Custom response for 404s in MVC 6

Question

I am migrating a Web API 2 application over to MVC 6 and I'm struggling to figure out how to return a custom response when status codes like 404 or 405 occur. In Web API, I used an HttpMessageHandler for 404 and a DelegatingHandler for 405.

I am aware of the concept of middleware, but it seems like the middleware isn't executing late enough in the pipeline to capture the 404. Also, generating responses in middleware is sub-optimal because the HttpResponse class only exposes raw HTTP parameters like ContentLength and Body . I would prefer to simply return an IActionResult that contains a custom response model like all my regular endpoints do.

Startup code:

applicationBuilder.UseMiddleware<NotFoundMiddleware>();
applicationBuilder.UseMvc();

Middleware:

public class NotFoundMiddleware
{
    private readonly RequestDelegate _next;

    public NotFoundMiddleware(RequestDelegate next)
    {
        _next = next.EnsureNotNull(nameof(next));
    }

    public Task Invoke(HttpContext context)
    {
        if (context.Response?.StatusCode == StatusCodes.Status404NotFound)
        {
            // context.Response.StatusCode is 200 because context.Response is set to an instance of DefaultHttpResponse
        }

        return _next.Invoke(context);
    }
}

Is there a good way of accomplishing what I need?

Answer 1

Rearrange how you are executing your logic

public class NotFoundMiddleware {
    private readonly RequestDelegate _next;

    public NotFoundMiddleware(RequestDelegate next) {
        _next = next.EnsureNotNull(nameof(next));
    }

    public async Task Invoke(HttpContext context) {
        //let the context go through the pipeline
        await _next.Invoke(context);

        if (context.Response?.StatusCode == StatusCodes.Status404NotFound) {
           //execute your logic on the way out of the pipeline.
        }

    }
}

and also make sure that you register it early in the pipeline