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I have a pretty big system implemented in C++ I need to interact with. The system has a pretty big API, a number of C++ DLLs. These DLLs export C++ classes, as opposed to a nice C style API. and I need to use them from a new C# project.

From what I know .NET has three ways of interacting with native software:

  1. P/Invoke - which works only on C APIs
  2. COM objects
  3. C++/CLI

So the way I understand it, I have three approaches accordingly:

  1. Writing a wrapper in C and calling it with P/Invoke. which seems way too much work.
  2. Writing a wrapper with COM. which I don't know how to do, and unless it's insanely easy i'm reluctant to learn what seems to me - a dying technology .
  3. Writing a wrapper in C++/CLI. which seems the least work, though still a lot.

My question:

  1. First of all I would like to know why doesn't .NET allow me simply to use the C++ classes "as is"? I'm assuming it's a matter of memory management. and if it is I'm more than willing to write finalizers, and implementing IDisposable . From what I know C++ classes are just really fancy structs, and since P/Invoke supports structs, and functions that take structs as the first parameter, why not support classes?

  2. Second, assuming I'm really lazy, and its a lot of boring, tedious, work, What would be the best way to use these DLL's? a possibility to call them directly from C# would be the best. If not then I'd love an automatic tool to produce the wrappers. Also, the DLLs might change, probably, just slightly but still, id rather not be forced to manually re-write wrappers.

For a really good answer, especially on the first part, or a good automatic tool, I'll reward a bounty...

Thank you

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  • 1. Because different DLL's have different calling conventions. Commented Apr 4, 2013 at 22:27
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    stackoverflow.com/questions/2120690/… Commented Apr 4, 2013 at 22:27
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    Cxxi sounds like what you want, but I'm not sure how usable it is yet. Commented Apr 4, 2013 at 22:28
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    I'd imagine the main reason for not supporting C++ classes is because C++ does not have a standardized ABI. And no, C++ classes aren't just fancy structs. A given class might be using [virtual] multiple inheritance for example. The behind the scenes implementation of that is completely up to the discretion of the compiler vendor for example. Commented Apr 4, 2013 at 22:41
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    My searches of the Internet's tubes seem to be that it is easiest to write your own managed C++ to act as wrappers for the unmanaged C++. Check out this write up of the P/Invoke versus the bridging managed C++ DLL at: codeproject.com/Articles/18032/How-to-Marshal-a-C-Class Commented Apr 4, 2013 at 22:48

2 Answers 2

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If you are willing to go the lazy way I would suggest using a tool to generate C# wrapper for your C++ classes. And of course the tool for generating such a wrapper is SWIG

For more information, see my old answer to similar question

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I'm accepting your answer for the SWIG suggestion. though we ended up using P/Invoke to a C Wrapper.
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The way I understand it, in C++ the calling conventions for methods in classes are quite messy. At the level of the .LIB files that are used by the linker, the function names get "decorated" (see e.g. Name mangling). Also, the layout of class variables in memory is presumably different between all the different implementations of C++ compared to C#, which is why Interop Marshaling is such a big topic. So my guess is that Microsoft judged that there are too many low level issues, and hence too hard, to make it worthwile.

I've used both P/Invoke and COM objects to link my C# to C++, but I haven't used C++/CLI. I've been dabbling in COM for years, and it's very much like it's own language. So if you've never done any COM I think you're right that it's not worth learning it at this stage. If a C++/CLI would let you interface to the objects, rather than having to squeeze everything through a C interface, then I think you're right that it'll be the best route.

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