Accessing Internal Methods in Unit Tests

In our code we often have the case where we want to test a method or property that shouldn’t be visible outside of the implementing assembly. In our build system our tests reside in separate assemblies (so that they don’t clog the production code) – which means we can’t access any methods or properties that aren’t public (I probably should add that we are using NUnit as our unit testing framework).

So far we used a workaround where we created a dummy class in our test assembly and derive it from the class we want to test. Then we add additional methods that call the protected methods we really want to test. This always seems a little dirty, and sometimes it isn’t possible at all.

This morning I came around the real solution: there is a little-known attribute InternalsVisibleTo that works like the friend class concept in C++ and makes internal methods and properties visible to another assembly. You specify the attribute in the AssemblyInfo.cs file and give it the name of the assembly that needs to access internal methods.


Assembly A.dll:

File AssemblyInfo.cs:

...[assembly: System.Runtime.CompilerServices.InternalsVisibleTo("ATests")]...

File ClassA.cs:

public class ClassA
    internal bool DoSomething()
        return true;

Assembly ATests.dll:

File ClassATests.cs:

using NUnit.Framework;
public class ClassATests
    public void TestSomething()
        ClassA a = new ClassA();

6 Responses to “Accessing Internal Methods in Unit Tests”

  1. Andreas Says:

    Looks like you solved the problem that so many did complicated solutions for. Thanks!

  2. sandrar Says:

    Hi! I was surfing and found your blog post… nice! I love your blog. 🙂 Cheers! Sandra. R.

  3. Martin Says:


    thanks for the hint. I always wondered, why are tests so complicated. Now the only thing left to keep me wondering is “Why is the attribute so little-known?”.

  4. sheir Says:

    Is there a similar way to access Private methods in a UT (nunit) class?
    I know that the VS Test allows something called a PrivateObject but have no idea is there is something similar in NUnit.

    • eb Says:

      I don’t think there’s something out of the box. However, you could use reflection to access private methods.

      • sheir Says:

        What I ended up recommending was to create an Internal wrapper method for the Private method, with same signature except, of course, for the scope. Also prefix the method name with UT (for unit test).

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