Welcome to this lesson on handling requests with middleware in ASP.NET Core. Middleware is a foundational concept in ASP.NET Core applications, playing a crucial role in processing requests and responses. In this lesson, we'll explore middleware in depth, including how to configure and utilize it effectively.

In ASP.NET Core, middleware refers to C# classes designed to handle HTTP requests and responses within an application pipeline. Middleware components can:

• Generate an HTTP response for an incoming HTTP request. This kind of middleware is called terminal middleware.
• Process and potentially modify an incoming HTTP request before passing it to the next middleware in the pipeline.
• Process and potentially modify an outgoing HTTP response before either passing it to the next middleware or sending it back to the client.

The middleware pipeline is bidirectional: requests pass through each middleware on the way in, and responses pass back through in the reverse order on the way out.

In ASP.NET Core, middleware is configured in the Program.cs file. Middleware components execute in the order they are added to the pipeline, making the order crucial for the correct flow of requests and responses.

As we mentioned previously, middleware components can be chained together, and the request passes from one middleware to another. Let's illustrate that:

In the diagram above, you see the following request flow:

1. The ASP.NET Core web server passes the request to the middleware pipeline.
2. The logging middleware logs the incoming request.
3. The authentication middleware associates a user with the current request.
4. The authorization middleware checks if the request is allowed to be executed for the user.
5. If the user is not allowed, the authorization middleware will short-circuit the pipeline. Otherwise, the request will reach the endpoint middleware.
6. The response passes back through each middleware that ran previously in the pipeline.
7. The response is returned to the ASP.NET Core web server.

Note that ASP.NET Core includes default middleware at the end of the pipeline that automatically sends a 404 response if no other middleware handles the request.

The HttpContext object in ASP.NET Core encapsulates all the information related to a single HTTP request and response. Created by the ASP.NET Core web server for each request, it includes details such as:

• Request properties: headers, query string, body.
• Response properties: status code, headers.
• User information.

As the request progresses through the middleware pipeline, the HttpContext object allows each middleware component to access and manipulate the request and response data as needed. After the request is fully processed, the HttpContext is updated with the response information. The web server then converts this updated HttpContext into a raw HTTP response and sends it back to the client.

Now that you understand what middleware is, it's time to see a few examples. Let's start by taking a look at middleware that addresses a common problem of web applications — serving static files. ASP.NET Core provides built-in middleware for serving static files from the wwwroot directory. To enable this middleware, use the app.UseStaticFiles() method in your middleware pipeline.

Here’s an example:

With this middleware in place, any static files placed in the wwwroot directory will be accessible via the web server. For example, a file at wwwroot/index.html would be served at /index.html. Additionally, GET requests to the root URL / will still return the message "Welcome to CodeSignal Learn!".

Instead of returning "Welcome to CodeSignal Learn!" on the root URL, you may want to add a simple welcome page as a starting point for your application. This can be done using a pre-built Welcome Page Middleware. Here's how you can add a "Welcome" page:

In this example, when you navigate to the root URL /, you'll see a simple welcome page.

In this lesson, we delved into middleware in ASP.NET Core, emphasizing its critical role in processing HTTP requests and responses. We examined the request flow in the middleware pipeline and the significance of the HttpContext object for accessing and modifying request and response data. Additionally, we provided practical examples, demonstrating how to serve static files using app.UseStaticFiles() and incorporating a welcome page with app.UseWelcomePage(). Up next, you will engage in hands-on practice by adding predefined middleware to your project to further solidify these concepts.