WebClientEx

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web;
using System.Net;

    public class WebClientEx : WebClient
    {
        public WebClientEx(CookieContainer container)
        {
            this.container = container;
        }

        public CookieContainer CookieContainer
        {
            get { return container; }
            set { container = value; }
        }

        private CookieContainer container = new CookieContainer();

        protected override WebRequest GetWebRequest(Uri address)
        {
            WebRequest r = base.GetWebRequest(address);
            var request = r as HttpWebRequest;

            if (request != null)
            {
                request.AllowAutoRedirect = true;
                request.CookieContainer = container;
            }
            return r;
        }

        protected override WebResponse GetWebResponse(WebRequest request, IAsyncResult result)
        {
            WebResponse response = base.GetWebResponse(request, result);
            ReadCookies(response);
            return response;
        }

        protected override WebResponse GetWebResponse(WebRequest request)
        {
            WebResponse response = null;

            try
            {
                response = base.GetWebResponse(request);
            }
            catch (Exception err)
            {
                if (err.GetType() == typeof(WebException))
                {
                    response = ((WebException)err).Response as HttpWebResponse;
                }
            }

            ReadCookies(response);
            return response;
        }

        public void BugFix_CookieDomain()
        {
            Type _ContainerType = typeof(CookieContainer);
            Hashtable table = (Hashtable)_ContainerType.InvokeMember("m_domainTable", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.GetField | System.Reflection.BindingFlags.Instance, null, container, new object[] { });

            ArrayList keys = new ArrayList(table.Keys);

            foreach (string keyObj in keys)
            {
                string key = (keyObj as string);
                if (key[0] == '.')
                {
                    string newKey = key.Remove(0, 1);
                    table[newKey] = table[keyObj];
                }
            }
        }

        private void ReadCookies(WebResponse r)
        {
            var response = r as HttpWebResponse;
            if (response != null)
            {
                CookieCollection cookies = response.Cookies;
                //container.Add(cookies);
                container.Add(r.ResponseUri, cookies);
                BugFix_CookieDomain();
            }
        }
    }