Understanding Middleware in .NET Core
What is Middleware in ASP.NET Core?
If you're starting to learn ASP.NET Core, you've probably heard the word "middleware" quite a few times. It might sound complicated at first, but don’t worry — in this post, we’ll break it down into simple terms.
You’ll learn:
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What middleware is
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Why it's important
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How it works
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How to create your own
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Best practices (and things to avoid)
What is Middleware in .NET Core?
Middleware is a piece of code that sits between the incoming HTTP request and the outgoing HTTP response in an ASP.NET Core app. It can:
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Inspect or modify the request
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Handle authentication or logging
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Pass the request to the next step
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Or even stop the request entirely
In simple words: Middleware is like a checkpoint that performs some logic on every request before it reaches your controller or API.
Real-Life Analogy: The Airport
Imagine you're at an airport going through various checks:
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Security Check (Middleware 1): Checks your ID
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Baggage Scan (Middleware 2): Scans your luggage
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Customs (Middleware 3): Verifies documents
Only if you clear all these checks are you allowed to board the plane (the final endpoint).
How Middleware Works: The Request-Response Lifecycle
Here’s a simplified flow of how middleware works:
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Request enters the ASP.NET Core app.
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The first middleware runs its logic.
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It can:
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Modify the request
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Perform logic (like logging or authentication)
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Pass it to the next middleware using
await next()
-
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Eventually, a response is returned.
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Each middleware gets a chance to:
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Modify the response
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Log something
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Handle any errors
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Why Use Middleware?
Middleware helps you:
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Handle authentication and authorization
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Add logging and monitoring
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Catch exceptions
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Add caching or compression
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Transform requests or responses
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Keep your app modular and testable
Built-in Middleware in ASP.NET Core
ASP.NET Core provides several built-in middleware components:
Middleware | Description |
|---|---|
UseRouting | Matches request to endpoints |
UseAuthentication | Authenticates users |
UseAuthorization | Checks user permissions |
UseStaticFiles | Serves static files |
UseExceptionHandler | Handles unhandled exceptions |
UseCors | Enables CORS |
UseSession | Enables session state |
UseEndpoints | Executes matched endpoint |
Creating Custom Middleware
public class CustomMiddleware
{
private readonly RequestDelegate _next;
public CustomMiddleware(RequestDelegate next)
{
_next = next;
}
public async Task InvokeAsync(HttpContext context)
{
// Custom logic before next middleware
Console.WriteLine("Request handled by custom middleware");
await _next(context); // Call next
// Custom logic after next middleware
Console.WriteLine("Response handled by custom middleware");
}
}
Advantage & Disadvantage of Middleware
| Advantage | Description |
|---|---|
| Modular | Each piece is self-contained and reusable |
| Lightweight | Less overhead compared to full frameworks |
| Highly configurable | Order of execution gives total control |
| Testable | Each middleware can be unit tested |
| Cross-cutting logic | Centralized logging, auth, error handling |
| Disadvantage | Description |
|---|---|
| Order-dependent | Wrong order can break functionality |
| Hard to debug | Complex chains are harder to trace |
| No built-in DI scope per middleware | Unless configured, can lead to DI issues |
| Unintentional short-circuiting | Missing await next() ends the pipeline early |
| Verbose for complex logic | For complex logic, can become hard to maintain |
What You Might Not Know About Middleware
1. Middleware Runs for Every Request
Even static file requests go through middleware. Optimize heavy middleware logic with condition checks (e.g., if (context.Request.Path.StartsWithSegments("/api"))).
2. Short-Circuiting the Pipeline
A middleware that doesn’t call await next() will stop the request from reaching subsequent components.
3. Middleware Order Matters a LOT
For example, calling UseAuthorization() before UseAuthentication() results in broken security behavior. Correct order:
4. You Can Branch Middleware with Map() and MapWhen()
Create conditional middleware branches for specific paths or conditions:
5. Middleware Can Modify Request and Response on the Fly
You can read/replace the request body, inject headers, or intercept the response before it's sent.
6. Middleware Runs Outside MVC/Minimal API
Middleware is framework-independent. It runs before minimal APIs or MVC controllers get invoked.
7. Streaming and Buffering in Middleware
To modify the response body, you might need to buffer it:
Middleware Order in .NET 6+
var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();
// 1. Global exception handler
app.UseExceptionHandler(/* ... or IExceptionHandler configured */);
// 2. HTTPS redirection / HSTS
app.UseHttpsRedirection();
if (!app.Environment.IsDevelopment())
{
app.UseHsts();
}
// 3. Static files
app.UseStaticFiles();
// 4. Routing (this & short-circuit routing possibilities)
app.UseRouting();
// 5. CORS (if used)
app.UseCors("MyCorsPolicy");
// 6. Authentication
app.UseAuthentication();
// 7. Authorization
app.UseAuthorization();
// 8. HTTP Logging (or earlier, depending on what you want to log)
app.UseHttpLogging();
// 9. Custom / user middleware(s) (logging, metrics, etc.)
// 10. Output Caching (if you want to cache responses)
app.UseOutputCache(); // or similar
// 11. Map endpoints (controllers, minimal APIs etc.)
app.MapControllers();
app.MapGet(...);
app.MapRazorPages();
// 12. Fallbacks / terminal middleware, e.g. return 404 or custom
Conclusion
Middleware is a core concept in ASP.NET Core and plays a critical role in how HTTP requests are handled in your application. By understanding how middleware works, the request pipeline, and the importance of middleware order, you’ll gain full control over how your app processes incoming requests and outgoing responses.
Whether you're using built-in middleware like UseAuthentication, or writing your own custom logic, middleware helps you keep your code modular, clean, and easy to maintain.
As you continue learning ASP.NET Core, start experimenting with custom middleware and observe how changing the order affects your app — that’s the best way to truly understand the pipeline in action.
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