How do you set the Content-Type header for an HttpClient request?

Question

I\'m trying to set the Content-Type header of an HttpClient object as required by an API I am calling.

I tried setting the Content-Ty

Answer 1

.Net tries to force you to obey certain standards, namely that the Content-Type header can only be specified on requests that have content (e.g. POST, PUT, etc.). Therefore, as others have indicated, the preferred way to set the Content-Type header is through the HttpContent.Headers.ContentType property.

With that said, certain APIs (such as the LiquidFiles Api, as of 2016-12-19) requires setting the Content-Type header for a GET request. .Net will not allow setting this header on the request itself -- even using TryAddWithoutValidation. Furthermore, you cannot specify a Content for the request -- even if it is of zero-length. The only way I could seem to get around this was to resort to reflection. The code (in case some else needs it) is

var field = typeof(System.Net.Http.Headers.HttpRequestHeaders)
    .GetField("invalidHeaders", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static) 
  ?? typeof(System.Net.Http.Headers.HttpRequestHeaders) 
    .GetField("s_invalidHeaders", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Static);
if (field != null)
{
  var invalidFields = (HashSet<string>)field.GetValue(null);
  invalidFields.Remove("Content-Type");
}
_client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "text/xml");

Edit:

As noted in the comments, this field has different names in different versions of the dll. In the source code on GitHub, the field is currently named s_invalidHeaders. The example has been modified to account for this per the suggestion of @David Thompson.

Answer 2

You need to do it like this:

    HttpContent httpContent = new StringContent(@"{ the json string }");
    httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));                
    HttpResponseMessage message = client.PostAsync(@"{url}", httpContent).Result;