TestProcessResponse.cs

using HarmonyLib;
using System;
using System.Net;

namespace TestProcessResponse
{
    class Program
    {
        static void Main(string[] args)
        {
            _ = args;
            var harmony = new Harmony("test");
            harmony.PatchAll();

            var t_CoreResponseData = AccessTools.TypeByName("System.Net.CoreResponseData");
            var m_SetResponse = AccessTools.Method("System.Net.HttpWebRequest:SetResponse", new[] { t_CoreResponseData });
            _ = harmony.Patch(m_SetResponse);

            var client = new WebClient();

            _ = client.DownloadString("http://www.google.com");
            _ = Console.ReadKey();
        }

        [HarmonyPatch(typeof(HttpWebRequest))]
        [HarmonyPatch("ProcessResponse")]
        class ProcessResponse_Patch
        {
            public static void Prefix()
            {
                Console.WriteLine("ProcessResponse called!");
            }
        }
    }
}