using System;

​

public class Program

    {

        public static void Main(string[] args)

        {

            var textInput = new TextInput();

            textInput.Add('a');

            Console.WriteLine("Adding 'a' as text input. string value: {0}", textInput.GetValue());

            textInput.Add('s');

            Console.WriteLine("Adding 's' as text input. string value: {0}", textInput.GetValue());

​

            var numericInput = new NumericInput();

            numericInput.Add('a');

            Console.WriteLine("Adding 'a' as numeric input. string value: {0}", numericInput.GetValue());

            numericInput.Add('2');

            Console.WriteLine("Adding '2' as numeric input. string value: {0}", numericInput.GetValue());

            numericInput.Add('3');

            Console.WriteLine("Adding '3' as numeric input. string value: {0}", numericInput.GetValue());

​

        }

    }

​

    public class TextInput

    {

        protected string text;

​

        public virtual void Add(char c)

        {

            text += c;

        }

​

        public string GetValue()

        {

            return text;

        }

    }

​

    public class NumericInput : TextInput

    {

        public override void Add(char c)

        {

            if (char.IsDigit(c))

                text += c;

        }

    }