如果您可以使用。net 4.7.1或更新版本，它将使用TLS 1.2作为基于操作系统功能的最低协议。 根据微软的建议:

To ensure .NET Framework applications remain secure, the TLS version should not be hardcoded. .NET Framework applications should use the TLS version the operating system (OS) supports.

硬编码ServicePointManager的替代方案。SecurityProtocol或上面提到的显式SchUseStrongCrypto密钥: 你可以告诉。net使用SystemDefaultTlsVersions键来使用默认的channel设置， 例如:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319] "SystemDefaultTlsVersions"=dword:00000001

I'm running under .NET 4.5.2, and I wasn't happy with any of these answers. As I'm talking to a system which supports TLS 1.2, and seeing as SSL3, TLS 1.0, and TLS 1.1 are all broken and unsafe for use, I don't want to enable these protocols. Under .NET 4.5.2, the SSL3 and TLS 1.0 protocols are both enabled by default, which I can see in code by inspecting ServicePointManager.SecurityProtocol. Under .NET 4.7, there's the new SystemDefault protocol mode which explicitly hands over selection of the protocol to the OS, where I believe relying on registry or other system configuration settings would be appropriate. That doesn't seem to be supported under .NET 4.5.2 however. In the interests of writing forwards-compatible code, that will keep making the right decisions even when TLS 1.2 is inevitably broken in the future, or when I upgrade to .NET 4.7+ and hand over more responsibility for selecting an appropriate protocol to the OS, I adopted the following code:

SecurityProtocolType securityProtocols = ServicePointManager.SecurityProtocol;
if (securityProtocols.HasFlag(SecurityProtocolType.Ssl3) || securityProtocols.HasFlag(SecurityProtocolType.Tls) || securityProtocols.HasFlag(SecurityProtocolType.Tls11))
{
    securityProtocols &= ~(SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11);
    if (securityProtocols == 0)
    {
        securityProtocols |= SecurityProtocolType.Tls12;
    }
    ServicePointManager.SecurityProtocol = securityProtocols;
}

This code will detect when a known insecure protocol is enabled, and in this case, we'll remove these insecure protocols. If no other explicit protocols remain, we'll then force enable TLS 1.2, as the only known secure protocol supported by .NET at this point in time. This code is forwards compatible, as it will take into consideration new protocol types it doesn't know about being added in the future, and it will also play nice with the new SystemDefault state in .NET 4.7, meaning I won't have to re-visit this code in the future. I'd strongly recommend adopting an approach like this, rather than hard-coding any particular security protocol states unconditionally, otherwise you'll have to recompile and replace your client with a new version in order to upgrade to a new security protocol when TLS 1.2 is inevitably broken, or more likely you'll have to leave the existing insecure protocols turned on for years on your server, making your organisation a target for attacks.

在经历了一番挣扎之后，注册表更改机制为我工作了。实际上，我的应用程序是32位的。所以我必须改变路径下的值。

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft.NETFramework\v4.0.30319

值类型需要为DWORD且值大于0。最好使用1。

有两种可能的情况，

如果你的应用程序运行在。net framework 4.5或更低版本上，你可以很容易地将新代码部署到产品中，那么你可以使用下面的解决方案。 你可以在调用api之前添加下面的代码行， ServicePointManager。SecurityProtocol = SecurityProtocolType.Tls12;// .net 4.5 如果您无法部署新代码，而希望使用生产环境中存在的相同代码进行解析，那么您有两个选择。

方案一:

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001

然后创建一个扩展名为.reg的文件并安装。

注意:此设置将应用于注册表级别，并适用于该机器上的所有应用程序，如果您希望仅限制为单个应用程序，则可以使用选项2

选项2:这可以通过更改配置文件中的一些配置设置来完成。 您可以在配置文件中添加其中任何一个。

<runtime>
    <AppContextSwitchOverrides value="Switch.System.Net.DontEnableSchUseStrongCrypto=false"/>
  </runtime>

or

<runtime>
  <AppContextSwitchOverrides value="Switch.System.Net.DontEnableSystemDefaultTlsVersions=false"
</runtime>

一些在其他答案上留下评论的人注意到，将System.Net.ServicePointManager.SecurityProtocol设置为特定的值意味着你的应用程序将无法利用未来的TLS版本，这些TLS版本在未来的。net更新中可能成为默认值。与其指定一个固定的协议列表，不如执行以下操作:

对于。net 4.7或更高版本，不要设置System.Net.ServicePointManager.SecurityProtocol。默认值(SecurityProtocolType.SystemDefault)将允许操作系统使用它知道的和已配置的任何版本，包括在应用程序创建时可能不存在的任何新版本。

对于。net Framework的早期版本，您可以打开或关闭您所了解和关心的协议，而让其他协议保持原样。

启用TLS 1.1和TLS 1.2而不影响其他协议:

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

注意使用|=来打开这些标志而不关闭其他标志。

关闭SSL3而不影响其他协议。

System.Net.ServicePointManager.SecurityProtocol &= ~SecurityProtocolType.Ssl3;