Converting Hexadecimal String to/from Byte Array in C#

public static class Conversion
{
   static readonly byte offset_0 = (byte)'0';
   static readonly byte offset_A = (byte)'A';
   static readonly byte offset_A16 = (byte)(offset_A - 10);
   static readonly byte offset_a = (byte)'a';
   static readonly byte offset_a16 = (byte)(offset_a - 10);

   public static byte[] StringToByteArray(string hex, int start, int length)
   {
      // Characters are evaluated in pairs, so 'length' must be even.
      // TODO: Handle the odd case.
      Debug.Assert(length % 2 == 0);

      byte[] bytes = new byte[length / 2];

      int idx = 0;
      int end = start + length;

      for (int i = start; i < end; i += 2)
      {
         byte highNibble = (byte)hex[i];
         if (highNibble >= offset_a)
            highNibble -= offset_a16;
         else if (highNibble >= offset_A)
            highNibble -= offset_A16;
         else
            highNibble -= offset_0;

         byte lowNibble = (byte)hex[i + 1];
         if (lowNibble >= offset_a)
            lowNibble -= offset_a16;
         else if (lowNibble >= offset_A)
            lowNibble -= offset_A16;
         else
            lowNibble -= offset_0;

         bytes[idx++] = (byte)(16 * highNibble + lowNibble);
      }
      return bytes;
   }
}

        public static byte[] GetBytes(string hexString) {

            if (null == hexString)
                return new byte[0];
            if (hexString.Length < 2) // Need at least 2 chars to make one byte
                return new byte[0];

            // Set the initial capacity to handle hexString being all valid hex chars
            List<byte> byteList = new List(hexString.Length/2);
            char c1 = 'x'; // Has to be initialized to compile though this value never used
            bool have1 = false;

            // Step through the string, appending a byte every two valid characters
            for (int i = 0; i < hexString.Length; i++) {
                char c = hexString[i];
                // System.Uri.IsHexDigit from comments on this page.
                //  http://www.codeproject.com/KB/recipes/hexencoding.aspx?fid=15401&fr=26#xx0xx
                if (Uri.IsHexDigit(c)) 
                    if (have1) {
                        // "c" is the 2nd char.  Get the byte & reset var.
                        byteList.Add(HexToByte(new string(new char[] {c1, c})));
                        have1 = false;
                    } else {
                        // This is the first char
                        have1 = true;
                        c1 = c;
                    }
            }
            return byteList.ToArray();
        }

List<byte> byteList = new List(hexString.Length/2);

List<byte> byteList = new List<byte>(hexString.Length / 2);

    private Byte[] hexStringToByteArray(string hexinput)
    {
        if (hexinput == String.Empty)
            return null;
        if (hexinput.Length % 2 == 1)
            hexinput = "0" + hexinput;
        int arr_size = hexinput.Length / 2;
        Byte[] myBytes = new Byte[arr_size];
        for (int i = 0; i < arr_size; i++)
            myBytes[i] = Convert.ToByte(hexinput.Substring(i * 2, 2), 16);            
        return myBytes;
    }

    private void test()
    {
        string hex = "15900fF654654651516ABCDEFFF";
        byte[] res = this.hexStringToByteArray(hex);
        string disp = String.Empty;
        foreach (byte b in res)
            disp += b.ToString("X2") + " - " + b.ToString() + Environment.NewLine;
        MessageBox.Show(disp);
    }

private static byte[] HexToData(string hexString)
{
    if (hexString == null)
        return null;

    if (hexString.Length % 2 == 1)
        hexString = '0' + hexString; // Up to you whether to pad the first or last byte

    byte[] data = new byte[hexString.Length / 2];

    for (int i = 0; i < data.Length; i++)
        data[i] = Convert.ToByte(hexString.Substring(i * 2, 2), 16);

    return data;
}