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ASP.NET Core Middleware

rick-anderson

Learn about ASP.NET Core middleware and the request pipeline.

>= aspnetcore-2.1

riande

mvc

07/15/2020

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fundamentals/middleware/index

ASP.NET Core Middleware

::: moniker range=">= aspnetcore-3.0"

By Rick Anderson and Steve Smith

Middleware is software that's assembled into an app 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.

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

Request delegates are configured using xref:Microsoft.AspNetCore.Builder.RunExtensions.Run%2A, xref:Microsoft.AspNetCore.Builder.MapExtensions.Map%2A, and xref:Microsoft.AspNetCore.Builder.UseExtensions.Use%2A 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, also called middleware components. Each middleware component in the request pipeline is responsible for invoking the next component in the pipeline or short-circuiting the pipeline. When a middleware short-circuits, it's called a terminal middleware because it prevents further middleware from processing the request.

xref:migration/http-modules explains the difference between request pipelines in ASP.NET Core and ASP.NET 4.x and provides additional middleware samples.

Create a middleware pipeline with IApplicationBuilder

The ASP.NET Core request pipeline consists of a sequence of request delegates, called one after the other. The following diagram demonstrates the concept. The thread of execution follows the black arrows.

Each delegate can perform operations before and after the next delegate. Exception-handling delegates should 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-csharp]

Chain multiple request delegates together with xref:Microsoft.AspNetCore.Builder.UseExtensions.Use%2A. The next parameter represents the next delegate in the pipeline. 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 the following example demonstrates:

[!code-csharp]

When a delegate doesn't pass a request to the next delegate, it's called short-circuiting the request pipeline. Short-circuiting is often desirable because it avoids unnecessary work. For example, Static File Middleware can act as a terminal middleware by processing a request for a static file and short-circuiting the rest of the pipeline. Middleware added to the pipeline before the middleware that terminates further processing still processes code after their next.Invoke statements. However, see the following warning about attempting to write to a response that has already been sent.

[!WARNING] Don't call next.Invoke after the response has been sent to the client. Changes to xref:Microsoft.AspNetCore.Http.HttpResponse after the response has started throw an exception. For example, setting headers and a status code 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.

xref:Microsoft.AspNetCore.Http.HttpResponse.HasStarted%2A is a useful hint to indicate if headers have been sent or the body has been written to.

xref:Microsoft.AspNetCore.Builder.RunExtensions.Run%2A delegates don't receive a next parameter. The first Run delegate is always terminal and terminates the pipeline. Run is a convention. Some middleware components may expose Run[Middleware] methods that run at the end of the pipeline:

[!code-csharp] [!INCLUDEabout the series]

In the preceding example, the Run delegate writes "Hello from 2nd delegate." to the response and then terminates the pipeline. If another Use or Run delegate is added after the Run delegate, it's not called.

Middleware order

The following diagram shows the complete request processing pipeline for ASP.NET Core MVC and Razor Pages apps. You can see how, in a typical app, existing middlewares are ordered and where custom middlewares are added. You have full control over how to reorder existing middlewares or inject new custom middlewares as necessary for your scenarios.

The Endpoint middleware in the preceding diagram executes the filter pipeline for the corresponding app type—MVC or Razor Pages.

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

The following Startup.Configure method adds security-related middleware components in the typical recommended order:

[!code-csharp]

In the preceding code:

Middleware that is not added when creating a new web app with individual users accounts is commented out.
Not every middleware needs to go in this exact order, but many do. For example:
- UseCors, UseAuthentication, and UseAuthorization must go in the order shown.
- UseCors currently must go before UseResponseCaching due to this bug.

In some scenarios, middleware will have different ordering. For example, caching and compression ordering is scenario specific, and there's multiple valid orderings. For example:

app.UseResponseCaching();
app.UseResponseCompression();

With the preceding code, CPU could be saved by caching the compressed response, but you might end up caching multiple representations of a resource using different compression algorithms such as gzip or brotli.

The following ordering combines static files to allow caching compressed static files:

app.UseResponseCaching();
app.UseResponseCompression();
app.UseStaticFiles();

The following Startup.Configure method adds middleware components for common app scenarios:

Exception/error handling
- When the app runs in the Development environment:
  - Developer Exception Page Middleware (xref:Microsoft.AspNetCore.Builder.DeveloperExceptionPageExtensions.UseDeveloperExceptionPage%2A) reports app runtime errors.
  - Database Error Page Middleware reports database runtime errors.
- When the app runs in the Production environment:
  - Exception Handler Middleware (xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A) catches exceptions thrown in the following middlewares.
  - HTTP Strict Transport Security Protocol (HSTS) Middleware (xref:Microsoft.AspNetCore.Builder.HstsBuilderExtensions.UseHsts%2A) adds the Strict-Transport-Security header.
HTTPS Redirection Middleware (xref:Microsoft.AspNetCore.Builder.HttpsPolicyBuilderExtensions.UseHttpsRedirection%2A) redirects HTTP requests to HTTPS.
Static File Middleware (xref:Microsoft.AspNetCore.Builder.StaticFileExtensions.UseStaticFiles%2A) returns static files and short-circuits further request processing.
Cookie Policy Middleware (xref:Microsoft.AspNetCore.Builder.CookiePolicyAppBuilderExtensions.UseCookiePolicy%2A) conforms the app to the EU General Data Protection Regulation (GDPR) regulations.
Routing Middleware (xref:Microsoft.AspNetCore.Builder.EndpointRoutingApplicationBuilderExtensions.UseRouting%2A) to route requests.
Authentication Middleware (xref:Microsoft.AspNetCore.Builder.AuthAppBuilderExtensions.UseAuthentication%2A) attempts to authenticate the user before they're allowed access to secure resources.
Authorization Middleware (xref:Microsoft.AspNetCore.Builder.AuthorizationAppBuilderExtensions.UseAuthorization%2A) authorizes a user to access secure resources.
Session Middleware (xref:Microsoft.AspNetCore.Builder.SessionMiddlewareExtensions.UseSession%2A) establishes and maintains session state. If the app uses session state, call Session Middleware after Cookie Policy Middleware and before MVC Middleware.
Endpoint Routing Middleware (xref:Microsoft.AspNetCore.Builder.EndpointRoutingApplicationBuilderExtensions.UseEndpoints%2A with xref:Microsoft.AspNetCore.Builder.RazorPagesEndpointRouteBuilderExtensions.MapRazorPages%2A) to add Razor Pages endpoints to the request pipeline.

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseDatabaseErrorPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();
    app.UseCookiePolicy();
    app.UseRouting();
    app.UseAuthentication();
    app.UseAuthorization();
    app.UseSession();

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

In the preceding example code, each middleware extension method is exposed on xref:Microsoft.AspNetCore.Builder.IApplicationBuilder through the xref:Microsoft.AspNetCore.Builder?displayProperty=fullName namespace.

xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A is the first middleware component added to the pipeline. Therefore, the Exception Handler Middleware catches any exceptions that occur in later calls.

Static File Middleware is called early in the pipeline so that 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 Static File Middleware, including those under wwwroot, are publicly available. For an approach to secure static files, see xref:fundamentals/static-files.

If the request isn't handled by the Static File Middleware, it's passed on to the Authentication Middleware (xref:Microsoft.AspNetCore.Builder.AuthAppBuilderExtensions.UseAuthentication%2A), which performs authentication. Authentication doesn't short-circuit unauthenticated requests. Although Authentication Middleware authenticates requests, authorization (and rejection) occurs only after MVC selects a specific Razor Page or MVC controller and action.

The following example demonstrates a middleware order where requests for static files are handled by Static File Middleware before Response Compression Middleware. Static files aren't compressed with this middleware order. The Razor Pages responses can be compressed.

public void Configure(IApplicationBuilder app)
{
    // Static files aren't compressed by Static File Middleware.
    app.UseStaticFiles();

    app.UseRouting();

    app.UseResponseCompression();

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

For Single Page Applications (SPAs), the SPA middleware xref:Microsoft.Extensions.DependencyInjection.SpaStaticFilesExtensions.UseSpaStaticFiles%2A usually comes last in the middleware pipeline. The SPA middleware comes last:

To allow all other middlewares to respond to matching requests first.
To allow SPAs with client-side routing to run for all routes that are unrecognized by the server app.

For more details on SPAs, see the guides for the React and Angular project templates.

Forwarded Headers Middleware order

[!INCLUDE]

Branch the middleware pipeline

xref:Microsoft.AspNetCore.Builder.MapExtensions.Map%2A 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-csharp]

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 segments are removed from HttpRequest.Path and appended to HttpRequest.PathBase for each request.

Map supports nesting, for example:

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

Map can also match multiple segments at once:

[!code-csharp]

xref:Microsoft.AspNetCore.Builder.MapWhenExtensions.MapWhen%2A 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-csharp]

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=main	Branch used = main

xref:Microsoft.AspNetCore.Builder.UseWhenExtensions.UseWhen%2A also branches the request pipeline based on the result of the given predicate. Unlike with MapWhen, this branch is rejoined to the main pipeline if it doesn't short-circuit or contain a terminal middleware:

[!code-csharp]

In the preceding example, a response of "Hello from main pipeline." is written for all requests. If the request includes a query string variable branch, its value is logged before the main pipeline is rejoined.

Built-in middleware

ASP.NET Core ships with the following middleware components. The Order column provides notes on middleware placement in the request processing pipeline and under what conditions the middleware may terminate request processing. When a middleware short-circuits the request processing pipeline and prevents further downstream middleware from processing a request, it's called a terminal middleware. For more information on short-circuiting, see the Create a middleware pipeline with IApplicationBuilder section.

Middleware	Description	Order
Authentication	Provides authentication support.	Before `HttpContext.User` is needed. Terminal for OAuth callbacks.
Authorization	Provides authorization support.	Immediately after the Authentication Middleware.
Cookie Policy	Tracks consent from users for storing personal information and enforces minimum standards for cookie fields, such as `secure` and `SameSite`.	Before middleware that issues cookies. Examples: Authentication, Session, MVC (TempData).
CORS	Configures Cross-Origin Resource Sharing.	Before components that use CORS. `UseCors` currently must go before `UseResponseCaching` due to this bug.
Diagnostics	Several separate middlewares that provide a developer exception page, exception handling, status code pages, and the default web page for new apps.	Before components that generate errors. Terminal for exceptions or serving the default web page for new apps.
Forwarded Headers	Forwards proxied headers onto the current request.	Before components that consume the updated fields. Examples: scheme, host, client IP, method.
Health Check	Checks the health of an ASP.NET Core app and its dependencies, such as checking database availability.	Terminal if a request matches a health check endpoint.
Header Propagation	Propagates HTTP headers from the incoming request to the outgoing HTTP Client requests.
HTTP Method Override	Allows an incoming POST request to override the method.	Before components that consume the updated method.
HTTPS Redirection	Redirect all HTTP requests to HTTPS.	Before components that consume the URL.
HTTP Strict Transport Security (HSTS)	Security enhancement middleware that adds a special response header.	Before responses are sent and after components that modify requests. Examples: Forwarded Headers, URL Rewriting.
MVC	Processes requests with MVC/Razor Pages.	Terminal if a request matches a route.
OWIN	Interop with OWIN-based apps, servers, and middleware.	Terminal if the OWIN Middleware fully processes the request.
Response Caching	Provides support for caching responses.	Before components that require caching. `UseCORS` must come before `UseResponseCaching`.
Response Compression	Provides support for compressing responses.	Before components that require compression.
Request Localization	Provides localization support.	Before localization sensitive components.
Endpoint Routing	Defines and constrains request routes.	Terminal for matching routes.
SPA	Handles all requests from this point in the middleware chain by returning the default page for the Single Page Application (SPA)	Late in the chain, so that other middleware for serving static files, MVC actions, etc., takes precedence.
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 a file.
URL Rewrite	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.

Additional resources

Lifetime and registration options contains a complete sample of middleware with scoped, transient, and singleton lifetime services.
xref:fundamentals/middleware/write
xref:test/middleware
xref:migration/http-modules
xref:fundamentals/startup
xref:fundamentals/request-features
xref:fundamentals/middleware/extensibility
xref:fundamentals/middleware/extensibility-third-party-container

::: moniker-end

::: moniker range="< aspnetcore-3.0"

By Rick Anderson and Steve Smith

Middleware is software that's assembled into an app 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.

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

xref:migration/http-modules explains the difference between request pipelines in ASP.NET Core and ASP.NET 4.x and provides additional middleware samples.

Create a middleware pipeline with IApplicationBuilder

[!code-csharp]

The first xref:Microsoft.AspNetCore.Builder.RunExtensions.Run%2A delegate terminates the pipeline.

[!code-csharp]

[!WARNING] Don't call next.Invoke after the response has been sent to the client. Changes to xref:Microsoft.AspNetCore.Http.HttpResponse after the response has started throw an exception. For example, changes such as setting headers and a status code 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.

xref:Microsoft.AspNetCore.Http.HttpResponse.HasStarted%2A is a useful hint to indicate if headers have been sent or the body has been written to.

Middleware order

The following Startup.Configure method adds security related middleware components in the recommended order:

[!code-csharp]

In the preceding code:

Middleware that is not added when creating a new web app with individual users accounts is commented out.
Not every middleware needs to go in this exact order, but many do. For example, UseCors and UseAuthentication must go in the order shown.

The following Startup.Configure method adds middleware components for common app scenarios:

Exception/error handling
- When the app runs in the Development environment:
  - Developer Exception Page Middleware (xref:Microsoft.AspNetCore.Builder.DeveloperExceptionPageExtensions.UseDeveloperExceptionPage%2A) reports app runtime errors.
  - Database Error Page Middleware (Microsoft.AspNetCore.Builder.DatabaseErrorPageExtensions.UseDatabaseErrorPage) reports database runtime errors.
- When the app runs in the Production environment:
  - Exception Handler Middleware (xref:Microsoft.AspNetCore.Builder.ExceptionHandlerExtensions.UseExceptionHandler%2A) catches exceptions thrown in the following middlewares.
  - HTTP Strict Transport Security Protocol (HSTS) Middleware (xref:Microsoft.AspNetCore.Builder.HstsBuilderExtensions.UseHsts%2A) adds the Strict-Transport-Security header.
HTTPS Redirection Middleware (xref:Microsoft.AspNetCore.Builder.HttpsPolicyBuilderExtensions.UseHttpsRedirection%2A) redirects HTTP requests to HTTPS.
Static File Middleware (xref:Microsoft.AspNetCore.Builder.StaticFileExtensions.UseStaticFiles%2A) returns static files and short-circuits further request processing.
Cookie Policy Middleware (xref:Microsoft.AspNetCore.Builder.CookiePolicyAppBuilderExtensions.UseCookiePolicy%2A) conforms the app to the EU General Data Protection Regulation (GDPR) regulations.
Authentication Middleware (xref:Microsoft.AspNetCore.Builder.AuthAppBuilderExtensions.UseAuthentication%2A) attempts to authenticate the user before they're allowed access to secure resources.
Session Middleware (xref:Microsoft.AspNetCore.Builder.SessionMiddlewareExtensions.UseSession%2A) establishes and maintains session state. If the app uses session state, call Session Middleware after Cookie Policy Middleware and before MVC Middleware.
MVC (xref:Microsoft.AspNetCore.Builder.MvcApplicationBuilderExtensions.UseMvc%2A) to add MVC to the request pipeline.

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseDatabaseErrorPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();
    app.UseCookiePolicy();
    app.UseAuthentication();
    app.UseSession();
    app.UseMvc();
}

public void Configure(IApplicationBuilder app)
{
    // Static files aren't compressed by Static File Middleware.
    app.UseStaticFiles();

    app.UseResponseCompression();

    app.UseMvcWithDefaultRoute();
}

Use, Run, and Map

Configure the HTTP pipeline using xref:Microsoft.AspNetCore.Builder.UseExtensions.Use%2A, xref:Microsoft.AspNetCore.Builder.RunExtensions.Run%2A, and xref:Microsoft.AspNetCore.Builder.MapExtensions.Map%2A. 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.

[!code-csharp]

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 segments are removed from HttpRequest.Path and appended to HttpRequest.PathBase for each request.

[!code-csharp]

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=main	Branch used = main

Map supports nesting, for example:

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

Map can also match multiple segments at once:

[!code-csharp]

Built-in middleware

Middleware	Description	Order
Authentication	Provides authentication support.	Before `HttpContext.User` is needed. Terminal for OAuth callbacks.
Cookie Policy	Tracks consent from users for storing personal information and enforces minimum standards for cookie fields, such as `secure` and `SameSite`.	Before middleware that issues cookies. Examples: Authentication, Session, MVC (TempData).
CORS	Configures Cross-Origin Resource Sharing.	Before components that use CORS.
Diagnostics	Several separate middlewares that provide a developer exception page, exception handling, status code pages, and the default web page for new apps.	Before components that generate errors. Terminal for exceptions or serving the default web page for new apps.
Forwarded Headers	Forwards proxied headers onto the current request.	Before components that consume the updated fields. Examples: scheme, host, client IP, method.
Health Check	Checks the health of an ASP.NET Core app and its dependencies, such as checking database availability.	Terminal if a request matches a health check endpoint.
HTTP Method Override	Allows an incoming POST request to override the method.	Before components that consume the updated method.
HTTPS Redirection	Redirect all HTTP requests to HTTPS.	Before components that consume the URL.
HTTP Strict Transport Security (HSTS)	Security enhancement middleware that adds a special response header.	Before responses are sent and after components that modify requests. Examples: Forwarded Headers, URL Rewriting.
MVC	Processes requests with MVC/Razor Pages.	Terminal if a request matches a route.
OWIN	Interop with OWIN-based apps, servers, and middleware.	Terminal if the OWIN Middleware fully processes the request.
Response Caching	Provides support for caching responses.	Before components that require caching.
Response Compression	Provides support for compressing responses.	Before components that require compression.
Request Localization	Provides localization support.	Before localization sensitive components.
Endpoint 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 a file.
URL Rewrite	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.

Additional resources

::: moniker-end

38 KiB Raw Blame History

ASP.NET Core Middleware

Create a middleware pipeline with IApplicationBuilder

Middleware order

Forwarded Headers Middleware order

Branch the middleware pipeline

Built-in middleware

Additional resources

Create a middleware pipeline with IApplicationBuilder

Middleware order

Use, Run, and Map

Built-in middleware

Additional resources

38 KiB

Raw Blame History