C# "internal" access modifier when doing unit testing

I'm trying to figure out if I should start using more of internal access modifier.

I know that if we use internal and set the assembly variable InternalsVisibleTo, we can test functions that we don't want to declare public from the testing project.

This makes me think that I should just always use internal because at least each project (should?) have its own testing project.

Why shouldn't one do this? When should one use private?

Internal classes need to be tested and there is an assembly attribute:

using System.Runtime.CompilerServices;

[assembly:InternalsVisibleTo("MyTests")]

Add this to the project info file, e.g. Properties\AssemblyInfo.cs, for the project under test. In this case "MyTests" is the test project.

Stack Overflow Q#358196, score: 1563