您可以从

As you can see from the docs, TLS 1.2 is not in the enumeration for SslProtocols, it was added to the enum in .NET 4.5 (thanks @orhun).

.NET 3.5上没有针对TLS 1.2兼容性的解决方法.

There is no workaround for TLS 1.2 compatibility on .NET 3.5.

不幸的是，您必须升级到.NET 4.5或更高版本才能获得TLS 1.2兼容性.

Unfortunately you will have to upgrade to .NET 4.5 or later to get TLS 1.2 compatibility.

编辑10/11/17

我上面的答案不再准确.

My above answer is no longer accurate. In May of 2017, Microsoft released a package to allow TLS 1.2 in .NET 3.5.1.