What are middleware and how do you create custom middleware?

💡 Concept: Middleware in ASP.NET Core

Middleware are components in the HTTP request pipeline that process requests and responses.

📘 Quick Intro

Middleware components execute in a sequence and can handle, modify, or short-circuit requests.

🧠 Analogy

Like an assembly line where each station performs a specific task on the product.

🔧 Technical Explanation

Middleware are classes with an Invoke or InvokeAsync method.
They have access to HttpContext and the next middleware.
Registered in Startup.cs via IApplicationBuilder.
Can perform actions before and after calling the next middleware.
Enable cross-cutting concerns like logging, authentication, error handling.

🎯 Use Cases

✅ Authentication and authorization.
✅ Logging and diagnostics.
✅ Custom error handling.
✅ Response compression and caching.

💻 Code Example


// Custom middleware example
public class CustomMiddleware {
    private readonly RequestDelegate _next;

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

    public async Task InvokeAsync(HttpContext context) {
        // Logic before next middleware
        await context.Response.WriteAsync(""Before next middleware\n"");

        await _next(context);

        // Logic after next middleware
        await context.Response.WriteAsync(""After next middleware\n"");
    }
}

// Register in Startup.cs
public void Configure(IApplicationBuilder app) {
    app.UseMiddleware();
    // other middleware
}

❓ Interview Q&A

Q1: What is middleware?
A: Software components that handle HTTP requests and responses.

Q2: How do you create custom middleware?
A: Create a class with Invoke or InvokeAsync method and register it.

Q3: What is the order of middleware execution?
A: Middleware executes in the order registered.

Q4: Can middleware short-circuit requests?
A: Yes, by not calling the next middleware.

Q5: What is the use of next middleware delegate?
A: To call the next middleware in the pipeline.

Q6: Can middleware modify responses?
A: Yes.

Q7: Where do you register middleware?
A: In Startup.cs Configure method.

Q8: What are some built-in middleware?
A: Authentication, Static Files, Routing.

Q9: Is middleware pipeline configurable?
A: Yes.

Q10: Can middleware be asynchronous?
A: Yes.

📝 MCQs

Q1. What is middleware?

Software handling HTTP requests
Database service
UI component
Background task

Q2. How do you create custom middleware?

Using filters
Class with Invoke method
Middleware attribute
Static class

Q3. What controls middleware order?

Alphabetical
Registration order
Random
Priority attribute

Q4. Can middleware short-circuit requests?

No
Yes
Sometimes
Never

Q5. What does next middleware do?

Stops pipeline
Calls next component
Restarts pipeline
Ignores request

Q6. Can middleware modify response?

No
Yes
Sometimes
Never

Q7. Where to register middleware?

Program.cs
Startup.cs Configure
Controllers
Middleware folder

Q8. Name built-in middleware

Authentication, Routing
Logging, Caching
Validation, Filters
Services, Repositories

Q9. Is middleware pipeline configurable?

No
Yes
Maybe
Rarely

Q10. Can middleware be async?

No
Yes
Sometimes
Never

💡 Bonus Insight

Middleware provides a powerful way to customize and control the HTTP request pipeline in ASP.NET Core.

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