【SSL】WebClient 请求 https 页面出错:未能创建 SSL/TLS 安全通道

using (WebClient client = new WebClient())
{
    string address="https://xxx.com";
    client.Headers.Add(HttpRequestHeader.ContentType, "text/xml");

    System.Text.ASCIIEncoding  encoding=new System.Text.ASCIIEncoding();
    var response = client.UploadData(address, "POST", encoding.GetBytes(msg));
}

ServicePointManager.ServerCertificateValidationCallback += new System.Net.Security.RemoteCertificateValidationCallback(bypassAllCertificateStuff);

private static bool bypassAllCertificateStuff(object sender, X509Certificate cert, X509Chain chain, System.Net.Security.SslPolicyErrors error)
{
   return true;
}

ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

 1 public class Test
 2 {
 3     public void TestFun
 4     {
 5　　　　　　　ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);
 6 　　　　　　 using (WebClient client = new WebClient())
 7 　　　　　　{
 8    　　　　　  string address="https://xxx.com";
 9 　　　　　　　  client.Headers.Add(HttpRequestHeader.ContentType,"text/xml");
10    　　　　　　System.Text.ASCIIEncoding  encoding=new System.Text.ASCIIEncoding();
11    　　　　　　var response = client.UploadData(address, "POST", encoding.GetBytes(msg));
12 　　　　　　}
13     } 
14     private bool CheckValidationResult(object sender, System.Security.Cryptography.X509Certificates.X509Certificate certificate, System.Security.Cryptography.X509Certificates.X509Chain chain, System.Net.Security.SslPolicyErrors errors)
15     {   // 总是接受 认证平台 服务器的证书
16         return true;
17     }
18 }