Save/Load CookieContainer Sample

Form1.cs

using System;
using System.IO;
using System.Net;
using System.Runtime.Serialization.Formatters.Binary;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private const    string DatName = "cookie.dat";
        private readonly string Desktop = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);

        private WebClientEx wc = new WebClientEx();

        public Form1()
        {
            InitializeComponent();
        }

        private async void SaveCookiesAsync(string path)
        {
            using (var stream = File.OpenWrite(path))
            {
                await Task.Run(() =>
                {
                    var bf = new BinaryFormatter();
                    bf.Serialize(stream, wc.CookieContainer);
                });
                await stream.FlushAsync();
            }
        }

        private async Task<CookieContainer> LoadCookiesAsync(string path)
        {
            try
            {
                using (var stream = File.OpenRead(path))
                {
                    return await Task.Run(() =>
                    {
                        var bf = new BinaryFormatter();
                        return (CookieContainer)bf.Deserialize(stream);
                    });
                }
            }
            catch (FileNotFoundException e)
            {
                System.Diagnostics.Debug.WriteLine(e.Message);
                return null;
            }
        }

        private async void Form1_Load(object sender, EventArgs e)
        {
            string path = Path.Combine(Desktop, DatName);
            if (File.Exists(path))
            {
                wc.CookieContainer = await LoadCookiesAsync(path);
            }

            await wc.DownloadStringTaskAsync("");
            SaveCookiesAsync(path);
        }
    }
}

WebClientEx.cs

using System;
using System.Net;

namespace WindowsFormsApplication1
{
    public class WebClientEx : WebClient
    {
        public CookieContainer CookieContainer { get; set; } = new CookieContainer();
        public string Referer { get; private set; }

        protected override WebRequest GetWebRequest(Uri address)
        {
            var req = base.GetWebRequest(address);
            if (req is HttpWebRequest)
            {
                ((HttpWebRequest)req).CookieContainer = CookieContainer;
            }
            return req;
        }

        protected override WebResponse GetWebResponse(WebRequest request)
        {
            var res = base.GetWebResponse(request);
            Referer = (res as HttpWebResponse)?.ResponseUri.AbsoluteUri;
            return res;
        }

        protected override WebResponse GetWebResponse(WebRequest request, IAsyncResult result)
        {
            var res = base.GetWebResponse(request, result);
            Referer = (res as HttpWebResponse)?.ResponseUri.AbsoluteUri;
            return res;
        }
    }
}