在进行HTTPS连接时，需要指定SecurityProtocol。对于.NET 4.0的代码，需要强制转换。

...
System.Net.ServicePointManager.SecurityProtocol = (SecurityProtocolType) 192 |
                                                   (SecurityProtocolType) 768 |
                                                   (SecurityProtocolType) 3072;
...

在 .net 4.5以上的代码使用以下代码即可。

System.Net.ServicePointManager.SecurityProtocol = 
    System.Net.SecurityProtocolType.Ssl3 
    | System.Net.SecurityProtocolType.Tls12 
    | SecurityProtocolType.Tls11 
    | SecurityProtocolType.Tls;