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title author description manager ms.author ms.date ms.prod ms.technology ms.topic uid
ASP.NET Core Middleware rick-anderson Learn about ASP.NET Core middleware and the request pipeline. wpickett riande 01/22/2018 asp.net-core aspnet article fundamentals/middleware/index

ASP.NET Core Middleware

By Rick Anderson and Steve Smith

View or download sample code (how to download)

What is middleware?

Middleware is software that's assembled into an application pipeline to handle requests and responses. Each component:

  • Chooses whether to pass the request to the next component in the pipeline.
  • Can perform work before and after the next component in the pipeline is invoked.

Request delegates are used to build the request pipeline. The request delegates handle each HTTP request.

Request delegates are configured using Run, Map, and Use extension methods. An individual request delegate can be specified in-line as an anonymous method (called in-line middleware), or it can be defined in a reusable class. These reusable classes and in-line anonymous methods are middleware, or middleware components. Each middleware component in the request pipeline is responsible for invoking the next component in the pipeline, or short-circuiting the chain if appropriate.

Migrating HTTP Modules to Middleware explains the difference between request pipelines in ASP.NET Core and ASP.NET 4.x and provides more middleware samples.

Creating a middleware pipeline with IApplicationBuilder

The ASP.NET Core request pipeline consists of a sequence of request delegates, called one after the other, as this diagram shows (the thread of execution follows the black arrows):

Request processing pattern showing a request arriving, processing through three middlewares, and the response leaving the application. Each middleware runs its logic and hands off the request to the next middleware at the next() statement. After the third middleware processes the request, the request passes back through the prior two middlewares in reverse order for additional processing after their next() statements before leaving the application as a response to the client.

Each delegate can perform operations before and after the next delegate. A delegate can also decide to not pass a request to the next delegate, which is called short-circuiting the request pipeline. Short-circuiting is often desirable because it avoids unnecessary work. For example, the static file middleware can return a request for a static file and short-circuit the rest of the pipeline. Exception-handling delegates need to be called early in the pipeline, so they can catch exceptions that occur in later stages of the pipeline.

The simplest possible ASP.NET Core app sets up a single request delegate that handles all requests. This case doesn't include an actual request pipeline. Instead, a single anonymous function is called in response to every HTTP request.

[!code-csharpMain]

The first app.Run delegate terminates the pipeline.

You can chain multiple request delegates together with app.Use. The next parameter represents the next delegate in the pipeline. (Remember that you can short-circuit the pipeline by not calling the next parameter.) You can typically perform actions both before and after the next delegate, as this example demonstrates:

[!code-csharpMain]

[!WARNING] Don't call next.Invoke after the response has been sent to the client. Changes to HttpResponse after the response has started will throw an exception. For example, changes such as setting headers, status code, etc, will throw an exception. Writing to the response body after calling next:

  • May cause a protocol violation. For example, writing more than the stated content-length.
  • May corrupt the body format. For example, writing an HTML footer to a CSS file.

HttpResponse.HasStarted is a useful hint to indicate if headers have been sent and/or the body has been written to.

Ordering

The order that middleware components are added in the Configure method defines the order in which they're invoked on requests, and the reverse order for the response. This ordering is critical for security, performance, and functionality.

The Configure method (shown below) adds the following middleware components:

  1. Exception/error handling
  2. Static file server
  3. Authentication
  4. MVC

ASP.NET Core 2.x

public void Configure(IApplicationBuilder app)
{
    app.UseExceptionHandler("/Home/Error"); // Call first to catch exceptions
                                            // thrown in the following middleware.

    app.UseStaticFiles();                   // Return static files and end pipeline.

    app.UseAuthentication();               // Authenticate before you access
                                           // secure resources.

    app.UseMvcWithDefaultRoute();          // Add MVC to the request pipeline.
}

ASP.NET Core 1.x

public void Configure(IApplicationBuilder app)
{
    app.UseExceptionHandler("/Home/Error"); // Call first to catch exceptions
                                            // thrown in the following middleware.

    app.UseStaticFiles();                   // Return static files and end pipeline.

    app.UseIdentity();                     // Authenticate before you access
                                           // secure resources.

    app.UseMvcWithDefaultRoute();          // Add MVC to the request pipeline.
}

In the code above, UseExceptionHandler is the first middleware component added to the pipeline—therefore, it catches any exceptions that occur in later calls.

The static file middleware is called early in the pipeline so it can handle requests and short-circuit without going through the remaining components. The static file middleware provides no authorization checks. Any files served by it, including those under wwwroot, are publicly available. See Working with static files for an approach to secure static files.

ASP.NET Core 2.x

If the request isn't handled by the static file middleware, it's passed on to the Identity middleware (app.UseAuthentication), which performs authentication. Identity doesn't short-circuit unauthenticated requests. Although Identity authenticates requests, authorization (and rejection) occurs only after MVC selects a specific Razor Page or controller and action.

ASP.NET Core 1.x

If the request isn't handled by the static file middleware, it's passed on to the Identity middleware (app.UseIdentity), which performs authentication. Identity doesn't short-circuit unauthenticated requests. Although Identity authenticates requests, authorization (and rejection) occurs only after MVC selects a specific controller and action.

The following example demonstrates a middleware ordering where requests for static files are handled by the static file middleware before the response compression middleware. Static files are not compressed with this ordering of the middleware. The MVC responses from UseMvcWithDefaultRoute can be compressed.

public void Configure(IApplicationBuilder app)
{
    app.UseStaticFiles();         // Static files not compressed
                                  // by middleware.
    app.UseResponseCompression();
    app.UseMvcWithDefaultRoute();
}

Use, Run, and Map

You configure the HTTP pipeline using Use, Run, and Map. The Use method can short-circuit the pipeline (that is, if it doesn't call a next request delegate). Run is a convention, and some middleware components may expose Run[Middleware] methods that run at the end of the pipeline.

Map* extensions are used as a convention for branching the pipeline. Map branches the request pipeline based on matches of the given request path. If the request path starts with the given path, the branch is executed.

[!code-csharpMain]

The following table shows the requests and responses from http://localhost:1234 using the previous code:

Request Response
localhost:1234 Hello from non-Map delegate.
localhost:1234/map1 Map Test 1
localhost:1234/map2 Map Test 2
localhost:1234/map3 Hello from non-Map delegate.

When Map is used, the matched path segment(s) are removed from HttpRequest.Path and appended to HttpRequest.PathBase for each request.

MapWhen branches the request pipeline based on the result of the given predicate. Any predicate of type Func<HttpContext, bool> can be used to map requests to a new branch of the pipeline. In the following example, a predicate is used to detect the presence of a query string variable branch:

[!code-csharpMain]

The following table shows the requests and responses from http://localhost:1234 using the previous code:

Request Response
localhost:1234 Hello from non-Map delegate.
localhost:1234/?branch=master Branch used = master

Map supports nesting, for example:

app.Map("/level1", level1App => {
       level1App.Map("/level2a", level2AApp => {
           // "/level1/level2a"
           //...
       });
       level1App.Map("/level2b", level2BApp => {
           // "/level1/level2b"
           //...
       });
   });

Map can also match multiple segments at once, for example:

app.Map("/level1/level2", HandleMultiSeg);

Built-in middleware

ASP.NET Core ships with the following middleware components, as well as a description of the order in which they should be added:

Middleware Description Order
Authentication Provides authentication support. Before HttpContext.User is needed. Terminal for OAuth callbacks.
CORS Configures Cross-Origin Resource Sharing. Before components that use CORS.
Diagnostics Configures diagnostics. Before components that generate errors.
ForwardedHeaders/HttpOverrides Forwards proxied headers onto the current request. Before components that consume the updated fields (examples: Scheme, Host, ClientIP, Method).
Response Caching Provides support for caching responses. Before components that require caching.
Response Compression Provides support for compressing responses. Before components that require compression.
RequestLocalization Provides localization support. Before localization sensitive components.
Routing Defines and constrains request routes. Terminal for matching routes.
Session Provides support for managing user sessions. Before components that require Session.
Static Files Provides support for serving static files and directory browsing. Terminal if a request matches files.
URL Rewriting Provides support for rewriting URLs and redirecting requests. Before components that consume the URL.
WebSockets Enables the WebSockets protocol. Before components that are required to accept WebSocket requests.

Writing middleware

Middleware is generally encapsulated in a class and exposed with an extension method. Consider the following middleware, which sets the culture for the current request from the query string:

[!code-csharpMain]

Note: The sample code above is used to demonstrate creating a middleware component. See Globalization and localization for ASP.NET Core's built-in localization support.

You can test the middleware by passing in the culture, for example http://localhost:7997/?culture=no.

The following code moves the middleware delegate to a class:

[!code-csharpMain]

The following extension method exposes the middleware through IApplicationBuilder:

[!code-csharpMain]

The following code calls the middleware from Configure:

[!code-csharpMain]

Middleware should follow the Explicit Dependencies Principle by exposing its dependencies in its constructor. Middleware is constructed once per application lifetime. See Per-request dependencies below if you need to share services with middleware within a request.

Middleware components can resolve their dependencies from dependency injection through constructor parameters. UseMiddleware<T> can also accept additional parameters directly.

Per-request dependencies

Because middleware is constructed at app startup, not per-request, scoped lifetime services used by middleware constructors are not shared with other dependency-injected types during each request. If you must share a scoped service between your middleware and other types, add these services to the Invoke method's signature. The Invoke method can accept additional parameters that are populated by dependency injection. For example:

public class MyMiddleware
{
    private readonly RequestDelegate _next;

    public MyMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext httpContext, IMyScopedService svc)
    {
        svc.MyProperty = 1000;
        await _next(httpContext);
    }
}

Additional resources