HttpMessageInvokerExtensions.cs

using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Reflection;
using System.Text;

namespace YourNamespace
{
	public static class HttpMessageInvokerExtensions
    {
	    /// <summary>
	    /// This helps implement <see cref="DelegatingHandler" />.
	    /// DelegatingHandler: https://msdn.microsoft.com/en-us/library/system.net.http.delegatinghandler(v=vs.118).aspx
	    /// -> extends HttpMessageHandler:  https://msdn.microsoft.com/en-us/library/system.net.http.httpmessagehandler(v=vs.118).aspx
	    /// HttpMessageInvoker: https://msdn.microsoft.com/en-us/library/system.net.http.httpmessageinvoker(v=vs.118).aspx
	    /// -> Has constructor that takes HttpMessageHandler
	    /// </summary>
	    private const string InternalHandlerFieldName = "_handler";
	    private const string InternalHandlerFieldNameAlt = "handler";
		private const BindingFlags InternalHandlerBindingFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;

		public static void AppendDelegatingHandler(this HttpMessageInvoker client, DelegatingHandler handler)
	    {
		    throw new NotImplementedException();
		}

	    internal static FieldInfo GetInternalSendAsyncHandlerFieldInfo(this HttpMessageInvoker client)
	    {
		    FieldInfo internalField = null;
		    var currentType = client.GetType();
		    while (currentType != null)
		    {
			    if (currentType == typeof(HttpMessageInvoker))
			    {
				    internalField = currentType.GetField(InternalHandlerFieldName, InternalHandlerBindingFlags);
				    if (internalField == null)
				    {
					    internalField = currentType.GetField(InternalHandlerFieldNameAlt, InternalHandlerBindingFlags);
				    }

				    break;
			    }
			    currentType = currentType.BaseType;
		    }

		    if (internalField == null)
		    {
			    throw new Exception("Could not find internal HttpMessageHandler field.");
			}

		    return internalField;

	    }
	}
}