using System;

using System.Collections.Generic;

using System.Threading.Tasks;

using System.Threading;

using System.Text;

using System.Linq;

using System.Collections;

​

public class HelloWorld

{   

    public static void Main(String[] args)

    {

        var s = new StringBuilder();

        for(var i = 0; i < 256; i++)

        {

            Console.WriteLine("{0} = {1}", i, (char) i);

            s.Append((char) i);

        }

​

        Console.WriteLine(s);

        Console.WriteLine(FilterString(s.ToString()).ToArray());

    }

    public static IEnumerable<char> FilterString(string input)

    {

        foreach(var c in input)

        {

            switch ((int)c)

            {

                case >= 48 when c <= 57:

                case >= 65 when c <= 90:

                case >= 97 when c <= 122:

                    yield return c;

                    break;

                default:

                    continue;

            }

        }

    }

}