Perfect! The source code for that class method is exactly what I need, thank you.

Just for others who have the same problem, you can do the following:

do$System.Encryption.MD5HashInit()
do$System.Encryption.MD5HashUpdate(stream.Read(...)) // Read in each chunk streamset hash = $System.Encryption.MD5HashFinal()
set checksum = $ZCONVERT(##class(%xsd.hexBinary).LogicalToXSD(hash), "L")

That's perfect thank you. To answer your and Enrico's question - I'm working against a FHIR spec and series of examples which present it in this way, so to avoid any potential problems I'm trying to match these as much as possible. It may be valid XML but I'm not taking any chances!

We're still using Healthshare 2017.2 unfortunately. Do you know if there is anything similar for our version?

Perfect, thank you, that works. I overlooked this because the docs said "ELEMENT" is the default behaviour so I assumed this was also the case for collection properties.

That's great thank you very much for your help