Question

I’m trying to add a c++ project to my c# code and I’m trying to implement the IUnknown interface. I keep getting an error about c2259: cannot instantiate abstract class.

I’ve tried to play with some things like making the class a ref class and changing the implementation but nothing seems to work. Below is some of the code I am using.

My interface class:

interface __declspec(uuid("c78b266d-b2c0-4e9d-863b-e3f74a721d47"))
IClientWrapper : public IUnknown
{
    public:
        virtual STDMETHODIMP get_CurrentIsReadOnly(bool *pIsReadOnly) = 0;
        virtual STDMETHODIMP get_CachedIsReadOnly(bool *pIsReadOnly) = 0;
};

My handler class:

#include "RotateHandler.h"

RotateHandler::RotateHandler()
{
}

RotateHandler::~RotateHandler()
{
}

STDMETHODIMP RotateHandler::CreateClientWrapper(IUIAutomationPatternInstance *pPatternInstance, IUnknown **pClientWrapper)
{
    *pClientWrapper = new RotateWrapper(pPatternInstance);  //here is error c2259
    if (*pClientWrapper == NULL)
        return E_INVALIDARG;
    return S_OK;
}

STDMETHODIMP RotateHandler::Dispatch(IUnknown *pTarget, UINT index, const struct UIAutomationParameter *pParams, UINT cParams)
{
    switch(index)
    {
        case Rotation_GetIsReadOnly:
            return ((ICustomProvider*)pTarget)->get_IsReadOnly((bool*)pParams[0].pData);
    }
    return E_INVALIDARG;
}

And my wrapper class:

#include "RotateWrapper.h"

RotateWrapper::RotateWrapper()
{
}

RotateWrapper::RotateWrapper(IUIAutomationPatternInstance *pInstance)
    : _pInstance(pInstance)
{
    _pInstance->AddRef();
}

RotateWrapper::~RotateWrapper()
{
    _pInstance->Release();
}

STDMETHODIMP RotateWrapper::get_CurrentIsReadOnly(bool *pIsReadOnly)
{
    return _pInstance->GetProperty(0, false, UIAutomationType_Bool, pIsReadOnly);
}

STDMETHODIMP RotateWrapper::get_CachedIsReadOnly(bool *pIsReadOnly)
{
    return _pInstance->GetProperty(0, true, UIAutomationType_Bool, pIsReadOnly);
}

Any help is appreciated.

My class definition goes like this:

public class RotateWrapper : public IClientWrapper

Answer 1

You need to implement the methods inherited from IUnknown: QueryInterface, AddRef, and Release. Failure to do that means your class still has pure virtual methods, and the compiler is correct to forbid you from instantiating such a class.