WebRequest webRequest = WebRequest.Create("验证码url");

            WebResponse webResponse = webRequest.GetResponse();

            Stream stream = webResponse.GetResponseStream();

            string cookie = webResponse.Headers.Get("Set-Cookie");

            Image img = Bitmap.FromStream(stream);

            pictureBox1.Image = img;

            using (MemoryStream ms1 = new MemoryStream())

            {

                img.Save(ms1, System.Drawing.Imaging.ImageFormat.Jpeg);

                byte[] arr1 = new byte[ms1.Length];

                ms1.Position = 0;

                ms1.Read(arr1, 0, (int)ms1.Length);

                ms1.Close();

                textBox1.Text = Convert.ToBase64String(arr1);

            }