基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系。

1,先加入命名空间：

using System.Net.Security;
using System.Security.Authentication;
using System.Security.Cryptography.X509Certificates;

2，再重载CheckValidationResult方法，返回true

public bool CheckValidationResult(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors)
{   // 总是接受
return true;
}

3,然后在HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create(url);前面加上如下一行代码：

ServicePointManager.ServerCertificateValidationCallback = new System.Net.Security.RemoteCertificateValidationCallback(CheckValidationResult);//验证服务器证书回调自动验证