Content is missing for some 404 responses

Question

I'm trying to return custom error responses from web.api. Let it be simple string "Oops!" formatted as json. So I created simple delegating handler which replaces error responses like this:

public class ErrorMessageHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(
       HttpRequestMessage request, CancellationToken cancellationToken)
    {
       var response = await base.SendAsync(request, cancellationToken);

       if (response.IsSuccessStatusCode)
           return response;

       var formatter = new JsonMediaTypeFormatter();
       var errorResponse = request.CreateResponse(response.StatusCode, "Oops!", formatter);
       return errorResponse;
    }
}

Next I make sure that this is the only one message handler in pipeline:

httpConfig.MessageHandlers.Clear();
httpConfig.MessageHandlers.Add(new ErrorMessageHandler());

// Only default route
httpConfig.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );

app.UseWebApi(httpConfig); // OWIN self-hosting

Controller is also simplest one:

public class ValuesController : ApiController
{
   public IHttpActionResult Get(int id)
   {
      if (id == 42)
        return Ok("Answer to the Ultimate Question of Life, the Universe, and Everything");

      return NotFound();
   }
}

And here goes interesting:

• /api/values/42 gives me 200 response with value string
• /api/values/13 gives me 404 response with my custom "Oops!" string
• /api/values/42/missing gives me empty 404 response

The last case is my problem. When I set a breakpoint on the last line of delegating handler I clearly see that errorResponse contains ObjectContent<string> with the correct value. But why this value is cleared away later?

Answer 1

The reason is this code in HttpMessageHandlerAdapter.InvokeCore (so basically in UseWebApi middleware):

response = await _messageInvoker.SendAsync(request, cancellationToken);
// ...
if (IsSoftNotFound(request, response)) {
       callNext = true;
}
else { 
    // ...
}

Where IsSoftNotFound is:

private static bool IsSoftNotFound(HttpRequestMessage request, HttpResponseMessage response)
{
    if (response.StatusCode == HttpStatusCode.NotFound)
    {
        bool routingFailure;
        if (request.Properties.TryGetValue<bool>(HttpPropertyKeys.NoRouteMatched, out routingFailure)
            && routingFailure)
        {
            return true;
        }
    }
    return false;
}

So basically in case of "soft" 404, where "soft" means no route matched (and is indicated by property with specific key in request.Properties) - middleware will call some next component. Otherwise - will just send response.

This IsSoftDelete is true for your case (because indeed no route is matched) and that next component (didn't have time to figure out what that is really) clears your response content.

To "fix" this issue - remove property with that key from request after request was handled by previous handler:

public class ErrorMessageHandler : DelegatingHandler {
    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken) {
        var response = await base.SendAsync(request, cancellationToken);

        if (response.IsSuccessStatusCode)
            return response;

        // here, can also check if 404
        request.Properties.Remove(HttpPropertyKeys.NoRouteMatched);
        var formatter = new JsonMediaTypeFormatter();
        var errorResponse = request.CreateResponse(response.StatusCode, "Oops!", formatter);
        return errorResponse;
    }
}