using System;

​

namespace ConsoleApp1

{

    public class TextInput

    {

        protected string _text = "";

​

        public void Add(char c)

        {

            Console.WriteLine("TextInput");

            _text += c;

        }

​

        public string GetValue()

        {

            return _text;

        }

    }

​

​

    public class NumericInput : TextInput

    {

        public new void Add(char c)

        {

            Console.WriteLine("NumericInput");

            if (!char.IsDigit(c)) return;

            _text += c;

        }

    }

​

​

    public class Program

    {

        public static void Main(string[] args)

        {

            TextInput input = new NumericInput();

            input.Add('1');

            input.Add('a');

            input.Add('0');

            Console.WriteLine(input.GetValue());

​

            Console.WriteLine(char.IsDigit('1'));

            Console.WriteLine(char.IsDigit('a'));

            Console.WriteLine(char.IsDigit('0'));

        }

    }

}