Routing to Controller Actions

By Ryan Nowak and Rick Anderson

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 do not 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 does not produce a value since there is 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 does not 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 adds 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?}");

is an example of a conventional routing. We call this style 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 used to map 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 is not 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 are 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 are 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 is not 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 are 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. Routes names must be unique application-wide.

Route names have no impact on URL matching or handling of requests; they are 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. 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 are decorated with the HttpGetAttribute.

Route templates applied to an action that begin with a / do not 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("/")]     // Does not 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.

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

For convenience, attribute routes support token replacement by enclosing a token in square-braces ([, ]]). The tokens [action], [area], and [controller] will be replaced with the values of the action name, area name, and controller name from the action where the route is defined. In this example the actions can 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() { ... }

   [HttpPost("{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]")] will generate a unique route name for each action.

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

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;

        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.

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 does not have a value can use a default value if it has one, or be skipped if it is 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 are 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 do not 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. See 🔧 HTML Helpers for more details.

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

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.UseMvc(routes =>
{
    routes.MapAreaRoute("blog_route", "Blog",
        "Manage/{controller}/{action}/{id?}");
    routes.MapRoute("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:

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

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 are 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
    {
        public IActionResult AddUser()
        {
            return View();
        }        
    }
}

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
    {
        public IActionResult AddUser()
        {
            return View();
        }        
    }
}
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Namespace2
{
    // Matches { area = Zebra, controller = Users, action = AddUser }
    [Area("Zebra")]
    public class UsersController : Controller
    {
        public IActionResult AddUser()
        {
            return View();
        }        
    }
}
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
    {
        public IActionResult AddUser()
        {
            return View();

        }
    }
}
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 is not 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.

app.UseMvc(routes =>
{
    routes.MapAreaRoute("duck_route", "Duck",
        "Manage/{controller}/{action}/{id?}");
    routes.MapRoute("default", "Manage/{controller=Home}/{action=Index}/{id?}");
});
using Microsoft.AspNetCore.Mvc;

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

        public IActionResult GenerateURLOutsideOfArea()
        {
            // Uses the empty value for area
            var url = Url.Action("Index", "Home", new { area = "" }); 
            // returns /Manage/Home/Index
            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 is not 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 is 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 is 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 are 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 does not 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.