Can't set Content-Type header on HttpResponseMessage headers?

I'm using the ASP.NET WebApi to create a RESTful API. I'm creating a PUT method within one of my controllers, and the code looks like this:

public HttpResponseMessage Put(int idAssessment, int idCaseStudy, string value) {
    var response = Request.CreateResponse();
    if (!response.Headers.Contains("Content-Type")) {
        response.Headers.Add("Content-Type", "text/plain");
    }

    response.StatusCode = HttpStatusCode.OK;
    return response;
}

When I PUT to that location with the browser via AJAX, it gives me this Exception:

Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.

But isn't Content-Type a perfectly valid header for a response? Why am I getting this exception?

Have a look at the HttpContentHeaders.ContentType Property:

response.Content.Headers.ContentType = new MediaTypeHeaderValue("text/plain");

if (response.Content == null)
{
    response.Content = new StringContent("");
    // The media type for the StringContent created defaults to text/plain.
}

Something is missing in ASP Web API: the EmptyContent type. It will allow sending an empty body while still allowing all content-specific headers.

Put the following class somewhere in your code :

public class EmptyContent : HttpContent
{
    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        return Task.CompletedTask;
    }
    protected override bool TryComputeLength(out long length)
    {
        length = 0L;
        return true;
    }
}

Then use it as you wish. You now have a content object for your extra headers.

response.Content = new EmptyContent();
response.Content.Headers.LastModified = file.DateUpdatedUtc;

Why use EmptyContent instead of new StringContent(string.Empty)?

• StringContent is a heavy class that executes lots of codes (because it inherits ByteArrayContent)
  • so let's save a few nanoseconds
• StringContent will add an extra useless/problematic header: Content-Type: plain/text; charset=...
  • so let's save a few network bytes