using System;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var enc = new TestEncoding();

        var bytes = enc.GetBytes("helloworld");

        var result = enc.GetChars(bytes);

        Console.WriteLine(result);

    }

}

​

public class TestEncoding : Encoding

{

    private char[] conversionArray = new[]{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};

    public override int GetByteCount(char[] chars, int index, int count)

    {

        return chars.Length;

    }

​

    public override int GetBytes(char[] chars, int charIndex, int charCount, byte[] bytes, int byteIndex)

    {

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

        {

            bytes[byteIndex + i] = GetByte(chars[charIndex + i]);

        }

​

        return charCount;

    }

​

    public override int GetCharCount(byte[] bytes, int index, int count)

    {

        return bytes.Length;

    }

​

    public override int GetChars(byte[] bytes, int byteIndex, int byteCount, char[] chars, int charIndex)

    {

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

        {

            chars[charIndex + i] = conversionArray[bytes[byteIndex + i]];

        }

​

        return byteCount;

    }

​

    public override int GetMaxByteCount(int charCount)

    {

        return charCount;

    }

​

    public override int GetMaxCharCount(int byteCount)

    {

        return byteCount;

    }

​

    private byte GetByte(char c)

    {

        for (var i = 0; i < conversionArray.Length; i++)

        {

            if (conversionArray[i] == c)

                return (byte)i;

        }

​

        throw new Exception("Invalid character");

    }

​

    private char GetChar(byte b)

    {

        return conversionArray[b];

    }

}