HttpWebRequest 基础连接已经关闭: 未能为 SSL/TLS 安全通道建立信任关系

请求对象前加入 

 ServicePointManager.ServerCertificateValidationCallback = ValidateServerCertificate;

然后实现该方法

private static bool ValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
　　return true;
}

就可以了