如何在.NET Standard 2.0中处理HttpClientHandler.SslProtocol的平台不支持异常

你可以通过RuntimeInformation.FrameworkDescription在运行时判断当前环境，从而针对性地跳过.NET Framework 4.7.1不支持的SslProtocols设置，以下是两种实现方式的修改方案：

1. 直接创建HttpClient的版本 #

由于原代码是静态只读字段初始化，需要将逻辑移到静态构造函数中，先判断框架再配置Handler：

private static readonly HttpClient _httpClient;

static YourClassName()
{
    var handler = new HttpClientHandler
    {
        AutomaticDecompression = System.Net.DecompressionMethods.GZip | System.Net.DecompressionMethods.Deflate,
        ServerCertificateCustomValidationCallback = null
    };

    // 仅在非.NET Framework环境下设置SslProtocols
    if (!RuntimeInformation.FrameworkDescription.StartsWith(".NET Framework"))
    {
        handler.SslProtocols = System.Security.Authentication.SslProtocols.Tls11;
    }

    _httpClient = new HttpClient(handler)
    {
        Timeout = TimeSpan.FromSeconds(45)
    };
}

2. IHttpClientFactory实现的版本 #

在配置PrimaryHttpMessageHandler时加入框架判断逻辑：

private static readonly Lazy<IHttpClientFactory> httpClientFactoryLazy = new Lazy<IHttpClientFactory>(() =>
{
    var services = new ServiceCollection();
    services.AddHttpClient("HttpClientFactory", client =>
    {
        client.Timeout = TimeSpan.FromSeconds(15);
    })
    .ConfigurePrimaryHttpMessageHandler(() =>
    {
        var handler = new HttpClientHandler();
        handler.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;

        // 跳过.NET Framework环境下的SslProtocols设置
        if (!RuntimeInformation.FrameworkDescription.StartsWith(".NET Framework"))
        {
            handler.SslProtocols = System.Security.Authentication.SslProtocols.Tls11;
        }

        return handler;
    });
    var serviceProvider = services.BuildServiceProvider();
    return serviceProvider.GetRequiredService<IHttpClientFactory>();
});

关键说明 #

• RuntimeInformation.FrameworkDescription的返回值在.NET Framework 4.7.1中为.NET Framework 4.7.1，在.NET Core/.NET 5+中为.NET Core X.X或.NET X，通过StartsWith判断足够可靠。
• 这种方式是运行时动态判断，完美适配.NET Standard项目被不同框架引用的场景，无需修改任何引用该Standard项目的外部项目。
• 不推荐用条件编译：因为.NET Standard项目编译时不绑定具体目标框架，条件编译无法区分最终运行时的环境，运行时判断才是正确的选择。

内容的提问来源于stack exchange，提问作者Nishad