In my previous post, I talked about how ClickOnce udpates applications but forgot to mention something specific to data files. ClickOnce deployed applications might use files that need to be migrated from one version to another. For example, an access database. ClickOnce supports this but with respect to downloading updates, data files are always downloaded (and need to be synchronized with the version on the client). ClickOnce doesn't even look at the hash when it sees that a file is marked as a data file.