Changing the response object from OWIN Middleware

1. Yes, deriving from OwinMiddleware is recommended. The reason some middleware classes don't derive from OwinMiddleware is that either they haven't switched over yet because the class was introduced recently. Or to avoid having the assembly take a dependency on the Microsoft.Owin assembly for some reason.

2. The probable reason setting stuff on the response after calling Invoke on Next doesn't work is that the response HTTP header gets sent as soon as anyone starts writing to the response body stream. So any changes to status code or HTTP headers after a middleware component starts writing to the response body won't have any effect.

What you can try doing is to use the OnSendingHeaders callback that OWIN provides. Here's how you can use it:

public override async Task Invoke(IOwinContext context)
{
   var response = context.Response;
   var request =  context.Request;

   response.OnSendingHeaders(state =>
   {
       var resp = (OwinResponse)state;
       resp.Headers.Add("X-MyResponse-Header", "Some Value");
       resp.StatusCode = 403;
       resp.ReasonPhrase = "Forbidden";
    }, response);

  var header = request.Headers["X-Whatever-Header"];

  await Next.Invoke(context);
}

Credit to biscuit314 for updating my answer.

I tried to edit Youssef's excellent answer to correct a minor bug and update the example with how the OwinMiddleware now works.

The edit was rejected (well, approved by one, rejected by one for being too minor, and rejected by two for being too major).

Here is that version of Youssef's code:

public override async Task Invoke(IOwinContext context)
{
  var response = context.Response;
  var request =  context.Request;

  response.OnSendingHeaders(state =>
    {
        var resp = (OwinResponse)state;
        resp.Headers.Add("X-MyResponse-Header", "Some Value");
        resp.StatusCode = 403;
        resp.ReasonPhrase = "Forbidden"; // if you're going to change the status code
                                         // you probably should also change the reason phrase
    }, response);

  var header = request.Headers["X-Whatever-Header"];

  await Next.Invoke(context);
}