Who’s Locking My Folder?

Recently, I ran across a problem deleting a folder used by my application because the folder “was being used by another process”. Using Process Explorer, I was able to determine that it was my application that was holding the lock on the directory.

Because of the steps to reproduce, I was able to narrow down the method that was locking the folder fairly quickly. Once I found the problem method, I set a break point at the start of the method and fired up my application. When the break point was hit, I stepped over the first statement then checked Process Explorer to see if the folder in question was locked. I repeated these steps for each statement in the method, until Process Explorer reported the folder as open by my application.

The offending statement was a call to show an open file dialog (via the OpenFileDialog class). I proceeded by tweaking/removing the various properties of the OpenFileDialog being set by my code. When I changed the RestoreDirectory property from false to true, then the folder was no longer being locked.

MSDN says that if the RestoreDirectory property is true, then the “dialog box restores the current directory to its original value if the user changed the directory while searching for files”. If RestoreDirectory is false, then it will not restore the current directory to its original value and will maintain the directory previously selected by the user. It is apparent that the file dialog(s) open a handle to the directory previously selected by the user in order to use it as the initial directory the next time a file dialog is shown.

Using Lutz Roeder’s .Net Reflector, I was able to determine that the directory handle is opened by the Windows API (and is not specific to the .Net runtime). Specifically, the OFN_NOCHANGEDIR flag of the OPENFILENAME structure corresponds to the RestoreDirectory property.

To be clear, I don’t consider this a bug. It is just one of those gotchas that can have you tearing your hair out if you don’t know about it. A sample project can be downloaded from here.