How do you implement middleware?

Question

Accepted Answer

**Middleware** — request/response pipeline.

**Basic middleware:**
```csharp
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);
        sw.Stop();
        context.Response.Headers.Add("X-Elapsed", sw.ElapsedMilliseconds.ToString());
    }
}
```

**Registration:**
```csharp
app.UseMiddleware<RequestTimingMiddleware>();
```

**Key:** Middleware for cross-cutting concerns.

C#: How do you implement middleware?