C# 图片转换为base64

 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);
            }