C# to C++/CLI to C DLL System.IO.FileNotFoundException

Question

I'm getting System.IO.FileNotFoundException: The specified module could not be found when running C# code that calls a C++/CLI assembly which in turn calls a pure C DLL. It happens as soon as an object is instantiated that calls the pure C DLL functions.

BackingStore is pure C. CPPDemoViewModel is C++/CLI calling BackingStore it has a reference to BackingStore.

I tried the simplest possible case - add a new C# unit test project that just tries to create an object defined in CPPDemoViewModel . I added a reference from the C# project to CPPDemoViewModel .

A C++/CLI test project works fine with just the added ref to CPPDemoViewModel so it's something about going between the languages.

I'm using Visual Studio 2008 SP1 with .Net 3.5 SP1. I'm building on Vista x64 but have been careful to make sure my Platform target is set to x86.

This feels like something stupid and obvious I'm missing but it would be even more stupid of me to waste time trying to solve it in private so I'm out here embarrassing myself!

This is a test for a project porting a huge amount of legacy C code which I'm keeping in a DLL with a ViewModel implemented in C++/CLI.

edit After checking directories, I can confirm that the BackingStore.dll has not been copied.

I have the standard unique project folders created with a typical multi-project solution.

WPFViewModelInCPP
  BackingStore
  CPPViewModel
  CPPViewModelTestInCS
    bin
      Debug
  Debug

The higher-level Debug appears to be a common folder used by the C and C++/CLI projects, to my surprise.

WPFViewModelInCPP\Debug contains BackingStore.dll, CPPDemoViewModel.dll, CPPViewModelTest.dll and their associated .ilk and .pdb files

WPFViewModelInCPP\CPPViewModelTestInCS\bin\Debug contains CPPDemoViewModel and CPPViewModelTestInCS .dll and .pdb files but not BackingStore. However, manually copying BackingStore into that directory did not fix the error.

CPPDemoViewModel has the property Copy Local set which I assume is responsible for copying its DLL when if is referenced. I can't add a reference from a C# project to a pure C DLL - it just says A Reference to Backing Store could not be added.

I'm not sure if I have just one problem or two.

I can use an old-fashioned copying build step to copy the BackingStore.dll into any given C# project's directories, although I'd hoped the new .net model didn't require that.

DependencyWalker is telling me that the missing file is GPSVC.dll which has been suggested indicates security setting issues. I suspect this is a red herring.

edit2 With a manual copy of BackingStore.dll to be adjacent to the executable, the GUI now works fine. The C# Test Project still has problems which I suspect is due to the runtime environment of a test project but I can live without that for now.

Answer 1

Are the C and C++ DLLs in the same directory as the C# assembly that's executing?

You may have to change your project output settings so that the C# assembly and the other DLLs all end up in the same folder.

I've often used the Dependency Walker in cases like this; it's a sanity check that shows that all the dependencies can actually be found.

Once your app is running, you may also want to try out Process Monitor on the code you are running, to see which DLLs are being referenced, and where they are located.