C＃编译器如何决定发出可重定向的程序集引用？

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"

Answer 3

I've noticed by experimenting that the C# compiler would make an reference compiler as retargetable if the referenced assembly is marked as retargetable (a modifier on the .assembly section in MSIL). I did not find how the compiler decides to make the assembly retargetable, yet.