Add Reference to c++ dll in C# Windows application

Question

HI,
In my C# windows application, I need to call one VC++ dll to read hardware. When I tried to add reference to dll got error " Reference "TestDll.dll" couldn't be added. Please make sure file is accessible, and that is valid assembly or COM component."
Any help appreciated.
Regards,
Vikram

Vulpes · Answer

It sounds like it's an unmanaged VC++ dll in which case it can't be added as a reference to a C# application. I assume it's not a COM dll either because, if it were, you could use COM interop.
If the dll exports a number of global functions, then you should be able to use Platfrom Invoke (DllImport attribute) to call them from C#. If that's the case, I'd check out this article:
http://www.c-sharpcorner.com/uploadfile/tanmayit08/unmanaged-cpp-dll-call-from-managed-C-Sharp-application/
However, if the dll is an unmanaged class library, then P/Invoke is usually impracticable and a better solution is to wrap the unmanaged classes in a C++/CLI (the managed version of C++) dll which you can then consume from C#. This is possible because, unlike the other .NET languages, a C++/CLI dll can contain a mixture of managed and unmanaged code.
If that's the case, then I'd check this article instead:
http://www.c-sharpcorner.com/UploadFile/SamTomato/managed-cpp-wrapper-for-unmanaged-code/

Sam Hobbs · Answer

For the benefit of anyone seeking answers in the future, see my article Native Windows Dynamic Link Libraries (DLLs) for a description of the type of DLLs that we can usually use P/Invoke with but not make references to for use in managed code.

Sam Hobbs · Answer

The article Managed C++ Wrapper For Unmanaged Code  is my article. I hope it is useful for you if you need it. There is a good chance however that you do not need to do that. Most DLLs can be used using P/Invoke (DllImport). A DLL for a device is the type of DLL that might be easier to use through a C++ wrapper.
As Vulpes indicates, the DLL is not likley a DLL that can be used by adding a reference to the project. The terminology to identify DLLs that can be used by adding a reference is confusing. Microsoft has made the terminology confusing. For this context, I think the best way to describe unmanaged DLLs that can be used by adding a reference is to say that the DLL is listed in the COM tab of the Add Reference dialog. Try using the Browse tab of the Add Reference dialog if you have not done that already. If that does not work then the DLL is probably a native DLL that you must either use Platform Invoke for or write a C++ wrapper for.
Do you have documentation of the DLL? You should. The documentation will make it totally clear whether the DLL is a native DLL.