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.

1
63
11/24/2008 6:49:25 PM

Accepted Answer

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.

83
6/29/2018 4:02:59 PM

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; 

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