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

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.

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.

Marshal C++ Class and use the PInvoke

C++ Code ,ClassName.h

class __declspec(dllexport) CClassName
{
 public:
    CClassName();
    ~CClassName();
    void function();
    
};

C++ Code, ClassName.cpp

CClassName::CClassName()
{
}

CClassName::~CClassName()
{
}

void CClassName::function()
{
    std::cout << "Bla bla bla" << std::endl;
    
}

C++ Code, ClassNameCaller.h file for the caller function

#include "ClassName.h"		

#ifdef __cplusplus
extern "C" {
#endif

extern __declspec(dllexport) CClassName* CreateClassName();

extern __declspec(dllexport) void DisposeClassName(CClassName* a_pObject);

extern __declspec(dllexport) void function(CClassName* a_pObject);


#ifdef __cplusplus
}
#endif

   

C++ Code, ClassNameCaller.cpp file for the caller function

#include "ClassNameCaller.h"


CClassName* CreateClassName()
{
	return new CClassName();
}

void DisposeClassName(CClassName* a_pObject)
{
	if(a_pObject!= NULL)
	{
		delete a_pObject;
		a_pObject= NULL;
	}
}

void function(CClassName* a_pObject)
{
	if(a_pObject!= NULL)
	{
		a_pObject->function();
	}
}

C# code

[DllImport("ClassNameDll.dll")]
static public extern IntPtr CreateClassName();

[DllImport("ClassNameDll.dll")]
static public extern void DisposeClassName(IntPtr pClassNameObject);

[DllImport("ClassNameDll.dll")]
static public extern void CallFunction(IntPtr pClassNameObject);
 
//use the functions
IntPtr pClassName = CreateClassName();

CallFunction(pClassName);

DisposeClassName(pClassName);

pClassName = IntPtr.Zero; 

Here is a sample how to call C++ class method from VB - for C# you only have to rewrite the sample program in Step 4.

You may need to write an intermediary DLL (in C++, perhaps) that handles this for you and exposes the interface you need. Your DLL would be in charge of loading the 3rd party DLL, creating an instance of this C++ object, and exposing its member functions as needed via whatever API you design. You would then use P/Invoke to get at your API and cleanly manipulate the object.

Note: For the API of your DLL, try keeping the data types limited to primitives (long, int, char*, etc.) to prevent module boundary issues.

The way I've done this is by creating a thin Managed C++ wrapper around my unmanaged C++ DLL. The managed wrapper contains "proxy" classes that wrap around the unmanaged code exposing the interface that's needed by the .NET application. This is a bit of double work but it allows quite seamless operation in normal environments. Things do get trickier with dependencies in some circumstances (such as ASP.NET) but you will probably not run into that.

I agree with JaredPar. Creating instances of unmanaged classes in managed code should not be possible.

Another thing is - if you could recompile the DLL in managed C++ or make a COM component out of it, it would be much easier/