using a class defined in a c++ dll in c# code

Question

I have a dll that was written in c++, I need to use this dll in my c# code. After searching I found that using P/Invoke would give me access to the function I need, but these functions are defined with in a class and use non-static private member variables. So I need to be able to create an instance of this class to properly use the functions. How can I gain access to this class so that I can create an instance? I have been unable to find a way to do this.

I guess I should note that the c++ dll is not my code.

Answer 1

There is no way to directly use a C++ class in C# code. You can use PInvoke in an indirect fashion to access your type.

The basic pattern is that for every member function in class Foo, create an associated non-member function which calls into the member function.

class Foo {
public:
  int Bar();
};
extern "C" Foo* Foo_Create() { return new Foo(); }
extern "C" int Foo_Bar(Foo* pFoo) { return pFoo->Bar(); }
extern "C" void Foo_Delete(Foo* pFoo) { delete pFoo; }

Now it's a matter of PInvoking these methods into your C# code

[DllImport("Foo.dll")]
public static extern IntPtr Foo_Create();

[DllImport("Foo.dll")]
public static extern int Foo_Bar(IntPtr value);

[DllImport("Foo.dll")]
public static extern void Foo_Delete(IntPtr value);

The downside is you'll have an awkward IntPtr to pass around but it's a somewhat simple matter to create a C# wrapper class around this pointer to create a more usable model.

Even if you don't own this code, you can create another DLL which wraps the original DLL and provides a small PInvoke layer.