How to call C DLL functions C++/CLI

Question

I have looked into lot of online searches and most have examples which talk about C# with C or C++ with C# but have never come across using C DLL calling from C++ (specifically from C++/CLI with WPF in my case).

Can someone please explain with an example how to call C DLL functions in C++. I have a C DLL which has all its functions defined as extern "C" funcName() and there is also a export functions .def file which has all the function names which need to be exported. Now having this C DLL how can I call its exported functions in a C++ code.

Thanks.

Answer 1

OK, so "WPF with C++" is not a language. WPF is a .NET library. C++/CLI is the language you are writing in.

Calling C dlls from C++/CLI is "pretty darn easy", depending on how the DLL was exported. You can either use the .NET libraries to do this, or the C libraries (I suggest the C libraries). There are two ways to link to a dll: implicit linking or explicit linking.

Implicit linking is much cleaner, imho. But you require access to a header file (Edit: possibly with declspec decorations, see BenVoigt's notes below) and a .lib file. If you have any of these, simply add the .lib file to the Additional Dependencies (right-click project->properties->Configuration Properties->Linker->Input) and add the header file path to the include (right-click project->properties->Configuration Properties->C/C++->General). Then you can include the header file (props to metacubed for mentioning this) as an extern "C" header file:

extern "C"
{
   #inlude "c_header.h"
}

(this is because C++ does name-mangling so that you can have overloading & namespaces & classes and stuff. It'll interpret your C header file as a C++ header file (and mangle all your names) if you don't use the extern "C")

http://msdn.microsoft.com/en-us/library/d14wsce5.aspx

Or you can link explicitly

http://msdn.microsoft.com/en-us/library/784bt7z7.aspx http://msdn.microsoft.com/en-us/library/zzk20sxw.aspx

But I suspect that figuring out that you're using C++/CLI, that'll tell you what to google. (Though C++ answers will work for you too).

For the .NET way, an easy trick is to look up the C# call (please note that this is also explicit linking, and thus doesn't require the .lib file and headers).

http://msdn.microsoft.com/en-us/library/eyzhw3s8.aspx

Now the tricky bit is that if you're using C++/CLI and want to get your fancy managed classes into C form, you'll have to marshal the code. Here is a handy link to a tabel that shows you how.

http://msdn.microsoft.com/en-us/library/ac7ay120%28vs.71%29.aspx

Answer 2

Calling a C dll function from a C++ dll is exactly the same, whether it is for WPF, C++/CLI or native C++.

1. Include the header file. See below for syntax.
2. Add the .lib as an import dependency to your project. Also set the linker to consume the DLL. See Linking implicitly for all the details.
3. Make sure that the consumed DLL is present in the run-time class path when the program executes.
4. Use the function defined in the header file.

The header file include should be declared as:

extern "C" {
    #include "c_header.h"
}

That's all!

EDIT: Also, check this out: Call a C function from C++ code.