Reference an unmanaged C++ DLL from Managed C++

Question

This question follows from my previous question: Create DLL from unmanaged C++, but you would not have to read it to understand this new question.

I now have a DLL that contains unmanaged C++ code consisting of a few functions, of which I only export one for outside use. Next, I need to use this DLL in a Managed C++ project (built with Common Language Runtime support). So far, I have added a reference to the existing unmanaged project's header file by setting the Additional Include Directories in Visual Studio 2010.

If I now try to add a reference to the DLL file: MyManagedProject > Properties > Common Properties > Add New Reference > Browse > MyUnmanagedDLL.dll, I get an error Could not add a reference to file MyUnmanagedDLL.dll because it is neither a .NET assembly nor a registered ActiveX control. However, if I take the previously mentioned DLL and I copy it to the same folder as my Managed C++'s executable, everything works fine.

I am not sure if this is the correct way to do it, as it seems to be a hackish solution. Also, it means that I have to copy the DLL across every time I make a change to its source code.

To summarise my problem:

1. I have created a DLL consisting of unmanaged C++ code, let's call it MyUnmanagedDLL.dll. I have also created a header file for this DLL.
2. I have a Managed C++ project (CLR), let's call it MyManagedProject.vxproj
3. How can I use MyUnmanagedDLL.dll in MyManagedProject.vxproj? Is there a correct way to reference it, or should I just copy it to the same directory as MyManagedProject.exe? (This works but I'm not sure if its good practise...)

Extra information: Windows 7, Visual Studio 2010 Ultimate, CMake 2.8.10.2

Answer 1

You cannot add the unmanaged DLL as a reference to your managed C++ project. You can only do that with managed DLLs. What you do instead is link to the unmanaged DLL in the same way as you link to an unmanaged DLL in an unmanaged C++ project:

• Use the header file for compilation.
• Supply the unmanaged DLL's .lib file to the linker, for example by adding it to the list of Additional Dependencies list in the linker configuration pages.
• Put the DLL in the same directory as the executable, so that it can be located by the loader.

Answer 2

Apparently, the only solution is to pass the unmanaged dll path in method decorated with DllImport Attribute.

What you can do to keep things in a neat way is to create a lib folder to put the unmanaged dll's and mark their property ''Copy to output directory'' as ''Copy Always'' (Right-Click over the unmanaged dll->Properties->Copy to output directory: Copy Always).

In your method DllImport attribute, you must specify the parameters as "lib/unmanaged.dll". In C#, you would have something like this.

[DllImport("lib/testLib.dll")]
private static extern int DisplayHelloFromDLL();

This approach has no difference from previous answers but it will help you to keep code a little bit more neat.