C#: What are middleware in ASP.NET Core?

1public class RequestTimingMiddleware
2{
3    private readonly RequestDelegate _next;
4    private readonly ILogger<RequestTimingMiddleware> _logger;
5
6    public RequestTimingMiddleware(
7        RequestDelegate next,
8        ILogger<RequestTimingMiddleware> logger)
9    {
10        _next = next;
11        _logger = logger;
12    }
13
14    public async Task InvokeAsync(HttpContext context)
15    {
16        var stopwatch = Stopwatch.StartNew();
17        await _next(context);  // Call next middleware
18        stopwatch.Stop();
19        _logger.LogInformation(
20            $"{context.Request.Path} took {stopwatch.ElapsedMilliseconds}ms");
21    }
22}

1// Extension method
2public static class RequestTimingExtensions
3{
4    public static IApplicationBuilder UseRequestTiming(
5        this IApplicationBuilder builder)
6    {
7        return builder.UseMiddleware<RequestTimingMiddleware>();
8    }
9}
10
11// Usage
12app.UseRequestTiming();