Json Post到 https的坑 - the underlying connection was closed an unexpected error occurred on a send（远程服务器未知错误导致关闭）

最近做了一个安装包，安装包会弹出dotnet的 窗体，这个安装包会去调用https的一个api。用测试程序测试窗体都是好的。一旦打入安装包后，就报错。研究了半天，原来是https惹的祸

解决方案:

1. .NET 4.6 and above. You don’t need to do any additional work to support TLS 1.2, it’s supported by default.DOTNET 4.6.1 以上版本，默认就可以了

2. .NET 4.5. TLS 1.2 is supported, but it’s not a default protocol. You need to opt-in to use it. The following code will make TLS 1.2 default, make sure to execute it before making a connection to secured resource:

　　　　ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 

1. .NET 4.0. TLS 1.2 is not supported, but if you have .NET 4.5 (or above) installed on the system then you still can opt in for TLS 1.2 even if your application framework doesn’t support it. The only problem is that SecurityProtocolType in .NET 4.0 doesn’t have an entry for TLS1.2, so we’d have to use a numerical representation of this enum value:

　　　　ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;

1. .NET 3.5 or below. TLS 1.2 is not supported (*) and there is no workaround. Upgrade your application to more recent version of the framework. DOTNET3.5不支持

各种tls版本兼容写法：

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls

其实默认配置DOTNET 4.6.1 以上版本就可以了，但是因为我是安装包，没法写配置文件.....

  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
  </startup>