All 17 ASP.NET Core Middleware & Request Pipeline questions
More in ASP.NET Core Middleware & Request Pipeline
What is middleware in ASP.NET Core, and how does it form the request pipeline? How do you register built-in middleware for serving static files in an ASP.NET Core application? Why does the order in which middleware is registered matter so much in ASP.NET Core? What's the difference between app.Use, app.Run, and app.Map in configuring the middleware pipeline? How does short-circuiting work in middleware, and when would you deliberately not call next()? What is the purpose of UseExceptionHandler and UseDeveloperExceptionPage middleware, and how should they differ between environments? How do you conditionally apply middleware only to certain requests using UseWhen or MapWhen? How does the HttpContext object serve as the shared state carrier through the middleware pipeline? What is middleware in ASP.NET Core, and how does the order in which it's registered affect request handling? What are Minimal APIs in ASP.NET Core, and when would you choose them over traditional MVC controllers?
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How do you write a custom middleware component as a reusable class instead of an inline lambda?

Intermediate

Answer

A class-based middleware implements a constructor accepting RequestDelegate next (and typically other injected services), and an InvokeAsync (or Invoke) method containing the actual per-request logic, then calling await next(context) to pass control to the next middleware. It's registered via a custom `app.UseMiddleware<T>()` call or, more idiomatically, an extension method wrapping that call.
public class RequestTimingMiddleware {
    private readonly RequestDelegate _next;
    public RequestTimingMiddleware(RequestDelegate next) => _next = next;

    public async Task InvokeAsync(HttpContext context) {
        var sw = Stopwatch.StartNew();
        await _next(context);
        Console.WriteLine($"{context.Request.Path} took {sw.ElapsedMilliseconds}ms");
    }
}

// Registration
app.UseMiddleware<RequestTimingMiddleware>();
Real-world example A team extracts an inline logging lambda into a proper RequestTimingMiddleware class once it grows complex enough to need dependency injection (like an ILogger) and unit testing, which inline lambdas can't easily support.

Common follow-ups: Why is a class-based middleware constructed once per app lifetime rather than per request?;How do you inject scoped services into middleware given this singleton-like construction?

Dependency Injection;Diagnostics & Performance
All 17 ASP.NET Core Middleware & Request Pipeline questions