It's impossible if the method is declared in the derived class as overrides. to do that, the method in the derived class should be declared as new:
public class Base {
public virtual string X() {
return "Base";
}
}
public class Derived1 : Base
{
public new string X()
{
return "Derived 1";
}
}
public class Derived2 : Base
{
public override string X() {
return "Derived 2";
}
}
Derived1 a = new Derived1();
Base b = new Derived1();
Base c = new Derived2();
a.X(); // returns Derived 1
b.X(); // returns Base
c.X(); // returns Derived 2