Unit testing internal methods

Posted on: January 12, 2009

Here’s a little gem.

Most people like to keep their unit test assemblies and code separate from the code being tested (maintainability is such a nice thing). This however raises the issue of how to write tests for methods and classes that are marked internal.

Lots of different approaches exist, such as editing the *.csproj file by hand to include (inject) the code file containing the tests in to the code being tested. This, however is not a very smooth approach.

Thankfully there is a much easier approach using a technique called Friend Assemblies. Basically it’s a way of saying that Assembly A and Assembly B are friends, meaning that B can peek into A’s little secrets marked Internal.
Here’s how to set this up:

1. Add the Assembly Attribute ‘InternalsVisibleTo’ to the assembly being tested:
This goes into the file ‘AssemblyInfo.cs’, an autogenerated file in the Properties folder of your project.

[assembly: InternalsVisibleTo(“Tests.dll”)]

2. Make sure that the assembly containing the tests reference the assembly being tested, and that the output matches the name you typed in the Assembly Attribute (in this case ‘Tests.dll’).

One good thing to know is that sometimes your assemblies fail to make friends because the output of you test assembly has not been explicitly named. In such cases you should make sure to compile the test assembly using the ‘/out:’ switch.

As always, MSDN has the full story here:

and here:


2 Responses to "Unit testing internal methods"

Nice one! I’ve got a question though:
Doesn’t adding the InternalsVisibleTo attribute to the assemblyinfo file expose all internal methods from all classes in that assembly?
Maybe it would be better to add it only where you need it?

are you talking about adding it as a class attribute?

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