编写代码，使用HttpWebRequest访问网页，错误提示：
未能创建 SSL/TLS 安全通道
页面本来是Http网址，突然转换为https网址，所以遇到这个问题也算正常。将解决方法记录下来给需要的人。
csharp
//只需要在 HttpWebRequest 发送请求前加入下面的代码
ServicePointManager.ServerCertificateValidationCallback += (s, cert, chain, sslPolicyErrors) => true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
//.Net 4.0版本使用下面代码
ServicePointManager.ServerCertificateValidationCallback += (s, cert, chain, sslPolicyErrors) => true;
ServicePointManager.SecurityProtocol = (SecurityProtocolType)192 | (SecurityProtocolType)768 | (SecurityProtocolType)3072;