Return content with IHttpActionResult for non-OK response

You can use this:

return Content(HttpStatusCode.BadRequest, "Any object");

I ended up going with the following solution:

public class HttpActionResult : IHttpActionResult
{
    private readonly string _message;
    private readonly HttpStatusCode _statusCode;

    public HttpActionResult(HttpStatusCode statusCode, string message)
    {
        _statusCode = statusCode;
        _message = message;
    }

    public Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
    {
        HttpResponseMessage response = new HttpResponseMessage(_statusCode)
        {
            Content = new StringContent(_message)
        };
        return Task.FromResult(response);
    }
}

... which can be used like this:

public IHttpActionResult Get()
{
   return new HttpActionResult(HttpStatusCode.InternalServerError, "error message"); // can use any HTTP status code
}

I'm open to suggestions for improvement. :)

You can also do:

return InternalServerError(new Exception("SOME CUSTOM MESSAGE"));

You can use HttpRequestMessagesExtensions.CreateErrorResponse (System.Net.Http namespace), like so:

public IHttpActionResult Get()
{
   return ResponseMessage(Request.CreateErrorResponse(HttpStatusCode.InternalServerError, "Message describing the error here"));
}

It is preferable to create responses based on the request to take advantage of Web API's content negotiation.