InternalsVisibleTo with strong-named assemblies

The InternalsVisibleTo attribute is commonly used to expose internal members of an assembly to a test project. This lets you test internal methods without reflection, making your tests more maintainable.

If the assembly does not have a strong name, adding the InternalsVisibleTo attribute is straightforward:

// Project
[assembly: InternalsVisibleTo("TestProject")] // The TestPrject will have access to the internal types and members

public static class Sample
{
    internal void Fibonacci(int n)
    {
        // TODO
    }
}

// Testproject
[TestClass]
public class Test
{
    [TestMethod]
    public void TestFibo()
    {
        Sample.Fibonacci(0); // No error
    }
}

However, if the assembly has a strong name, the InternalsVisibleTo attribute must include the PublicKey. Retrieving the PublicKey (not the PublicKeyToken) is not immediately obvious, so here are the steps.

This assumes you already have a .snk file.

1. Find sn.exe in the Windows SDK (C:\Program Files\Microsoft SDKs\Windows\v6.0A\Bin)

2. Extract the public key of the snk file

   Shellcopy

   sn.exe -p file.snk file.PublicKey

3. Get the human-readable version of the public key

   Shellcopy

   sn.exe -tp SomeName.PublicKey

   This prints a long hexadecimal string to the console.

4. Use the public key in the InternalsVisibleTo attribute:

   C#copy

   [assembly: InternalsVisibleTo("ProjectTest, PublicKey=002400000...")]

If the public key does not match, you will get a compilation error.

Do you have a question or a suggestion about this post? Contact me!