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How to write custom middleware in asp.net core?

ICSM Computer 241 30 Mar 2026

1 Answers

ICSM Computer 30 Mar 2026

Step-by-Step: Create Custom Middleware

1. Create Middleware Class

using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using System;

public class CustomMiddleware
{
    private readonly RequestDelegate _next;

    public CustomMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        // BEFORE request processing
        Console.WriteLine("Request Incoming: " + context.Request.Path);

        // Call next middleware
        await _next(context);

        // AFTER response processing
        Console.WriteLine("Response Outgoing: " + context.Response.StatusCode);
    }
}

2. Create Extension Method (Best Practice)

using Microsoft.AspNetCore.Builder;

public static class CustomMiddlewareExtensions
{
    public static IApplicationBuilder UseCustomMiddleware(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<CustomMiddleware>();
    }
}

3. Register Middleware in Pipeline

In `Program.cs` (.NET 6+)

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.UseCustomMiddleware();

app.MapGet("/", () => "Hello World!");

app.Run();

Execution Flow

Middleware works like a pipeline:

Request → Middleware1 → Middleware2 → Controller → Middleware2 → Middleware1 → Response

Example: Simple Request Timer Middleware

using System.Diagnostics;

public class RequestTimingMiddleware
{
    private readonly RequestDelegate _next;

    public RequestTimingMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        var stopwatch = Stopwatch.StartNew();

        await _next(context);

        stopwatch.Stop();

        Console.WriteLine($"Request took {stopwatch.ElapsedMilliseconds} ms");
    }
}

Example: Custom Authentication Check

public class ApiKeyMiddleware
{
    private readonly RequestDelegate _next;

    public ApiKeyMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (!context.Request.Headers.TryGetValue("ApiKey", out var apiKey))
        {
            context.Response.StatusCode = 401;
            await context.Response.WriteAsync("Unauthorized");
            return;
        }

        await _next(context);
    }
}

Important Concepts

1. Order Matters

Middleware executes in the order added:

app.UseMiddleware<A>();
app.UseMiddleware<B>();

Flow:

A → B → Controller → B → A

2. Short-Circuiting

Middleware can stop pipeline:

return; // stops next middleware

3. Dependency Injection Support

public class MyMiddleware
{
    private readonly RequestDelegate _next;
    private readonly ILogger<MyMiddleware> _logger;

    public MyMiddleware(RequestDelegate next, ILogger<MyMiddleware> logger)
    {
        _next = next;
        _logger = logger;
    }
}

When to Use Custom Middleware

Logging requests/responses
Global exception handling
Authentication/authorization
Rate limiting
Request validation

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