Question

So I have an unmanaged DLL that exports only a C style factory method that returns a new instance of a class (simplified here to look simple).

hello.h

#if defined(HWLIBRARY_EXPORT) // inside DLL
#   define HWAPI   __declspec(dllexport)
#else // outside DLL
#   define HWAPI   __declspec(dllimport)
#endif

struct HelloWorld{
   public:
    virtual void sayHello() = 0;
    virtual void release() = 0;
};
extern "C" HWAPI HelloWorld* GetHW();

hello.cpp

  #include "hello.h"

struct HelloWorldImpl : HelloWorld
{
    void sayHello(){
        int triv;
        std::cout<<"Hello World!";
        std::cin>>triv;
    };
    void release(){
        this->HelloWorldImpl::~HelloWorldImpl();
    };
};
HelloWorld* GetHW(){
    HelloWorld* ptr = new HelloWorldImpl();
    return ptr;
};

Now, I can use dllimport to access GetHW() but is there a way to access the member functions of the returned ‘struct’… ie, sayHello and release?

Answer 1

Maybe there are some other ways and tricks, but since your class is not a POD the safe and tried way for me is to export each member function as a nonmember function with an additional pointer argument.

extern "C" HWAPI HelloWorld* GetHW();
extern "C" HWAPI void SayHello(HelloWorld* p); //{p->SayHello();}
extern "C" HWAPI void Releasr(HelloWorld* p); //{p->Release();}

In C#, you import all these, then create a wrapper class that will do something like this:

class HelloWorld
{
   public HelloWorld() {ptr = Imports.GetHW();}
   public void SayHello {Imports.SayHello(ptr); }
   public ~HelloWorld() {Imports.Release(ptr);}
   private IntPtr ptr;
}