觉得应该有用，保留下。
目标框架
符号
.NET Framework
NETFRAMEWORK, NET20, NET35, NET40, NET45, NET451, NET452, NET46, NET461, NET462, NET47, NET471, NET472, NET48
.NET Standard
NETSTANDARD, NETSTANDARD1_0, NETSTANDARD1_1, NETSTANDARD1_2, NETSTANDARD1_3, NETSTANDARD1_4, NETSTANDARD1_5, NETSTANDARD1_6, NETSTANDARD2_0, NETSTANDARD2_1
.NET Core
NETCOREAPP, NETCOREAPP1_0, NETCOREAPP1_1, NETCOREAPP2_0, NETCOREAPP2_1, NETCOREAPP2_2, NETCOREAPP3_0, NETCOREAPP3_1
例如
#if NET461
`````
#else
`````
#endif
