Explicitly Set Content-Type Headers For Get Operation in HttpClient

It's possible - and very dirty - to override the library behavior with a bit of reflection and by introducing a DelegatingHandler that you give as argument to the HttpClient constructor. See the code below.

public class HmacAuthenticatingHandler : DelegatingHandler
{
    public HmacAuthenticatingHandler(HttpMessageHandler innerHandler) 
       : base(innerHandler)
    {
    }

    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {

        // HACK: Set 'Content-Type' even for GET requests
        var invalidHeaders = (HashSet<string>)typeof(HttpHeaders)
            // use "_invalidHeaders" for System.Net.Http v2.2+
            .GetField("invalidHeaders", BindingFlags.NonPublic | BindingFlags.Instance)
            .GetValue(request.Headers);
        invalidHeaders.Remove("Content-Type");

        request.Headers.Remove("Content-Type");
        request.Headers.Add("Content-Type", "application/json");

        var response = await base.SendAsync(request, cancellationToken);
        return response;
    }
}

Based on my findings i concluded the HttpClient is very restrictive in terms of the protocol rules. I also reflected through the implementation DLL and i couldn't find anything that it would indicate that it allows protocol violations.

GET requests shouldn't have content-type headers, and the HttpClient is enforcing that rule.

I think the exception message when you try to set the content-type header is self-descriptive:

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

Also if you use set the content body you get one more self-descriptive message:

System.Net.ProtocolViolationException: Cannot send a content-body with this verb-type.

Since you are willing to violate HTTP rules for GET requests i am pretty sure your only option is to stick with the less restrictive WebClient, which works in that scenario.

Although of no help to you right now, it does look like a future release of the .NET framework may support this type of protocol violation with the addition of an AddWithoutValidation method:

https://msdn.microsoft.com/en-us/library/hh204926