https创建请求UrL报错: 未能为 SSL/TLS 安全通道建立信任关系

1）导入命名空间

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

2）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）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);