New Post: InvalidFormatException not releasing file

I think the problem is with IsComplete. I think you're assessment of the disposing is correct. You have to manage your own streams if you give them to SharpCompress as well as use using blocks liberally.

IsComplete doesn't force the parsing of the archive fully like other operations. This seems like a bug and I think I've now fixed it:
https://sharpcompress.codeplex.com/SourceControl/changeset/2f8ce7087d9b