C# SSL认证设置

//.net 4.0 设置： 
ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

//.net 4.5 设置： 
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls1.2;

附上官方说明

https://docs.microsoft.com/zh-cn/dotnet/api/system.net.securityprotocoltype?view=netframework-4.7.2