Compiler Error C2261
'string' : assembly reference is invalid and cannot be resolved
A value was not valid.
InternalsVisibleToAttribute is used to specify a friend assembly. For example, if a.dll wants to specify b.dll as a friend assembly, you would specify (in a.dll): InternalsVisibleTo("b"). The runtime then allows b.dll to access everything in a.dll (except private types).
For more on the correct syntax when specifying friend assemblies, see Friend Assemblies (C++).
Example
The following sample generates C2261.
// C2261.cpp
// compile with: /clr /c
using namespace System::Runtime::CompilerServices;
[assembly: InternalsVisibleTo("a,a,a")]; // C2261
[assembly: InternalsVisibleTo("a.a")]; // OK
[assembly: InternalsVisibleTo("a")]; // OK