AppDomain and locked assembly

[Derek Young]

I have a unit test I've written that tests a lot of managed
C++/unmanaged C++/C# interactions. I'm running it under NUnit which
creates a separate AppDomain for the test, runs it, then unloads the
AppDomain.

This works fine but one of my assemblies stays loaded/locked and
prevents the directory from being deleted. The AppDomain unloads fine
and every other DLL and PDB is unloaded correctly.

If I use Process Explorer I can see that there is a file handle open
to the assembly. The DLL is not shown in the list of loaded DLLs
however.
The AppDomain unload call should guarantee that all references to the
DLL are removed, correct? How is it possible that the process can
maintain a handle to an assembly whose AppDomain has been unloaded?

FWIW, I changed one of my DLLs to compile without any managed code
(and without /clr). The problem went away. Compiling it with /clr
but without other code changes causes the problem to come back. I'm
not sure if this is relevant.

-- derek