cannot make publicly visible class from internal class?

Question

I have an internal class:

internal abstract class Reader
{
    //internal abstract void methods
}

I cannot make this class:

public abstract class CustomFileReader : Reader
{
}

Because it is more visible than the class it inherits from. I want to do this to enforce that you must inherit from a publicly accessible inheritor of the Reader class and not the base class. Is this possible or do I have to expose the base class?

Answer 1

You can't have an internal class in the inheritance tree of a public class, but you can force users to derive from CustomFileReader instead of Reader, by making the only constructor of Reader internal:

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

public abstract class CustomFileReader : Reader
{
    // Public and protected constructors here
}

Now anything which tries to inherit directly from Reader outside your assembly will be told that it can't find an accessible base class constructor.