I can't understand why C# doesn't have isxdigit as part of the standard library somewhere, so here's my little version of the venerable C isxdigit:
/// <summary>
/// Is a character 0-9 a-f A-F ?
/// </summary>
public static bool IsXDigit(char c)
{
  if ('0' <= c && c <= '9') return true;
  if ('a' <= c && c <= 'f') return true;
  if ('A' <= c && c <= 'F') return true;
  return false;
}

Thanks for reading! And if you want to get in touch, I'd love to hear from you: chris.hulbert at gmail.