API response is missing but not middleware response updates

Question

I was experimenting asp.net core (net9.0) custom middleware with below sample code. In API response I get the text written by middleware but the API response itself is missing. Any insights to help me understand on what is actually happening ?

This is response I get for the API call

Middleware 1. Passing to the next middleware! Middleware 2. Passing to the next middleware! Terminal middleware 1! Middleware 2. Received from previous middleware. Middleware 1. Received from previous middleware.

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);
WebApplication app = builder.Build();

app.UseHsts();

// custom nonterminal middleware, only to be added by app.Use();
app.Use(async (context, next) =>
{
    await context.Response.WriteAsync("Middleware 1. Passing to the next middleware!\r\n");

    String? authHeader = context.Request.Headers.Authorization;
    if (String.IsNullOrWhiteSpace(authHeader) || !authHeader.ToLower().StartsWith("bearer"))
    {
        await context.Response.WriteAsync("Middleware 1. Terminating pipeline since auth header is not present.\r\n");
    }
    else
    {
        // Call the next middleware in the pipeline
        await next.Invoke();

        await context.Response.WriteAsync("Middleware 1.  Received from previous middleware.\r\n");
    }
});

app.Use(async (context, next) =>
{
    await context.Response.WriteAsync("Middleware 2. Passing to the next middleware!\r\n");

    // Call the next middleware in the pipeline
    await next.Invoke();

    await context.Response.WriteAsync("Middleware 2.  Received from previous middleware.\r\n");
});

// custom terminal middleware, only to be added by app.Run();
app.Run(async context =>
{
    await context.Response.WriteAsync("Terminal middleware 1!\r\n");
});

app.Run(async context =>
{
    // never reached since terminal Middleware 1 will not forward to this middleware
    await context.Response.WriteAsync("Hello from terminal middleware 2!\r\n");
});

app.MapGet("/", () => "Hello World!");

app.Run();

Answer 1

I was experimenting asp.net core (net9.0) custom middleware with below sample code. In API response I get the text written by middleware but the API response itself is missing. Any insights to help me understand on what is actually happening ?

If I understand correctly, you expect a response from a controller.

The run delegate terminates the pipeline as described in your code comments. The controller is never reached because that's how your code is designed.

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/middleware/?view=aspnetcore-9.0#run-delegates

Answer 2

your middleware registration is list:

1. middleware 1
2. middleware 2
3. run 1
4. run 2
5. map

as .Run() does not call Next() to run the next middleware from the list, your run 1 middleware its the end of the line. so after run 1 is called it returns to middleware 2, when when coupled returns to middleware 1