Internal abstract class: how to hide usage outside assembly?

I have a common assembly/project that has an abstract base class, then several derived classes that I want to make public to other assemblies.

I don't want the abstract base class to show up in these other assemblies in Intellisense, so I thought I'd make it internal, but I get this error:

Inconsistent accessibility: base class 'Settings' is less accessible than class 'IrcSettings' ....

I don't really get this. I am forced to make the abstract Settings class public, and thus visible outside this assembly.

How can I make this class internal instead?

As I understand, you want your abstract class to only be implemented by other classes in the same assembly (e.g. it is internal) but the derived classes could be public.

The way to do this is to make the abstract base class public, but give it an internal default constructor:

public abstract class MyClass
{
    internal MyClass() { }
}

This will allow MyClass (and hence its members) to be visible and usable to classes outside your assembly, but classes outside your assembly cannot inherit from it (will get a compile error).

Edit: If classes which can be seen by external assemblies inherit from MyClass, you cannot prevent MyClass from also being seen - e.g., showing up in Intellisense. However, you can prevent them from being used by following the above.

The abstract base class has to be public, as the entire inheritance heirarchy for a class has to be visible. This ensures the polymorphism works and is valid; however all the base classes' members can be internal (including the constructor), and hence not usable outside your assembly