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Middleware in ASP.NET Core

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Middleware in ASP.NET Core is a fundamental concept used to build the HTTP request pipeline. Each middleware component in the pipeline:

  • Processes incoming HTTP requests.

  • Optionally short-circuits the pipeline (e.g., for authentication).

  • Passes control to the next middleware.

  • Optionally processes the outgoing HTTP response.

What is Middleware?

Middleware is software that's assembled into an application pipeline to handle requests and responses. Each component:

  • Can perform work before and after the next delegate (middleware) is invoked.

  • Can terminate the request pipeline early.

  • Can perform asynchronous operations.

Request Pipeline Structure

Here’s a simple visualization:






HTTP Request --> Middleware 1 --> Middleware 2 --> Middleware 3 --> ... 


| Controller/Endpoint | 


<-- Middleware 3 <-- Middleware 2 <-- Middleware 1


HTTP Response 









Middleware Lifecycle



Each middleware is a RequestDelegate:







csharp



public delegate Task RequestDelegate(HttpContext context); 









Creating Custom Middleware



You can create custom middleware by either:


1. Class-based Middleware



public class MyMiddleware


{


private readonly RequestDelegate _next;


public MyMiddleware(RequestDelegate next)


{


_next = next;


}


public async Task InvokeAsync(HttpContext context)


{


// Pre-processing logic


Console.WriteLine("Request: " + context.Request.Path);


await _next(context);


// Call the next middleware


// Post-processing logic


Console.WriteLine("Response: " + context.Response.StatusCode);


}


}






Register it in Startup.cs:







csharp



public void Configure(IApplicationBuilder app, IWebHostEnvironment env)


{


app.UseMiddleware();


}         






2. Inline Middleware (Lambda)



app.Use(async (context, next) =>


{


Console.WriteLine("Before");


await next.Invoke();


Console.WriteLine("After");


}

);







Built-in Middleware Examples



ASP.NET Core includes many built-in middleware components:







    
        

            Middleware
            Description
        

    
    
        

            UseStaticFiles
            Serves static files like images, JS, CSS
        

        

            UseRouting
            Routes requests to endpoints
        

        

            UseAuthentication
            Authenticates the user
        

        

            UseAuthorization
            Authorizes access based on policies/roles
        

        

            UseEndpoints
            Maps endpoints to controllers or Razor pages
        

        

            UseExceptionHandler
            Global exception handling
        

        

            UseHttpsRedirection
            Redirects HTTP requests to HTTPS
        

        

            UseCors
            Configures Cross-Origin Resource Sharing (CORS)
        

        

            UseSession
            Enables session state
        

    









Order Matters



The order of middleware registration in Configure() is critical. For example:



app.UseRouting();


app.UseAuthentication();


app.UseAuthorization();


app.UseEndpoints(...);






Putting UseAuthentication() after UseEndpoints() would result in no authentication.




Best Practices


    
    
  • 
    
    
    Keep middleware focused on one responsibility.
    
    
    • 
    
  • 
    
    
    Handle exceptions early in the pipeline.
    
    
    • 
    
  • 
    
    
    Use inline middleware for short logic, classes for complex logic.
    
    
    • 
    
  • 
    
    
    Avoid long blocking operations – use async/await.
    
    
    • 







                        

                               
                                                            
                        


                    



                

            

        

    






        
        
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