Inconsistent accessibility: base class is less accessible than child class

You are placing your nested classes within another internal class.

For example, given:

class Program
{
    static void Main(string[] args)
    {
    }

    internal class A { }
    public class B : A { }
}

It will compile because the internal modifier of the wrapping class makes the public modifier on class B moot. Rather, type B's accessibility is limited by its wrapped class Program -- its accessibility domain is internal as well.

If you update it to be:

class Program
{
    static void Main(string[] args)
    {
    }
}

internal class A { }
public class B : A { }

It will throw the inconsistent visibility compiler error. Or if you redefine Program to be public instead of internal it will also throw the error. In this case, B's accessibility domain is now public and no longer limited by Program's internal accessibility domain.

---

From the C# specification 3.5.2 Accessibility Domains:

The accessibility domain of a nested member M declared in a type T within a program P is defined as follows (noting that M itself may possibly be a type):

If the declared accessibility of M is public, the accessibility domain of M is the accessibility domain of T.

And the MSDN's description of Accessibility Domain:

If the member is nested within another type, its accessibility domain is determined by both the accessibility level of the member and the accessibility domain of the immediately containing type.

If the wrapping type Program is internal, then the nested type B being public will have its accessibility to match Program, thus it is treated as internal and no compiler error is thrown.