I think you can and my colleage thinks you cannot!
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
I cannot even imagine why one should be allowed to create a private virtual method. By definition, a derived class could not see it.
Your colleague is right. You can't declare private virtual methods because there's no point (since there'd be no way to override them)...
But you can override protected virtual methods.