InternalsVisibleTo does not work

If your assembly is signed with a strong name look at this answer.

Otherwise check that the name of your test assembly really is "MyTests.dll" (it doesn't have to match the project name, though it will by default).

Let's break it down a bit as many of us have experienced this slight mix-up in the past...

Assembly A has your internal class. Assembly B has your unit tests.

You wish to grant the internals of assembly A visibility in assembly B.

You need to put the InternalsVisibleTo assembly attribute inside assembly A and grant access to assembly B.

You still need your test project to reference your main project.

This can be easy to overlook and if you have no existing test code this may appear like the InternalsVisibleTo is not functioning.

In my case I was coding to an Interface. As interfaces can only specify public properties the internal properties were not present on the interface.

Make sure you're not doing the same thing as I was!

I changed:

Assert.IsNotNull((exportFileManager)?.ThumbnailFileExporter);

To:

Assert.IsNotNull((exportFileManager as ExportFileManager)?.ThumbnailFileExporter);

I had the same issue after renaming a namespace. Basically the in .cs files was the new namespace but in the .csproj and AssemblyInfo.cs it was the old namespace.

namespace newNamespace {
   ....
}

So, I changed in .csproj thw following to the new namespace:

    <RootNamespace>oldnamespace</RootNamespace>
    <AssemblyName>oldnamespace</AssemblyName>

And in AssemblyInfo.cs:

[assembly: AssemblyTitle("oldnamespace")]
[assembly: AssemblyProduct("oldnamespace")]