ASP.NET Core Middleware

What is Middleware?

Put simply, you use middleware components to compose the functionality of your ASP.NET Core application.  Anything your application does, even something as basic as serving up static files, is performed by middleware.  An application without any middleware will simply return a 404 - Not Found response for every request.  This gives you complete control over how requests are handled and lets you keep your application as lean as possible.

In this post, we’ll look at the following topics, which cover the basics of middleware:

Next we’ll dive into the ASP.NET Core source code to see what is happening under the hood.

The Middleware Pipeline

As requests are received, they are processed by each middleware component in a construct commonly called the middleware pipeline.  The term “pipeline” implies that requests enter one end, are processed by each middleware component in turn, and exit the other end.  In reality, the run-time behavior is a variation of the Chain of Responsibility design pattern, with each middleware component having the option of acting on the HttpContext before and after calling the next component in the list:

AspNetCore Middleware1

Note: there is much more going on between the Client and first Middleware, which I will cover in a future post about hosting.  For this discussion, the additional detail has been omitted to focus on the immediate topic of middleware.

Middleware components may also choose not to call the following component, instead short-circuiting the rest of the chain and returning immediately.

Using Standard Middleware

ASP.NET Core comes with many middleware components out of the box, and you add them to your application in the Startup.Configure method as shown below:

In this case we are using the StaticFileMiddleware and WelcomePageMiddleware.  Middleware components are executed in the order that they are added to the application.  In this example, requests are processed by the StaticFileMiddleware first.  If a file exists that matches the request, it will be returned and no other middleware components are called.  If no matching file is found, the request passes to the WelcomePageMiddleware, which returns a welcome page for all requests it receives.

The standard middleware components typically have extension methods you can use to add them to the pipeline, such as UseStaticFiles() and UseWelcomePage(), which we saw above.

There are too many standard middleware components for me to document in this post, so for now I will simply list them and the project where they can be found on github.

Component Project
AnalysisMiddleware Diagnostics
AntiforgeryMiddleware Antiforgery
AuthenticationMiddleware Security
ClaimsTransformationMiddleware Security
CookieAuthenticationMiddleware Security
CookiePolicyMiddleware Security
CorsMiddleware CORS
DatabaseErrorPageMiddleware Diagnostics
DefaultFilesMiddleware StaticFiles
DeveloperExceptionPageMiddleware Diagnostics
DirectoryBrowserMiddleware StaticFiles
ElmCaptureMiddleware Diagnostics
ElmPageMiddleware Diagnostics
ExceptionHandlerMiddleware Diagnostics
FacebookMiddleware Security
ForwardedHeadersMiddleware BasicMiddleware
GoogleMiddleware Security
HttpMethodOverrideMiddleware BasicMiddleware
IISPlatformHandlerMiddleware IISIntegration
JwtBearerMiddleware Security
MapMiddleware HttpAbstractions
MapWhenMiddleware HttpAbstractions
MicrosoftAccountMiddleware Security
MigrationsEndPointMiddleware Diagnostics
OAuthMiddleware Security
OpenIdConnectMiddleware Security
ProxyMiddleware Proxy
RequestLocalizationMiddleware Localization
RequestServicesContainerMiddleware Hosting
ResponseBufferingMiddleware BasicMiddleware
RouterMiddleware Routing
RuntimeInfoMiddleware Diagnostics
SessionMiddleware Session
StaticFileMiddleware StaticFiles
StatusCodePagesMiddleware Diagnostics
TwitterMiddleware Security
WebSocketMiddleware WebSockets
WelcomePageMiddleware Diagnostics

Inline Middleware with Use and Run

In addition to using predefined middleware components, you can define middleware inline with the Use and Run extension methods.

Use adds middleware, which may call the next middleware component in the pipeline.  Run adds terminating middleware, that is, middleware that will never call subsequent middleware.

In the following example, the middleware defined inline with Use writes out a message, then calls the next middleware, and then writes out another message after the next middleware returns.  The middleware defined with Run simply writes out a message and returns.  If any middleware is added after Run, it will never be called.

Running this application and browsing to it, we see the expected messages:

middleware-use-run

Branching with Map and MapWhen

There are two middleware components we can use to branch the pipeline:  MapMiddleware and MapWhenMiddleware.

MapMiddleware lets you branch based on the route and can be added to the pipeline with the Map extension method.  In the following example, “Hello… ” will always be written, then, if the first path segment is either “/foo” or “/bar”, “Foo” or “Bar” will be written respectively.  If neither matches, “World” will be written.  Finally, a closing exclamation point is written before the response is returned.

MapWhenMiddleware lets you branch based on a predicate and can be added to the pipeline with the MapWhen extension method.  In this example, we use MapWhen to implement the commonly used FizzBuzz interview question.

Custom Middleware

You only need to meet two requirements to create your own middleware components:

  • Your middleware class must have a non-static public constructor with a RequestDelegate parameter.  This will be a reference to the next component in the pipeline.
  • Your middleware class must have a public method named Invoke that takes an HttpContext and returns a Task.

Let’s take a look at a simple custom middleware component.  The ArnoldQuotesMiddleware writes out a random movie quote from Arnold Schwarzenegger. This is a terminating component;  it does not call the next component in the pipeline because Arnold always gets the last word.  I have included the code you would use to hold the reference to the next middleware component, but the actual call is commented out.

Here is the constructor for the middleware class.  Note that we can include other parameters, which will be provided from the services collection (DI).

And here is the Invoke method, which writes out a quote from a list.  Invoke can take additional parameters in addition the the HttpContext, and they will be provided using services registered with the DI container.

We’ll also create an extension method to follow convention and make it easier to use our middleware:

Now we can use the extension method to add our ArnoldQuotesMiddleware:

You can download the code for this project here:

If you are curious about the two requirements we need to meet for our custom middleware, take a look at the implementation of the UseMiddleware<TMiddleware> method in the ASP.NET Core source code.

Configuring the Middleware Pipeline

What happens when we add middleware to the IApplicationBuilder instance in the Configure method?  IApplicationBuilder defines only one method to add middleware, so whether we use an extension method or define middleware inline, all roads lead to this method:

As mentioned earlier, RequestDelegate describes a function that processes an HttpContext asynchronously:

A middleware component can be composed of any number of classes, but at some point, a RequestDelegate is needed to add the middleware to the application.  You can think of the RequestDelegate as the entry point into the middleware component.

Going back to the IApplicationBuilder.Use method, you can think of its argument as a configuration function, which takes the entry point to the next component in the pipeline, does whatever work is needed to construct and configure the current middleware component, and returns a RequestDelegate, which will be used to access the middleware currently being configured.

Looking at the implementation of Use in ApplicationBuilder, we see it simply adds the provided configuration function to a private list:

Building the Middleware Pipeline

Now that we’ve seen how functions are added to configure middleware, let’s take a look at how the pipeline is built.  IApplicationBuilder declares a method named Build(), which is called by the host during startup:

Looking at the implementation of Build() in ApplicationBuilder we see it does the following:

  • Sets up a RequestDelegate to return a 404 response.  This will be the last RequestDelegate in the pipeline and explains why you get a 404 response if you don’t add any middleware to your application.
  • Iterates through the middleware configuration functions in reverse order.  For each configuration function, the currently referenced RequestDelegate is passed in as the next component in the pipeline.  The RequestDelegate returned by the configuration function is then assigned to that reference since it is now the first in the pipeline.
  • Finally, after all of the configuration functions have been processed, the first RequestDelegate is returned and this will be called to process any requests received by the application.

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