将微信二维码网络地址转成base64码串

qrcode_为获取的二维码网络地址

WebClient mywebclient = new WebClient();
 byte[] Bytes = mywebclient.DownloadData(qrcode_);
 mywebclient.Dispose();
//转换为base64
string telqr_ = Convert.ToBase64String(Bytes);