Use of internal FriendAccessAllowedAttribute in WPF

Question

During a run through Reflector to find the root cause of a bug in a WPF application, I stumbled across an internal attribute FriendAccessAllowedAttribute that is littered across many different classes, members, etc. However, I'm unable to locate what exactly uses this attribute (if anything). My guess is this is part of the C++/CLI infrastructure, but a search of MSDN, the C++/CLI spec, and the CLI spec reveals nothing. Is anybody aware of what it is used for?

Answer 1

As I recall, it was a way to control dependencies between friend assemblies in the Framework. Basically, when you grant assembly A "friend" access to assembly B, you don't want all internal members of B to be made available to A, because then any change to such a member would potentially be breaking (also, it would increase the security surface area that needs to be reviewed). Pretty sure this attribute is honored in Silverlight, not sure about the full Framework.

Answer 2

Interestingly, that attribute is attributed with itself!

Decompiled from C:\Windows\Microsoft.NET\Framework\v4.0.30319\mscorlib.dll, version 4.0:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Enum |
                AttributeTargets.Constructor | AttributeTargets.Method |
                AttributeTargets.Property | AttributeTargets.Field |
                AttributeTargets.Event | AttributeTargets.Interface, 
                AllowMultiple = false, Inherited = false)]
[FriendAccessAllowed]
internal sealed class FriendAccessAllowedAttribute : Attribute
{
}

So I presume it can only be used by selected assemblies which are assigned friends of mscorlib.

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EDIT Microsoft's reference source for the attribute shows the definition is alongside InternalsVisibleToAttribute and includes the comment string:

If AllInternalsVisible is not true for a friend assembly, the FriendAccessAllowed attribute indicates which internals are shared with that friend assembly.