C# 特定框架适用特定代码

 #if NET40 2 Console.WriteLine("Hello from .NET Core 4"); 3 #endif 

TFM获取：https://docs.microsoft.com/zh-cn/dotnet/standard/frameworks

预处理指令 #IF https://docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/preprocessor-directives#conditional-compilation