Determine the compiler version at compile time

Add the following to the project file

  <PropertyGroup>
    <DefineConstants>NETFX$(TargetFrameworkVersion.Replace("v", "").Replace(".", "_"));$(DefineConstants)</DefineConstants>
  </PropertyGroup>

This will define NETFX4_6, NETFX4_5_2 etc. for the framework versions 4.6 and 4.5.2.

You can then selectively compile code based on compiler version

    #if NETFX4_6
        Console.WriteLine("Ver 4.6");
    #endif
    #if NETFX4_5_2
        Console.WriteLine("Ver 4.5.2");
    #endif
    #if NETFX4_5_1
        Console.WriteLine("Ver 4.5.1");
    #endif