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Middleware in ASP.NET Core: Overview & Best Practices
Middleware in ASP.NET Core is a core concept for handling HTTP requests. Middleware are components forming the processing pipeline—each responsible for a specific aspect of request/response handling. Typical examples: authentication, logging, response compression, etc.
Key Concepts:

• Requests pass through middleware in order of registration (in
  
  Code:

  Program.cs
  or
  
  Code:

  Startup
  ).
  
• Each middleware can process, short-circuit, or modify the request/response.
  
• The response traverses the pipeline in the reverse order.
  

Why Use Middleware?

• Modular design: isolates concerns (authentication, logging, etc.).
  
• Order matters: Misordering (e.g., authorization before authentication) breaks logic.
  
• Pipeline flexibility: You can insert, remove, and test components with ease.
  

Built-In Middleware:

• Code:

  UseExceptionHandler
  /
  
  Code:

  UseDeveloperExceptionPage
  — error handling.
  
• Code:

  UseHttpsRedirection
  — forces HTTPS usage.
  
• Code:

  UseStaticFiles
  — serves static files from wwwroot/other folders.
  
• Code:

  UseRouting
  and
  
  Code:

  UseEndpoints
  — maps requests to controllers, Razor Pages, SignalR hubs.
  
• Code:

  UseAuthentication
  /
  
  Code:

  UseAuthorization
  — handles security.
  
• Code:

  UseCors
  ,
  
  Code:

  UseResponseCompression
  ,
  
  Code:

  UseSession
  ,
  
  Code:

  UseResponseCaching
  — for cross-origin policies, compression, session and cache management.
  

Custom Middleware:

• Middleware can be written as inline delegates or separate classes.
  
• Use class-based middleware for testability and Dependency Injection.
  
• Common mistakes:
  • Forgetting to call
    
    Code:

    next()
    blocks the pipeline.
    
  • Blocking operations in async middleware reduces scalability.
    
  • Incorrect DI (injecting scoped services in constructors).
    

Patterns & Advanced Use:

• Pipeline pattern: Each middleware “wraps” the next—like Russian matryoshka dolls.
  
• Short-circuiting: Skip further processing for failed auth, validation, etc.
  
• Performance: Place quick/early terminating middleware up front; resource-intensive ones at the end.
  

Testing & Diagnostics:

• Use logging (
  
  Code:

  ILogger
  ) liberally for flow and error tracing.
  
• Use tools like
  
  Code:

  TestServer
  and mock
  
  Code:

  HttpContext
  objects for integration and isolation testing.
  

Comparison:

• Middleware pipelines are conceptually similar to Express.js (Node.js) or Rack (Ruby), but ASP.NET Core's are strongly-typed and leverage the .NET ecosystem.
  

Useful code snippets and best practices included:

• Exception handling
  
• Registering middleware
  
• Writing custom middleware (class & inline styles)
  
• Async handling
  
• Dependency injection in InvokeAsync