HttpClientHandler

namespace ConsoleApplication1
{
  using System.Net.Http;
  using System.Threading;
  using System.Threading.Tasks;
  using static System.Console;

  class Program
  {
    static void Main(string[] args)
    {
      Task.Run(async () => { await RunAsync(); }).Wait();
      WriteLine("Press any key to finish.");
      ReadKey();
    }

    public static async Task RunAsync()
    {
      WriteLine("RunAsync called.");
      using (var client = new MyHttpClient())
        await client.GetAsync("http://www.google.com");
    }
  }

  public class MyHttpClient : HttpClient
  {
    public MyHttpClient()
      : base(new MyHandler())
    {
      WriteLine("Client instantiated.");
    }

    public override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
      WriteLine("HttpClient.SendAsync");
      return base.SendAsync(request, cancellationToken);
    }
  }

  public class MyHandler : HttpClientHandler
  {
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
      WriteLine("MyHandler.SendAsync");
      return base.SendAsync(request, cancellationToken);
    }
  }
}