Closing Files Accessed via .NET

I am using several .NET libraries written by a colleague. I use them to loop through a collection of data files in a proprietary format. The process works great, except that Matlab never releases the last file I accessed. I dispose of the object using obj.Dispose when I'm done with it and have even tried using fclose('all'). The only thing that works is closing Matlab entirely. Has anyone had a similar experience? How did you resolve it?