Making code internal but available for unit testing from other projects

If it is an internal class then it must not be getting used in isolation. Therefore you shouldn't really be testing it apart from testing some other class that makes use of that object internally.

Just as you shouldn't test private members of a class, you shouldn't be testing internal classes of a DLL. Those classes are implementation details of some publicly accessible class, and therefore should be well exercised through other unit tests.

The idea is that you only want to test the behavior of a class because if you test internal implementation details then your tests will be brittle. You should be able to change the implementation details of any class without breaking all your tests.

If you find that you really need to test that class, then you might want to reexamine why that class is internal in the first place.

Below are ways to use in .NET Core applications.

1. Add AssemblyInfo.cs file and add [assembly: InternalsVisibleTo("AssemblytoVisible")]
2. Add this in .csproj file (the project which contains the Internal classes)

<ItemGroup>
  <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
    <_Parameter1>Test_Project_Name</_Parameter1> <!-- The name of the project that you want the Internal class to be visible To it -->
  </AssemblyAttribute>
</ItemGroup>

For more information please follow https://improveandrepeat.com/2019/12/how-to-test-your-internal-classes-in-c/

If you're using .NET, the InternalsVisibleTo assembly attribute allows you to create "friend" assemblies. These are specific strongly named assemblies that are allowed to access internal classes and members of the other assembly.

Note, this should be used with discretion as it tightly couples the involved assemblies. A common use for InternalsVisibleTo is for unit testing projects. It's probably not a good choice for use in your actual application assemblies, for the reason stated above.

Example:

[assembly: InternalsVisibleTo("NameAssemblyYouWantToPermitAccess")]
namespace NameOfYourNameSpace
{