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.