Why can't my .NET CF application find a DLL In the same directory as the executable?

Question

I have a .NET CF 1.1 application that has been running perfectly fine for years. Occasionally, I get a help desk ticket with the following error message (generic):

Method not found: 
    MethodName
    AssemblyNamespace.Class

The DLL is there, and it's the same version as my other devices. What could change that would make it not find the method. Does this error imply that the assembly was loaded, or did it break before that even happened?

Does it matter how I added the reference in Visual Studio (2003, btw)? I have Copy Local set to true, and therefore I made the assumption that it needs to be in the same directory as my executable.

Any help you can offer is appreciated. Thanks.

EDIT: I believe this DLL is also in the windows directory of the device, possibly a different version. It contains a lot of hardware specific functions (ie backlight, keyboard state, etc.)

Answer 1

No it does not matter how you add the reference in Visual studio 2003. (100% sure)
Now for the not so sure : I think that someone else has the same assembly in the GAC of the phone. So the GAC assembly gets called rather than the one you deployed and that version of the assembly lacks the requested function.

Answer 2

Depends on how you reference the file in your code. You could be relying on the current working directory which depending on how the app is launched or what they do after, it might not be what you expect it to be.

Answer 3

Sometimes .NetCf throws this exception when the device is running out of memory, instead of the expected "Out of Memory" exception. I have observed this behaviour more often when loading native dlls using P/Invoke than loading pure netcf dlls.