GuidUtils.cs

public static class GuidUtils
{
	/// <summary>
	/// Creates a name-based UUID using the algorithm from RFC 4122 §4.3, using SHA1
	/// (version 5). This is useful for creating predictive Guid based on content.
	/// </summary>
	/// <param name="namespaceId">A known namespace to create the UUID within</param>
	/// <param name="name">The name (within the given namespace) to make the Guid from</param>
	/// <returns></returns>
	public static Guid Create(Guid namespaceId, string name)
	{
		if (name is null)
			throw new ArgumentNullException(nameof(name));

		return Create(namespaceId, Encoding.UTF8.GetBytes(name));
	}

	/// <summary>
	/// Creates a name-based UUID using the algorithm from RFC 4122 §4.3, using SHA1
	/// (version 5). This is useful for creating predictive Guid based on content.
	/// </summary>
	/// <param name="namespaceId">A known namespace to create the UUID within</param>
	/// <param name="nameBytes">The name (within the given namespace) to make the Guid from</param>
	/// <returns>A UUID derived from the namespace and name</returns>
	public static Guid Create(Guid namespaceId, byte[] nameBytes)
	{
		const int version = 5;

		// convert the namespace UUID to network order (step 3)
		byte[] namespaceBytes = namespaceId.ToByteArray();
		SwapByteOrder(namespaceBytes);

		// compute the hash of the namespace ID concatenated with the name (step 4)
		byte[] data = namespaceBytes.Concat(nameBytes).ToArray();
		byte[] hash;
		using (var algorithm = SHA1.Create())
			hash = algorithm.ComputeHash(data);

		// most bytes from the hash are copied straight to the bytes of the new GUID (steps 5-7, 9, 11-12)
		byte[] newGuid = new byte[16];
		Array.Copy(hash, 0, newGuid, 0, 16);

		// set the four most significant bits (bits 12 through 15) of the time_hi_and_version field to the appropriate 4-bit version number from Section 4.1.3 (step 8)
		newGuid[6] = (byte)((newGuid[6] & 0x0F) | (version << 4));

		// set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively (step 10)
		newGuid[8] = (byte)((newGuid[8] & 0x3F) | 0x80);

		// convert the resulting UUID to local byte order (step 13)
		SwapByteOrder(newGuid);
		return new Guid(newGuid);
	}

	/// <summary>
	/// Converts a Guid to/from network order (MSB-first)
	/// </summary>
	/// <param name="guid"></param>
	private static void SwapByteOrder(byte[] guid)
	{
		SwapBytes(guid, 0, 3);
		SwapBytes(guid, 1, 2);
		SwapBytes(guid, 4, 5);
		SwapBytes(guid, 6, 7);
	}

	private static void SwapBytes(byte[] guid, int left, int right)
	{
		byte temp = guid[left];
		guid[left] = guid[right];
		guid[right] = temp;
	}
}