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?

No hints from MSBuild target files? I wonder what you need to pass to the compiler from the command line. — leppie, Jul 10 '12 at 07:49

score 3 · Accepted Answer · answered Jul 11 '12 at 04:17

3

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.

answered Jul 11 '12 at 04:17

David Kean

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Thank you. This is what I was looking for. I was hoping to get rid of the message `Could not load file or assembly 'PostSharp, Version=3.0.0.0, Culture=neutral, PublicKeyToken=53d2effcf2ee70dc, Retargetable=Yes' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)` when providing (through IHostAssemblyStore) another assembly than what the CLR requested, but I still get the error even with a retargetable reference. Is there any workaround to this? – Gael Fraiteur Jul 11 '12 at 05:55
Retargable won't let you key jump a user assembly, like I'm assuming you are attempting. It is entirely for CLR internal purposes. I'm not an expert on the hosting APIs, but I think LoadFile might allow you to do it. – David Kean Jul 11 '12 at 08:15
Thanks. I will try another solution: same short name, same strong-name key, but different version number. Normal binding policies should do the trick. – Gael Fraiteur Jul 11 '12 at 12:07

leppie · Answer 2 · 2012-07-10T08:01:42.323

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"

edited Jul 10 '12 at 08:01

answered Jul 10 '12 at 07:55

leppie

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The `TargetFrameworkAttribute` is also present for Client and Full Framework builds for v4, it isn't around for v3.5. This might be why they added the attribute. As of v4, it also offers a very easy way to determine if an assembly was built for the full framework or the client profile. – Adam Houldsworth Jul 10 '12 at 08:05
@AdamHouldsworth: Thanks, and I guess it is obsolete now given the client profile is gone in 4.5? ;p – leppie Jul 10 '12 at 08:07
Even in .NET 4 the difference between the client profile and full download was a couple of MB lol, not worth it - probably why they canned it in favour of PCL. – Adam Houldsworth Jul 10 '12 at 08:08
Thanks. The reference to the portable mscorlib.dll may be the point. – Gael Fraiteur Jul 10 '12 at 10:58
The TargetFrameworkAttribute is actually meaningless when it comes to portability. Old style portable (ie mscorlib-based), it is based on the retargetable reference to mscorlib. New style portable (ie System.Runtime-based) it soley based on the fact that System.Runtime, et al are present at runtime. – David Kean Jul 11 '12 at 04:21
1

We discuss this at depth over here: http://channel9.msdn.com/Shows/Going+Deep/NET-45-David-Kean-and-Marcea-Trofin-Portable-Libraries. – David Kean Jul 11 '12 at 04:23
@DavidKean: Thanks, going to check that out now :) – leppie Jul 11 '12 at 04:51

score 0 · Answer 3 · answered Jul 10 '12 at 11:00

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.

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

3 Answers3

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