4 个解决办法，我用的第四个方法就解决了， 注册表手动添加的 重启后不管用，第四个方法 直接用程序改一下方便

首先得保证服务器是否支持 tls1.2 去注册表里查或者百度怎么查,基本大多数都用的是1.2  

1.   

代码前加这个 

ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
ServicePointManager.ServerCertificateValidationCallback = (sender, certificate, chain, errors) => true;

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

或者

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3
| SecurityProtocolType.Tls
| (SecurityProtocolType)0x300 //Tls11
| (SecurityProtocolType)0xC00; //Tls12

2. 复制下面注册表代码导入到注册表。新建txt，将后缀txt改为reg（注册表项），导入

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp]
"DefaultSecureProtocols"=dword:00000800

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\WinHttp]
"DefaultSecureProtocols"=dword:00000800

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Client]
"DisabledByDefault"=dword:00000000
"Enabled"=dword:00000001

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Server]
"DisabledByDefault"=dword:00000000
"Enabled"=dword:00000001

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client]
"DisabledByDefault"=dword:00000000
"Enabled"=dword:00000001

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server]
"DisabledByDefault"=dword:00000000
"Enabled"=dword:00000001

3. 升级 到 fromwork4.6 版本

4.

https://www.nartac.com/Products/IISCrypto/Download

去下载这个软件 打开软件后 都选上确定 重启就好了