using System;

​

public class Program

{

    public class B {

        private int _x;

        public virtual void SetX(int x) {

            if (x < 0) throw new Exception("less 0");

            else _x = x;

        }

    }

    public class A : B {        

        public override void SetX(int x) {

            if (x > 100) throw new Exception("more 100");

            base.SetX(x);

        }

    }

    public static void Foo(B b) { b.SetX(100500); }

    public static void Main()

    {

        var b = new B();

        var a = new A();

        Foo(b);

        Foo(a);

    }

}