What is RequestDelegate in .NET Core?

In simple terms, RequestDelegate in .NET Core is a function that handles an incoming HTTP request.

Think of it like this:

• A user opens your website.
• The request enters your ASP.NET Core application.
• RequestDelegate receives that request and decides what to do with it.

It can either:

• Generate a response and send it back to the user, or
• Pass the request to the next middleware in the pipeline.

Real-life analogy

Imagine you're at a restaurant.

• A customer = HTTP request
• A waiter = RequestDelegate
• The waiter either:
• Serves the customer directly, or
• Takes the order to the next person (chef, cashier, etc.).

Similarly, a RequestDelegate either handles the request itself or forwards it to the next middleware.

Simple example

app.Run(async context =>
{
    await context.Response.WriteAsync("Hello World!");
});

Here, app.Run() creates a RequestDelegate.

When someone visits the website:

• The request comes in.
• The RequestDelegate runs.
• It writes "Hello World!" to the response.
• The request ends.

Another example with middleware

app.Use(async (context, next) =>
{
    Console.WriteLine("Before");

    await next(); // Pass request to the next RequestDelegate

    Console.WriteLine("After");
});

Here:

• "Before" runs first.
• await next() sends the request to the next RequestDelegate.
• After the next one finishes, "After" runs.

In one sentence

RequestDelegate is a method that processes an HTTP request and either returns a response or passes the request to the next middleware in the ASP.NET Core request pipeline.