.NET6 minimal API: Manipulating response after .MapGet

Question

I need to manipulate the JSON after each .MapGet() in the same way (apply some transformations).

I always thought that we can setup middleware on response pipeline as well as on request pipeline by placing .UseMiddleware() (or .Use()) after .MapGet() like that:

[...]
app.UseMiddleware<MyRequestMiddleware>(); //It should be invoked before .MapGet()
app.MapGet(...);
app.UseMiddleware<MyResponseMiddleware>(); //It should be invoked after .MapGet()
await app.RunAsync();

In reality MyRequestMiddleware and MyResponseMiddleware are both pipelined to run before .MapGet(). How can I achieve "response pipeline" to run some transformation function for every endpoint? I know I can create a DI injected service and invoke the transformation in each endpoint but I'd rather not change the .MapGet() implementations and separate the transformations in another layer.

Answer 1

In ASP.NET Core middlewares that run before also run after (in a way). The request handling pipeline is build in the way that earlier middleware is (optionally) calling the next one (docs).

So if you want a middleware which does something after the next one then you just implement it in the following way:

public class MyMiddleware
{
    private readonly RequestDelegate _next;

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

    public async Task InvokeAsync(HttpContext context)
    {
        await _next(context);

        // your code that runs after the following middleware
    }
}

Or

app.Use(async (context, next) =>
{
    await next(context);
    // your code that runs after the following middleware
});

In this pattern you can write a single middleware which does something before AND after.

app.MapGet(...) does not register middleware, it registers route handler. The middleware which handles routes is registered with UseRouting if I remember correctly (also explicit call to it is not required).

See also How do I get the incoming request body and the outgoing response body in ASP.NET Core middleware?.