.NET6 minimal API: Manipulating response after .MapGet

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.

Solution

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
    }
}

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).