.net is *evil*: example.

dot-net-guid.rb

  # Convert a string in the expected format, into 16 bytes, emulating .Net's
  # Guid constructor.
  # "00000000-0000-0000-0000-000000000000" =>
  #  [CLSCompliantAttribute(false)]
  #  public Guid(
  #	  uint a,
  #	  ushort b,
  #	  ushort c,
  #	  byte d,
  #	  byte e,
  #	  byte f,
  #	  byte g,
  #	  byte h,
  #	  byte i,
  #	  byte j,
  #	  byte k
  #  )
  # => 16B
  def dot_net_guid(string)
    pattern = /\A([0-9a-f]{8})-([0-9a-f]{4})-([0-9a-f]{4})-([0-9a-f]{4})-([0-9a-f]{12})\z/i
    parts = pattern.match(string)
    raise ArgumentError unless parts
    
    guid = "".encode("ASCII-8BIT")
    guid << parts[1,1].pack("H*").unpack("l>").pack("l") +
      parts[2,1].pack("H*").unpack("s>").pack("s") +
      parts[3,1].pack("H*").unpack("s>").pack("s") +
      parts[4,2].pack("H*H*")
  end