Most C# developers will know what “public”, “private”, “protected” and “internal” access modifiers mean and would have had to use them in their code, but rarely do we come across the “protected internal” access modifier.
Now, protected means the type or member can be accessed by derived classes, NOT restricted to the same assembly, whereas internal means the type or member can be accessed by code ONLY in the same assembly. It’s easy (and common) to think of the access modifiers in a linear sense where private is the most restrictive, and protected, internal and ultimately public becomes less and less restrictive:
This is certainly the case within the SAME assembly, but when you have multiple assemblies this no longer holds true as types might be derived outside of the assembly it’s declared in:
And that is what protected internal gives you – accessibility from any derived classes anywhere, as well as any class from within the same assembly.