Moles and InternalsVisibleTo

The InternalsVisibleTo assembly-level attribute is a pretty cool little bit of wizardry. By adding one (typically to your AssemblyInfo.cs file), you can declare a “friend” assembly that gets access to all types and members marked “internal.” If you’ve ever right clicked an internal type or member in VS and selected “Create Unit Test,” you know that VS will generate this for you. It’s great for unit testing – while some people believe that you should only unit test the external interfaces of your types, I don’t see any reason not to break it down further when you can.

Moles assemblies can take advantage of InternalsVisibleTo as well, but they need their own InternalsVisibleTo attribute in your assembly specifically for them. Fortunately, it’s not too tough.

For unsigned assemblies, giving Moles access to the internals is as simple as creating an InternalsVisibleTo attribute in your AssemblyInfo pointing to the Moles assembly name:

[assembly: InternalsVisibleTo(“MyProject.MyClasses.Moles”)]

An assembly can have more than one InternalsVisibleTo attribute, so you can point one to your test project and one to the Moles assembly.

If your assembly is signed, there are a couple other hoops to jump through. Information floating around on the Internet confuses things a bit, but the fact of the matter is that if the assembly you want to increase visibility to is signed, the friend assembly has to be signed as well, and the InternalsVisibleTo attribute has to include the public key of the friend assembly. Not the public key token, but the entire public key. Unfortunately, the IntelliSense documentation for InternalsVisibleTo has absolutely no reference to this and does not show how to format things properly. The correct format is like so (with an example of a real public key, so as not to confuse you with brackets and other things, and to show you just how long they are):

[assembly: InternalsVisibleTo(“MyProject.MyClasses.MyFriendAssembly, PublicKey=0024000004800000940000000602000000240000525341310004000001000100b3a1f96b163680bfd8f36cd4a2ee94accb680d97d407ed8898abc1710662205878d27c138902bd3a08a19c6b2afacdf95a4de2cbd3b6fd2cc18e08540ad79cdea4ab5e81ea3c8191fbfdb72efa90c6d750224ae05fae6ab9fe1ebc6958423dbb644a36c7019dc8388e925a802e33d1902ed293fd0f420a1dcb4e135cff7ee8c5”)]

But how do you sign a Moles assembly, and how do you get the public key from it? It’s actually pretty simple. If you mole a signed assembly, the Moles assembly will be signed with a key packaged up in the framework and used specifically for this purpose. To get the key, first generate the Moles assembly without trying to use InternalsVisibleTo. Once you’ve got it, open up a Visual Studio command prompt, and do (case sensitive) “sn –Tp MyProject.MyClasses.Moles.dll”. This will spit out the public key, which you can use to create the InternalsVisibleTo attribute on your assembly. Finally, rebuild both projects to regenerate the Moles assembly, and you’re good to go.