how to inter call object's interface class between C# app and C++dll

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C++

C#

In a my c++ dll do like this:

my dll headerfile(eg:test.h):

(1) define a callback interface class:

class CClassCallback
{
public:
   virtual HRESULT OnAdded() = 0;
   virtual HRESULT OnDeled() = 0;
   virtual HRESULT OnFailed() = 0;
};

(2) a struct like this:

struct SAType
{
  CString         strTest;
  int             nTest;
  CClassCallback* pClassCallback;
};

(3)a object interface class like:

class CTestInterface
{
public:
	virtual HRESULT Test1(SAType* pTest) = 0;
	virtual HRESULT Test2() = 0;
	virtual void Release() = 0;
};

(4) the only export dll function:

__declspec(dllexport) CTestInterface* ConstructInterface();

my dll cppfile(eg:test.cpp):

(1) derived a class from CTestInterface

class CTestObject:public CTestInterface
{
public:
	HRESULT Test1(SAtype& Test) 
	{
		m_sType = Test;
	}
	HRESULT Test2()
	{
		
	}
	void Release()
	{
		delete this; //delete this object
	}
private:
	void InnerDemoFuc()
	{
		if(true)
		{
//in some condition callback the app by a callback object
			m_sType.pClassCallback->OnAdded();
		}
	}
	SAtype m_sType;
}

(2) construct a object and return its interface:

CTestInterface* ConstructInterface()
{
    return new CTestObject();
}

that's ok for dll.(real code is more complex)

when I use my dll in MFC UI APP like this:
(1) derive a class from CClassCallback:

class CClassCallbackEx : public CClassCallback
{
public:
	CClassCallbackEx();
	
	virtual HRESULT OnAdded()
	{
		.....//do UI app job independly from dll..
	}
	virtual HRESULT OnDeled()
	{
	}
	virtual HRESULT OnFailed()
	{
	}

// can add other func ,var for app needs.
}

(2) Use the dll like this:

//construct a dll's object,and get its interface
        CTestInterface* m_pInterface = ConstructInterface(); 
 	SAType sAtype;
//new a callback object 
	sAtype->pClassCallback = new CClassCallbackEx();
	m_pInterface->Test1(sAtype);
			
.......

// all are ok ,delete the ojbject through interface
	m_pInterface->Relese();				
	m_pInterface=NULL;	
  	
       delete (CClassCallbackEx)sAtype->pClassCallback ;

run ok.
but when i want use my dll in c# + .net UI app,some questions:
(1) i cannot get the CTestObject instance's interface's member function,
because the interface class is not export.i try to get the object's virture function table,failed.
(2) i have to derive a class from c++ interface in c# code, and use in C++dll, means ojbect callback, not funccallback (OOP)
i donot know how to do.

other words:
(1) C# use a C++Dll ojbect through virture class
(2) C++dll use a C# object through virture class

Give me some suggestions,pls