated conventional routes often use catch-all route parameters like {*article} to capture the remaining portion of the URL path. This can make a route 'too greedy' meaning that it matches URLs that you intended to be matched by other routes. Put the 'greedy' routes later in the route table to solve this.

Fallback

As part of request processing, MVC will verify that the route values can be used to find a controller and action in your application. If the route values don't match an action then the route isn't considered a match, and the next route will be tried. This is called fallback, and it's intended to simplify cases where conventional routes overlap.

Disambiguating actions

When two actions match through routing, MVC must disambiguate to choose the 'best' candidate or else throw an exception. For example:

public class ProductsController : Controller
{
   public IActionResult Edit(int id) { ... }

   [HttpPost]
   public IActionResult Edit(int id, Product product) { ... }
}

This controller defines two actions that would match the URL path /Products/Edit/17 and route data { controller = Products, action = Edit, id = 17 }. This is a typical pattern for MVC controllers where Edit(int) shows a form to edit a product, and Edit(int, Product) processes the posted form. To make this possible MVC would need to choose Edit(int, Product) when the request is an HTTP POST and Edit(int) when the HTTP verb is anything else.

The HttpPostAttribute ( [HttpPost] ) is an implementation of IActionConstraint that will only allow the action to be selected when the HTTP verb is POST. The presence of an IActionConstraint makes the Edit(int, Product) a 'better' match than Edit(int), so Edit(int, Product) will be tried first.

You will only need to write custom IActionConstraint implementations in specialized scenarios, but it's important to understand the role of attributes like HttpPostAttribute - similar attributes are defined for other HTTP verbs. In conventional routing it's common for actions to use the same action name when they're part of a show form -> submit form workflow. The convenience of this pattern will become more apparent after reviewing the Understanding IActionConstraint section.

If multiple routes match, and MVC can't find a 'best' route, it will throw an AmbiguousActionException.

Route names

The strings "blog" and "default" in the following examples are route names:

app.UseMvc(routes =>
{
   routes.MapRoute("blog", "blog/{*article}",
               defaults: new { controller = "Blog", action = "Article" });
   routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");
});

The route names give the route a logical name so that the named route can be used for URL generation. This greatly simplifies URL creation when the ordering of routes could make URL generation complicated. Route names must be unique application-wide.

Route names have no impact on URL matching or handling of requests; they're used only for URL generation. Routing has more detailed information on URL generation including URL generation in MVC-specific helpers.

Attribute routing

Attribute routing uses a set of attributes to map actions directly to route templates. In the following example, app.UseMvc(); is used in the Configure method and no route is passed. The HomeController will match a set of URLs similar to what the default route {controller=Home}/{action=Index}/{id?} would match:

public class HomeController : Controller
{
   [Route("")]
   [Route("Home")]
   [Route("Home/Index")]
   public IActionResult Index()
   {
      return View();
   }
   [Route("Home/About")]
   public IActionResult About()
   {
      return View();
   }
   [Route("Home/Contact")]
   public IActionResult Contact()
   {
      return View();
   }
}

The HomeController.Index() action will be executed for any of the URL paths /, /Home, or /Home/Index.

Note

This example highlights a key programming difference between attribute routing and conventional routing. Attribute routing requires more input to specify a route; the conventional default route handles routes more succinctly. However, attribute routing allows (and requires) precise control of which route templates apply to each action.

With attribute routing the controller name and action names play no role in which action is selected. This example will match the same URLs as the previous example.

public class MyDemoController : Controller
{
   [Route("")]
   [Route("Home")]
   [Route("Home/Index")]
   public IActionResult MyIndex()
   {
      return View("Index");
   }
   [Route("Home/About")]
   public IActionResult MyAbout()
   {
      return View("About");
   }
   [Route("Home/Contact")]
   public IActionResult MyContact()
   {
      return View("Contact");
   }
}

Note

The route templates above don't define route parameters for action, area, and controller. In fact, these route parameters are not allowed in attribute routes. Since the route template is already associated with an action, it wouldn't make sense to parse the action name from the URL.

Attribute routing with Http[Verb] attributes

Attribute routing can also make use of the Http[Verb] attributes such as HttpPostAttribute. All of these attributes can accept a route template. This example shows two actions that match the same route template:

[HttpGet("/products")]
public IActionResult ListProducts()
{
   // ...
}

[HttpPost("/products")]
public IActionResult CreateProduct(...)
{
   // ...
}

For a URL path like /products the ProductsApi.ListProducts action will be executed when the HTTP verb is GET and ProductsApi.CreateProduct will be executed when the HTTP verb is POST. Attribute routing first matches the URL against the set of route templates defined by route attributes. Once a route template matches, IActionConstraint constraints are applied to determine which actions can be executed.

Tip

When building a REST API, it's rare that you will want to use [Route(...)] on an action method as the action will accept all HTTP methods. It's better to use the more specific Http*Verb*Attributes to be precise about what your API supports. Clients of REST APIs are expected to know what paths and HTTP verbs map to specific logical operations.

Since an attribute route applies to a specific action, it's easy to make parameters required as part of the route template definition. In this example, id is required as part of the URL path.

public class ProductsApiController : Controller
{
   [HttpGet("/products/{id}", Name = "Products_List")]
   public IActionResult GetProduct(int id) { ... }
}

The ProductsApi.GetProduct(int) action will be executed for a URL path like /products/3 but not for a URL path like /products. See Routing for a full description of route templates and related options.

Route Name

The following code defines a route name of Products_List:

public class ProductsApiController : Controller
{
   [HttpGet("/products/{id}", Name = "Products_List")]
   public IActionResult GetProduct(int id) { ... }
}

Route names can be used to generate a URL based on a specific route. Route names have no impact on the URL matching behavior of routing and are only used for URL generation. Route names must be unique application-wide.

Note

Contrast this with the conventional default route, which defines the id parameter as optional ({id?}). This ability to precisely specify APIs has advantages, such as allowing /products and /products/5 to be dispatched to different actions.

Combining routes

To make attribute routing less repetitive, route attributes on the controller are combined with route attributes on the individual actions. Any route templates defined on the controller are prepended to route templates on the actions. Placing a route attribute on the controller makes all actions in the controller use attribute routing.

[Route("products")]
public class ProductsApiController : Controller
{
   [HttpGet]
   public IActionResult ListProducts() { ... }

   [HttpGet("{id}")]
   public ActionResult GetProduct(int id) { ... }
}

In this example the URL path /products can match ProductsApi.ListProducts, and the URL path /products/5 can match ProductsApi.GetProduct(int). Both of these actions only match HTTP GET because they're marked with the HttpGetAttribute.

Route templates applied to an action that begin with / or ~/ don't get combined with route templates applied to the controller. This example matches a set of URL paths similar to the default route.

[Route("Home")]
public class HomeController : Controller
{
    [Route("")]      // Combines to define the route template "Home"
    [Route("Index")] // Combines to define the route template "Home/Index"
    [Route("/")]     // Doesn't combine, defines the route template ""
    public IActionResult Index()
    {
        ViewData["Message"] = "Home index";
        var url = Url.Action("Index", "Home");
        ViewData["Message"] = "Home index" + "var url = Url.Action; =  " + url;
        return View();
    }

    [Route("About")] // Combines to define the route template "Home/About"
    public IActionResult About()
    {
        return View();
    }   
}

Ordering attribute routes

In contrast to conventional routes, which execute in a defined order, attribute routing builds a tree and matches all routes simultaneously. This behaves as-if the route entries were placed in an ideal ordering; the most specific routes have a chance to execute before the more general routes.

For example, a route like blog/search/{topic} is more specific than a route like blog/{*article}. Logically speaking the blog/search/{topic} route 'runs' first, by default, because that's the only sensible ordering. Using conventional routing, the developer is responsible for placing routes in the desired order.

Attribute routes can configure an order, using the Order property of all of the framework provided route attributes. Routes are processed according to an ascending sort of the Order property. The default order is 0. Setting a route using Order = -1 will run before routes that don't set an order. Setting a route using Order = 1 will run after default route ordering.

Tip

Avoid depending on Order. If your URL-space requires explicit order values to route correctly, then it's likely confusing to clients as well. In general attribute routing will select the correct route with URL matching. If the default order used for URL generation isn't working, using route name as an override is usually simpler than applying the Order property.

Razor Pages routing and MVC controller routing share an implementation. Information on route order in the Razor Pages topics is available at Razor Pages route and app conventions: Route order.

Token replacement in route templates ([controller], [action], [area])

For convenience, attribute routes support token replacement by enclosing a token in square-brackets ([, ]). The tokens [action], [area], and [controller] are replaced with the values of the action name, area name, and controller name from the action where the route is defined. In the following example, the actions match URL paths as described in the comments:

[Route("[controller]/[action]")]
public class ProductsController : Controller
{
    [HttpGet] // Matches '/Products/List'
    public IActionResult List() {
        // ...
    }

    [HttpGet("{id}")] // Matches '/Products/Edit/{id}'
    public IActionResult Edit(int id) {
        // ...
    }
}

Token replacement occurs as the last step of building the attribute routes. The above example will behave the same as the following code:


public class ProductsController : Controller
{
    [HttpGet("[controller]/[action]")] // Matches '/Products/List'
    public IActionResult List() {
        // ...
    }

    [HttpGet("[controller]/[action]/{id}")] // Matches '/Products/Edit/{id}'
    public IActionResult Edit(int id) {
        // ...
    }
}

Attribute routes can also be combined with inheritance. This is particularly powerful combined with token replacement.

[Route("api/[controller]")]
public abstract class MyBaseController : Controller { ... }

public class ProductsController : MyBaseController
{
   [HttpGet] // Matches '/api/Products'
   public IActionResult List() { ... }

   [HttpPut("{id}")] // Matches '/api/Products/{id}'
   public IActionResult Edit(int id) { ... }
}

Token replacement also applies to route names defined by attribute routes. [Route("[controller]/[action]", Name="[controller]_[action]")] generates a unique route name for each action.

To match the literal token replacement delimiter [ or ], escape it by repeating the character ([[ or ]]).

Use a parameter transformer to customize token replacement

Token replacement can be customized using a parameter transformer. A parameter transformer implements IOutboundParameterTransformer and transforms the value of parameters. For example, a custom SlugifyParameterTransformer parameter transformer changes the SubscriptionManagement route value to subscription-management.

The RouteTokenTransformerConvention is an application model convention that:

  • Applies a parameter transformer to all attribute routes in an application.
  • Customizes the attribute route token values as they are replaced.
public class SubscriptionManagementController : Controller
{
    [HttpGet("[controller]/[action]")] // Matches '/subscription-management/list-all'
    public IActionResult ListAll() { ... }
}

The RouteTokenTransformerConvention is registered as an option in ConfigureServices.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
    {
        options.Conventions.Add(new RouteTokenTransformerConvention(
                                     new SlugifyParameterTransformer()));
    });
}

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

        // Slugify value
        return Regex.Replace(value.ToString(), "([a-z])([A-Z])", "$1-$2").ToLower();
    }
}

Multiple Routes

Attribute routing supports defining multiple routes that reach the same action. The most common usage of this is to mimic the behavior of the default conventional route as shown in the following example:

[Route("[controller]")]
public class ProductsController : Controller
{
   [Route("")]     // Matches 'Products'
   [Route("Index")] // Matches 'Products/Index'
   public IActionResult Index()
}

Putting multiple route attributes on the controller means that each one will combine with each of the route attributes on the action methods.

[Route("Store")]
[Route("[controller]")]
public class ProductsController : Controller
{
   [HttpPost("Buy")]     // Matches 'Products/Buy' and 'Store/Buy'
   [HttpPost("Checkout")] // Matches 'Products/Checkout' and 'Store/Checkout'
   public IActionResult Buy()
}

When multiple route attributes (that implement IActionConstraint) are placed on an action, then each action constraint combines with the route template from the attribute that defined it.

[Route("api/[controller]")]
public class ProductsController : Controller
{
   [HttpPut("Buy")]      // Matches PUT 'api/Products/Buy'
   [HttpPost("Checkout")] // Matches POST 'api/Products/Checkout'
   public IActionResult Buy()
}

Tip

While using multiple routes on actions can seem powerful, it's better to keep your application's URL space simple and well-defined. Use multiple routes on actions only where needed, for example to support existing clients.

Specifying attribute route optional parameters, default values, and constraints

Attribute routes support the same inline syntax as conventional routes to specify optional parameters, default values, and constraints.

[HttpPost("product/{id:int}")]
public IActionResult ShowProduct(int id)
{
   // ...
}

See Route Template Reference for a detailed description of route template syntax.

Custom route attributes using IRouteTemplateProvider

All of the route attributes provided in the framework ( [Route(...)], [HttpGet(...)] , etc.) implement the IRouteTemplateProvider interface. MVC looks for attributes on controller classes and action methods when the app starts and uses the ones that implement IRouteTemplateProvider to build the initial set of routes.

You can implement IRouteTemplateProvider to define your own route attributes. Each IRouteTemplateProvider allows you to define a single route with a custom route template, order, and name:

public class MyApiControllerAttribute : Attribute, IRouteTemplateProvider
{
   public string Template => "api/[controller]";

   public int? Order { get; set; }

   public string Name { get; set; }
}

The attribute from the above example automatically sets the Template to "api/[controller]" when [MyApiController] is applied.

Using Application Model to customize attribute routes

The application model is an object model created at startup with all of the metadata used by MVC to route and execute your actions. The application model includes all of the data gathered from route attributes (through IRouteTemplateProvider). You can write conventions to modify the application model at startup time to customize how routing behaves. This section shows a simple example of customizing routing using application model.

using Microsoft.AspNetCore.Mvc.ApplicationModels;
using System.Linq;
using System.Text;
public class NamespaceRoutingConvention : IControllerModelConvention
{
    private readonly string _baseNamespace;

    public NamespaceRoutingConvention(string baseNamespace)
    {
        _baseNamespace = baseNamespace;
    }

    public void Apply(ControllerModel controller)
    {
        var hasRouteAttributes = controller.Selectors.Any(selector =>
                                                selector.AttributeRouteModel != null);
        if (hasRouteAttributes)
        {
            // This controller manually defined some routes, so treat this 
            // as an override and not apply the convention here.
            return;
        }

        // Use the namespace and controller name to infer a route for the controller.
        //
        // Example:
        //
        //  controller.ControllerTypeInfo ->    "My.Application.Admin.UsersController"
        //  baseNamespace ->                    "My.Application"
        //
        //  template =>                         "Admin/[controller]"
        //
        // This makes your routes roughly line up with the folder structure of your project.
        //
        var namespc = controller.ControllerType.Namespace;
        if (namespc == null)
            return;
        var template = new StringBuilder();
        template.Append(namespc, _baseNamespace.Length + 1,
                        namespc.Length - _baseNamespace.Length - 1);
        template.Replace('.', '/');
        template.Append("/[controller]");

        foreach (var selector in controller.Selectors)
        {
            selector.AttributeRouteModel = new AttributeRouteModel()
            {
                Template = template.ToString()
            };
        }
    }
}

Mixed routing: Attribute routing vs conventional routing

MVC applications can mix the use of conventional routing and attribute routing. It's typical to use conventional routes for controllers serving HTML pages for browsers, and attribute routing for controllers serving REST APIs.

Actions are either conventionally routed or attribute routed. Placing a route on the controller or the action makes it attribute routed. Actions that define attribute routes cannot be reached through the conventional routes and vice-versa. Any route attribute on the controller makes all actions in the controller attribute routed.

Note

What distinguishes the two types of routing systems is the process applied after a URL matches a route template. In conventional routing, the route values from the match are used to choose the action and controller from a lookup table of all conventional routed actions. In attribute routing, each template is already associated with an action, and no further lookup is needed.

Complex segments

Complex segments (for example, [Route("/dog{token}cat")]), are processed by matching up literals from right to left in a non-greedy way. See the source code for a description. For more information, see this issue.

URL Generation

MVC applications can use routing's URL generation features to generate URL links to actions. Generating URLs eliminates hardcoding URLs, making your code more robust and maintainable. This section focuses on the URL generation features provided by MVC and will only cover basics of how URL generation works. See Routing for a detailed description of URL generation.

The IUrlHelper interface is the underlying piece of infrastructure between MVC and routing for URL generation. You'll find an instance of IUrlHelper available through the Url property in controllers, views, and view components.

In this example, the IUrlHelper interface is used through the Controller.Url property to generate a URL to another action.

using Microsoft.AspNetCore.Mvc;

public class UrlGenerationController : Controller
{
    public IActionResult Source()
    {
        // Generates /UrlGeneration/Destination
        var url = Url.Action("Destination");
        return Content($"Go check out {url}, it's really great.");
    }

    public IActionResult Destination()
    {
        return View();
    }
}

If the application is using the default conventional route, the value of the url variable will be the URL path string /UrlGeneration/Destination. This URL path is created by routing by combining the route values from the current request (ambient values), with the values passed to Url.Action and substituting those values into the route template:

ambient values: { controller = "UrlGeneration", action = "Source" }
values passed to Url.Action: { controller = "UrlGeneration", action = "Destination" }
route template: {controller}/{action}/{id?}

result: /UrlGeneration/Destination

Each route parameter in the route template has its value substituted by matching names with the values and ambient values. A route parameter that doesn't have a value can use a default value if it has one, or be skipped if it's optional (as in the case of id in this example). URL generation will fail if any required route parameter doesn't have a corresponding value. If URL generation fails for a route, the next route is tried until all routes have been tried or a match is found.

The example of Url.Action above assumes conventional routing, but URL generation works similarly with attribute routing, though the concepts are different. With conventional routing, the route values are used to expand a template, and the route values for controller and action usually appear in that template - this works because the URLs matched by routing adhere to a convention. In attribute routing, the route values for controller and action are not allowed to appear in the template - they're instead used to look up which template to use.

This example uses attribute routing:

// In Startup class
public void Configure(IApplicationBuilder app)
{
    app.UseMvc();
}
using Microsoft.AspNetCore.Mvc;

public class UrlGenerationController : Controller
{
    [HttpGet("")]
    public IActionResult Source()
    {
        var url = Url.Action("Destination"); // Generates /custom/url/to/destination
        return Content($"Go check out {url}, it's really great.");
    }

    [HttpGet("custom/url/to/destination")]
    public IActionResult Destination() {
        return View();
    }
}

MVC builds a lookup table of all attribute routed actions and will match the controller and action values to select the route template to use for URL generation. In the sample above, custom/url/to/destination is generated.

Generating URLs by action name

Url.Action (IUrlHelper . Action) and all related overloads all are based on that idea that you want to specify what you're linking to by specifying a controller name and action name.

Note

When using Url.Action, the current route values for controller and action are specified for you - the value of controller and action are part of both ambient values and values. The method Url.Action, always uses the current values of action and controller and will generate a URL path that routes to the current action.

Routing attempts to use the values in ambient values to fill in information that you didn't provide when generating a URL. Using a route like {a}/{b}/{c}/{d} and ambient values { a = Alice, b = Bob, c = Carol, d = David }, routing has enough information to generate a URL without any additional values - since all route parameters have a value. If you added the value { d = Donovan }, the value { d = David } would be ignored, and the generated URL path would be Alice/Bob/Carol/Donovan.

Warning

URL paths are hierarchical. In the example above, if you added the value { c = Cheryl }, both of the values { c = Carol, d = David } would be ignored. In this case we no longer have a value for d and URL generation will fail. You would need to specify the desired value of c and d. You might expect to hit this problem with the default route ({controller}/{action}/{id?}) - but you will rarely encounter this behavior in practice as Url.Action will always explicitly specify a controller and action value.

Longer overloads of Url.Action also take an additional route values object to provide values for route parameters other than controller and action. You will most commonly see this used with id like Url.Action("Buy", "Products", new { id = 17 }). By convention the route values object is usually an object of anonymous type, but it can also be an IDictionary<> or a plain old .NET object. Any additional route values that don't match route parameters are put in the query string.

using Microsoft.AspNetCore.Mvc;

public class TestController : Controller
{
    public IActionResult Index()
    {
        // Generates /Products/Buy/17?color=red
        var url = Url.Action("Buy", "Products", new { id = 17, color = "red" });
        return Content(url);
    }
}

Tip

To create an absolute URL, use an overload that accepts a protocol: Url.Action("Buy", "Products", new { id = 17 }, protocol: Request.Scheme)

Generating URLs by route

The code above demonstrated generating a URL by passing in the controller and action name. IUrlHelper also provides the Url.RouteUrl family of methods. These methods are similar to Url.Action, but they don't copy the current values of action and controller to the route values. The most common usage is to specify a route name to use a specific route to generate the URL, generally without specifying a controller or action name.

using Microsoft.AspNetCore.Mvc;

public class UrlGenerationController : Controller
{
    [HttpGet("")]
    public IActionResult Source()
    {
        var url = Url.RouteUrl("Destination_Route"); // Generates /custom/url/to/destination
        return Content($"See {url}, it's really great.");
    }

    [HttpGet("custom/url/to/destination", Name = "Destination_Route")]
    public IActionResult Destination() {
        return View();
    }
}

Generating URLs in HTML

IHtmlHelper provides the HtmlHelper methods Html.BeginForm and Html.ActionLink to generate <form> and <a> elements respectively. These methods use the Url.Action method to generate a URL and they accept similar arguments. The Url.RouteUrl companions for HtmlHelper are Html.BeginRouteForm and Html.RouteLink which have similar functionality.

TagHelpers generate URLs through the form TagHelper and the <a> TagHelper. Both of these use IUrlHelper for their implementation. See Working with Forms for more information.

Inside views, the IUrlHelper is available through the Url property for any ad-hoc URL generation not covered by the above.

Generating URLS in Action Results

The examples above have shown using IUrlHelper in a controller, while the most common usage in a controller is to generate a URL as part of an action result.

The ControllerBase and Controller base classes provide convenience methods for action results that reference another action. One typical usage is to redirect after accepting user input.

public IActionResult Edit(int id, Customer customer)
{
    if (ModelState.IsValid)
    {
        // Update DB with new details.
        return RedirectToAction("Index");
    }
    return View(customer);
}

The action results factory methods follow a similar pattern to the methods on IUrlHelper.

Special case for dedicated conventional routes

Conventional routing can use a special kind of route definition called a dedicated conventional route. In the example below, the route named blog is a dedicated conventional route.

app.UseMvc(routes =>
{
    routes.MapRoute("blog", "blog/{*article}",
        defaults: new { controller = "Blog", action = "Article" });
    routes.MapRoute("default", "{controller=Home}/{action=Index}/{id?}");
});

Using these route definitions, Url.Action("Index", "Home") will generate the URL path / with the default route, but why? You might guess the route values { controller = Home, action = Index } would be enough to generate a URL using blog, and the result would be /blog?action=Index&controller=Home.

Dedicated conventional routes rely on a special behavior of default values that don't have a corresponding route parameter that prevents the route from being "too greedy" with URL generation. In this case the default values are { controller = Blog, action = Article }, and neither controller nor action appears as a route parameter. When routing performs URL generation, the values provided must match the default values. URL generation using blog will fail because the values { controller = Home, action = Index } don't match { controller = Blog, action = Article }. Routing then falls back to try default, which succeeds.

Areas

Areas are an MVC feature used to organize related functionality into a group as a separate routing-namespace (for controller actions) and folder structure (for views). Using areas allows an application to have multiple controllers with the same name - as long as they have different areas. Using areas creates a hierarchy for the purpose of routing by adding another route parameter, area to controller and action. This section will discuss how routing interacts with areas - see Areas for details about how areas are used with views.

The following example configures MVC to use the default conventional route and an area route for an area named Blog:

app.UseEndpoints(endpoints =>
{
    endpoints.MapAreaControllerRoute("blog_route", "Blog",
        "Manage/{controller}/{action}/{id?}");
    endpoints.MapControllerRoute("default_route", "{controller}/{action}/{id?}");
});

When matching a URL path like /Manage/Users/AddUser, the first route will produce the route values { area = Blog, controller = Users, action = AddUser }. The area route value is produced by a default value for area, in fact the route created by MapAreaRoute is equivalent to the following:

MapAreaRoute creates a route using both a default value and constraint for area using the provided area name, in this case Blog. The default value ensures that the route always produces { area = Blog, ... }, the constraint requires the value { area = Blog, ... } for URL generation.

Tip

Conventional routing is order-dependent. In general, routes with areas should be placed earlier in the route table as they're more specific than routes without an area.

Using the above example, the route values would match the following action:

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace1
{
    [Area("Blog")]
    public class UsersController : Controller
    {
        // GET /manage/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}

The AreaAttribute is what denotes a controller as part of an area, we say that this controller is in the Blog area. Controllers without an [Area] attribute are not members of any area, and will not match when the area route value is provided by routing. In the following example, only the first controller listed can match the route values { area = Blog, controller = Users, action = AddUser }.

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace1
{
    [Area("Blog")]
    public class UsersController : Controller
    {
        // GET /manage/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace2
{
    // Matches { area = Zebra, controller = Users, action = AddUser }
    [Area("Zebra")]
    public class UsersController : Controller
    {
        // GET /zebra/users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }        
    }
}
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace3
{
    // Matches { area = string.Empty, controller = Users, action = AddUser }
    // Matches { area = null, controller = Users, action = AddUser }
    // Matches { controller = Users, action = AddUser }
    public class UsersController : Controller
    {
        // GET /users/adduser
        public IActionResult AddUser()
        {
            var area = ControllerContext.ActionDescriptor.RouteValues["area"];
            var actionName = ControllerContext.ActionDescriptor.ActionName;
            var controllerName = ControllerContext.ActionDescriptor.ControllerName;

            return Content($"area name:{area}" +
                $" controller:{controllerName}  action name: {actionName}");
        }
    }
}

Note

The namespace of each controller is shown here for completeness - otherwise the controllers would have a naming conflict and generate a compiler error. Class namespaces have no effect on MVC's routing.

The first two controllers are members of areas, and only match when their respective area name is provided by the area route value. The third controller isn't a member of any area, and can only match when no value for area is provided by routing.

Note

In terms of matching no value, the absence of the area value is the same as if the value for area were null or the empty string.

When executing an action inside an area, the route value for area will be available as an ambient value for routing to use for URL generation. This means that by default areas act sticky for URL generation as demonstrated by the following sample.

using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace4
{
    [Area("Duck")]
    public class UsersController : Controller
    {
        // GET /Manage/users/GenerateURLInArea
        public IActionResult GenerateURLInArea()
        {
            // Uses the 'ambient' value of area.
            var url = Url.Action("Index", "Home");
            // Returns /Manage/Home/Index
            return Content(url);
        }

        // GET /Manage/users/GenerateURLOutsideOfArea
        public IActionResult GenerateURLOutsideOfArea()
        {
            // Uses the empty value for area.
            var url = Url.Action("Index", "Home", new { area = "" });
            // Returns /Manage
            return Content(url);
        }
    }
}

Understanding IActionConstraint

Note

This section is a deep-dive on framework internals and how MVC chooses an action to execute. A typical application won't need a custom IActionConstraint

You have likely already used IActionConstraint even if you're not familiar with the interface. The [HttpGet] Attribute and similar [Http-VERB] attributes implement IActionConstraint in order to limit the execution of an action method.

public class ProductsController : Controller
{
    [HttpGet]
    public IActionResult Edit() { }

    public IActionResult Edit(...) { }
}

Assuming the default conventional route, the URL path /Products/Edit would produce the values { controller = Products, action = Edit }, which would match both of the actions shown here. In IActionConstraint terminology we would say that both of these actions are considered candidates - as they both match the route data.

When the HttpGetAttribute executes, it will say that Edit() is a match for GET and isn't a match for any other HTTP verb. The Edit(...) action doesn't have any constraints defined, and so will match any HTTP verb. So assuming a POST - only Edit(...) matches. But, for a GET both actions can still match - however, an action with an IActionConstraint is always considered better than an action without. So because Edit() has [HttpGet] it's considered more specific, and will be selected if both actions can match.

Conceptually, IActionConstraint is a form of overloading, but instead of overloading methods with the same name, it's overloading between actions that match the same URL. Attribute routing also uses IActionConstraint and can result in actions from different controllers both being considered candidates.

Implementing IActionConstraint

The simplest way to implement an IActionConstraint is to create a class derived from System.Attribute and place it on your actions and controllers. MVC will automatically discover any IActionConstraint that are applied as attributes. You can use the application model to apply constraints, and this is probably the most flexible approach as it allows you to metaprogram how they're applied.

In the following example, a constraint chooses an action based on a country code from the route data. The full sample on GitHub.

public class CountrySpecificAttribute : Attribute, IActionConstraint
{
    private readonly string _countryCode;

    public CountrySpecificAttribute(string countryCode)
    {
        _countryCode = countryCode;
    }

    public int Order
    {
        get
        {
            return 0;
        }
    }

    public bool Accept(ActionConstraintContext context)
    {
        return string.Equals(
            context.RouteContext.RouteData.Values["country"].ToString(),
            _countryCode,
            StringComparison.OrdinalIgnoreCase);
    }
}

You are responsible for implementing the Accept method and choosing an 'Order' for the constraint to execute. In this case, the Accept method returns true to denote the action is a match when the country route value matches. This is different from a RouteValueAttribute in that it allows fallback to a non-attributed action. The sample shows that if you define an en-US action then a country code like fr-FR will fall back to a more generic controller that doesn't have [CountrySpecific(...)] applied.

The Order property decides which stage the constraint is part of. Action constraints run in groups based on the Order. For example, all of the framework provided HTTP method attributes use the same Order value so that they run in the same stage. You can have as many stages as you need to implement your desired policies.

Tip

To decide on a value for Order think about whether or not your constraint should be applied before HTTP methods. Lower numbers run first.