CS1540 - accessing base class protected member from child class's nested class

[TahirAhmadov]

The following code: SharpLab

public class C {
    protected int _x;
    protected abstract class Base
    {
        protected Base(C owner)
        {
            this._owner = owner;
        }

        public void Foo() { this._owner._x = 1; }

        protected /*internal*/ readonly C _owner;
    }
}
public class D : C
{
    void Foo() { this._x = 1; }
    class Child : Base
    {
        public Child(D owner) : base(owner) 
        { 
            //this._owner = owner;
        }
        public void Bar() { this._owner._x = 1; }
        //protected readonly D _owner;
    }
}

Produces error CS1540: Cannot access protected member 'C._x' via a qualifier of type 'C'; the qualifier must be of type 'D.Child' (or derived from it)
Shouldn't this be legal? After all, Child is nested in D, and D is a subclass of C, so C protected members are visible to D and by extension to everything nested in D. Am I missing anything?

[bernd5]

The wording is:

A protected member is accessible within its class and by derived class instances

the problem here is, that C could be something else than a C or D it could be E which inherits from C, too.
Therefore the rule is violated (it is not within its class and not by by derived class instance).

You could change Bar() to:

public void Bar()
{ 
    if (this._owner is D d)
    {
        d._x = 1; 
    }
}

Or you could make Child a nested class of C - then C and Child would become Friends which allows unrestricted access.