How does the C# compiler decide to emit retargetable assembly references?

Question

Retargetable assembly references have been introduced for the .NET Compact Framework and are now used to support Portable Class Libraries.

Basically, the compiler emits

Answer 1

Not sure if this will help, but the following file was auto-generated and included in the build.

using System;
using System.Reflection;
[assembly: global::System.Runtime.Versioning.TargetFrameworkAttribute(
   ".NETPortable,Version=v4.0,Profile=Profile4", 
   FrameworkDisplayName = ".NET Portable Subset")]

This might hint to the compiler to do some magic.

Edit:

I think above makes a library portable. From the command line I can see /nostdlib+ is used, and a portable mscorlib.dll is referenced (which I assume has the same attribute as mentioned above).

"...\Program Files\Reference Assemblies\Microsoft\Framework.NETPortable\v4.0\Profile\Profile4\mscorlib.dll"