BitConverter.ToString() in reverse? [duplicate]

Answer 1

I believe the following will solve this robustly.

public static byte[] HexStringToBytes(string s)
{
    const string HEX_CHARS = "0123456789ABCDEF";

    if (s.Length == 0)
        return new byte[0];

    if ((s.Length + 1) % 3 != 0)
        throw new FormatException();

    byte[] bytes = new byte[(s.Length + 1) / 3];

    int state = 0; // 0 = expect first digit, 1 = expect second digit, 2 = expect hyphen
    int currentByte = 0;
    int x;
    int value = 0;

    foreach (char c in s)
    {
        switch (state)
        {
            case 0:
                x = HEX_CHARS.IndexOf(Char.ToUpperInvariant(c));
                if (x == -1)
                    throw new FormatException();
                value = x << 4;
                state = 1;
                break;
            case 1:
                x = HEX_CHARS.IndexOf(Char.ToUpperInvariant(c));
                if (x == -1)
                    throw new FormatException();
                bytes[currentByte++] = (byte)(value + x);
                state = 2;
                break;
            case 2:
                if (c != '-')
                    throw new FormatException();
                state = 0;
                break;
        }
    }

    return bytes;
}