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 the following MSIL:

.assembly extern retargetable mscorlib
{
    .publickeytoken = (7C EC 85 D7 BE A7 79 8E )                         
    .ver 2:0:5:0
}

How does the C# compiler understand it has to emit a retargetable reference, and how to force the C# compiler to emit such reference even outside of a portable class library?

Answer 1

For the assembly itself, it's an assembly flag, ie [assembly: AssemblyFlags(AssemblyNameFlags.Retargetable)].

Make note that this flag is meaningless outside of platform assemblies - custom assemblies cannot be retargetable.

For references, it's copied as part of the name from the assembly being referenced.

Answer 2

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"