This copy of Mono.Cecil is a snapshot of the upstream Mono.Cecil located at: http://github.com/jbevain/cecil To update the snapshot, simply run: ruby update.rb in mono/mcs/class/Mono.Cecil. You need to have an updated clone of the upstream cecil in a sibling folder of your mono clone. You may have to update mono/data/cecil.pc.in if Cecil's assembly version changed since the last update. This gets picked up by mono-tools to locate Cecil.