InternalsVisibleToAttribute

The new InternalsVisibleToAttribute [^] attribute in .NET 2.0 can be used to expose internal members in one assembly to another. This should be used with caution but can be useful in these circumstances:

1. When refactoring code into multiple assemblies, it might make sense to temporarily shut the compiler up by using this attribute instead of deciding what to do with internal types.
2. When unit testing, it might be necessary to access internal members of the assembly under test.

Today, I wanted to use this attribute for scenario 2. So I applied it to my assembly under test (names changed to protect the innocent):

  
[assembly: InternalsVisibleTo("Company.Product.UnitTest")]

  

But this yielded a compiler error:

  
Friend assembly reference 'Company.Product.UnitTest' is invalid.

  
Strong-name signed assemblies must specify a public key in their InternalsVisibleTo declarations.

  

Fair enough, since my assembly is strongly-named. So I take a quick squiz at the documentation for the InternalsVisibleToAttribute [^], noting in particular their example code:

  
[assembly:InternalsVisibleToAttribute("AssemblyB, PublicKey=32ab4ba45e0a69a1")]

  

So I alter my code to specify the public key token:

  
[assembly: InternalsVisibleTo("Company.Product.UnitTest, PublicKey=xxxxxxxxxxxxxxxx")]

  

But the build still failed:

  
Assembly reference 'Company.Product.UnitTest, PublicKey=xxxxxxxxxxxxxxxx' is invalid and cannot be resolved

  

At this point I was a little confused. Why are we specifying a public key token but calling it the public key? I googled [^] for information on the attribute and found other people specifying the public key token instead of the public key. I tried changing my code:

  
[assembly: InternalsVisibleTo("Ingenious.Mvc.UnitTest, PublicKeyToken=xxxxxxxxxxxxxxxx")]

  

But the build still failed:

  
Friend assembly reference 'Company.Product.UnitTest, PublicKeyToken=xxxxxxxxxxxxxxxx' is invalid.

  
Strong-name signed assemblies must specify a public key in their InternalsVisibleTo declarations.

  

The only thing left to try was specifying the whole public key, despite not finding anyone with the same problem and not being prompted by the documentation. So I used the secutil tool:

  
C:\> secutil -hex -s Company.Product.UnitTest.dll

  

Note that the -hex flag must be specified before the -s, otherwise secutil incorrectly treats -hex as the assembly name. Perhaps there is an easier way to extract the public key but it's a rare requirement so I'm not really sure what it is (update: see Jason's comment below for a simpler solution, or this tool). So I copied and pasted the verbose output into my attribute, removing the leading 0x hex indicator:

  
[assembly: InternalsVisibleTo("Company.Product.UnitTest, [assembly: InternalsVisibleTo("Company.Product.UnitTest, PublicKey=AE6A7182001B91...")]

  

Finally, things built and worked as intended. I'll drop off a note at the MS suggestion box and post a link back here once done.

Update: There is a suggestion here.

Update 2: API documentation has been updated here.