Question

I think you can and my colleage thinks you cannot!

Answer 1

You can’t even declare private virtual methods. The only time it would make any sense at all would be if you had:

public class Outer
{
    private virtual void Foo() {}

    public class Nested : Outer
    {
        private override void Foo() {}
    }
}

… that’s the only scenario in which a type has access to its parent’s private members. However, this is still prohibited:

Test.cs(7,31): error CS0621:
‘Outer.Nested.Foo()’: virtual or
abstract members cannot be private
Test.cs(3,26): error CS0621:
‘Outer.Foo()’: virtual or abstract
members cannot
be private