So what is wrong with the following code? 
It should compile, I’ve seen identical code on an internet blog suggesting it should work. I delete the offending method SerializationToStreamAsync and have Visual Studio generate it from the interface – it produces the same issue.
It should work – but it doesn’t, keep reading for the solution!
I resolved the issue in 3 easy steps, after numerous long hours of research….
First, I had to remove the existing System.Net references, second I loaded NuGet Package manager, and finally I browsed for and installed the v4.1.0 version of the System.Net.Http package and the problem was resolved – I could now compile my solution successfully!
using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Newtonsoft.Json;
namespace Salesforce.Force.UnitTests
{
internal class JsonContent : HttpContent
{
private readonly MemoryStream _stream = new MemoryStream();
public JsonContent(object value)
{
Headers.ContentType = new MediaTypeHeaderValue("application/json");
var jw = new JsonTextWriter(new StreamWriter(_stream))
{
Formatting = Formatting.Indented
};
var serializer = new JsonSerializer();
serializer.Serialize(jw, value);
jw.Flush();
_stream.Position = 0;
}
protected JsonContent(string content)
{
Headers.ContentType = new MediaTypeHeaderValue("application/json");
var sw = new StreamWriter(_stream);
sw.Write(content);
sw.Flush();
_stream.Position = 0;
}
protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
{
return _stream.CopyToAsync(stream);
}
protected override bool TryComputeLength(out long length)
{
length = _stream.Length;
return true;
}
public static HttpContent FromFile(string filepath)
{
string content = File.ReadAllText(filepath);
return new JsonContent(content);
}
}
}