How to convert hex to a byte array?

Something like this:

using System;

public static class Parser
{    
    static void Main()
    {
        string hex = "0xBAC893CAB8B7FE03C927417A2A3F6A6"
                     + "0BD30FF35E250011CB25507EBFCD5223B";
        byte[] parsed = ParseHex(hex);
        // Just for confirmation...
        Console.WriteLine(BitConverter.ToString(parsed));
    }

    public static byte[] ParseHex(string hex)
    {
        int offset = hex.StartsWith("0x") ? 2 : 0;
        if ((hex.Length % 2) != 0)
        {
            throw new ArgumentException("Invalid length: " + hex.Length);
        }
        byte[] ret = new byte[(hex.Length-offset)/2];

        for (int i=0; i < ret.Length; i++)
        {
            ret[i] = (byte) ((ParseNybble(hex[offset]) << 4) 
                             | ParseNybble(hex[offset+1]));
            offset += 2;
        }
        return ret;
    }        

    static int ParseNybble(char c)
    {
        if (c >= '0' && c <= '9')
        {
            return c-'0';
        }
        if (c >= 'A' && c <= 'F')
        {
            return c-'A'+10;
        }
        if (c >= 'a' && c <= 'f')
        {
            return c-'a'+10;
        }
        throw new ArgumentException("Invalid hex digit: " + c);
    }
}

(EDIT: Now slightly more efficient – no substrings required…)

It’s possible that ParseNybble could be more efficient. For example, a switch/case may be more efficient:

    static int ParseNybble(char c)
    {
        switch (c)
        {
            case '0': case '1': case '2': case '3': case '4':
            case '5': case '6': case '7': case '8': case '9':
                return c-'0';
            case 'A': case 'B': case 'C': case 'D': case 'E': case 'F':
                return c-'A'+10;
            case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
                return c-'a'+10;
        }
        throw new ArgumentException("Invalid hex digit: " + c);
    }

or possibly a lookup array:

    // Omitted for brevity... I'm sure you get the gist
    private static readonly int[] NybbleLookup = BuildLookup();

    private int ParseNybble(char c)
    {
        if (c > 'f')
        {
            throw new ArgumentException("Invalid hex digit: " + c);
        }
        int ret = NybbleLookup[c];
        if (ret == -1)
        {
            throw new ArgumentException("Invalid hex digit: " + c);
        }
        return ret;
    }

I haven’t benchmarked any of these, and I’ve no idea which would be the fastest. The current solution is probably the simplest though.