C / C++ / MFC

//worksheet wrapper class
  CXLWorksheet worksheet;
	//range wrapper class
  CXLRange range;
	CString strRange;

  VARIANT v, vRet, vNotPassed, vBOOL;
  VariantInit(&v);
  VariantInit(&vRet);
  VariantInit(&vNotPassed);
  VariantInit(&vBOOL);
  V_VT(&vNotPassed) = VT_ERROR;
  V_ERROR(&vNotPassed) = DISP_E_PARAMNOTFOUND;
	V_VT(&vBOOL) = VT_BOOL;

	LPDISPATCH pXLAppDispatch = NULL;
	LPUNKNOWN lpUnk; CLSID clsid; 
	if( S_OK == ::CLSIDFromProgID(L"Excel.Application", &clsid) )
	{
		if( S_OK == ::GetActiveObject(clsid, NULL, &lpUnk) )
		{
			VERIFY(lpUnk->QueryInterface(IID_IDispatch, (void**)&pXLAppDispatch) == S_OK);
			//m_XLApp is Excel application class
			m_XLApp.AttachDispatch( pXLAppDispatch );
			lpUnk->Release();
		}
	}


	V_BOOL(&vBOOL) = TRUE;
	m_XLApp.SetVisible(vBOOL);
	

	VARIANT vFilename; 
	V_VT(&vFilename) = VT_BSTR;

//strFileName is full path of Excel file name
	int Len = strFileName.GetLength();
	V_BSTR(&vFilename) = SysAllocString(
		T2COLE(strFileName.GetBuffer(Len+1)) );

//m_workbooks is workbooks wrapper class
	if( !m_workbooks.m_lpDispatch )
	{
		m_workbooks.AttachDispatch( m_XLApp.GetWorkbooks() );

		TRY
		{
		vRet = m_workbooks.Open(vFilename, 
			vNotPassed,vNotPassed,vNotPassed,vNotPassed,
			vNotPassed,vNotPassed,vNotPassed,vNotPassed,
			vNotPassed,vNotPassed,vNotPassed );
		}
		CATCH_ALL(e)
		{
			//* Create a new one instead
			vRet = m_workbooks.Add(vNotPassed);
		}
		END_CATCH_ALL

	//m_workbook is workbook wrapper class
		if( V_VT(&vRet) == VT_DISPATCH )
			m_workbook.AttachDispatch( V_DISPATCH(&vRet) );
	}

	LPDISPATCH lpDisp;
	//for this example, nXLWorksheetNo == 1
	lpDisp = m_workbook.GetWorksheets(nXLWorksheetNo);
	worksheet.AttachDispatch(lpDisp);

	//in this example, nCols == 10	
	for( int nCol=0; nCol<nCols; nCol++ )
	{
		//* Work on an entire column at a time
			//* Add Column (field) title

		//strRange == "A1"
		strRange = pTable->m_strXLTopLeftCell;
		XLAdjustCol(strRange, nCol);
		V_VT(&v) = VT_BSTR;
		V_BSTR(&v) = SysAllocString(
			T2COLE(strRange.GetBuffer(0)) );
		LPDISPATCH pDisp = worksheet.Range1(v);
		VariantClear(&v);
		range.AttachDispatch(pDisp);
		VariantInit(&v);
		V_VT(&v) = VT_BSTR;
		//in this example, strHeader is "nVar_VSL" (no significance to bug)
		V_BSTR(&v) = strHeader.AllocSysString();
		if( V_VT(&v) != VT_EMPTY )
		{
			try
			{
			//here is where the bug is
			//range throws exception if user changes to another sheet
			//why doesn't it just keep writing to current sheet?
			range.SetValue(v);
			}
			catch(COleDispatchException *pEx)
			{
				//most of the time crashes when nCol==0 or 8
				TRACE("nCol = %d\n",nCol);
				pEx->Delete();
			}
			VariantClear(&v);
		}
                   }
LPDISPATCH CXLWorksheet::Range1(const VARIANT& Cell1)
{
	LPDISPATCH result;
	static BYTE parms[] =
		VTS_VARIANT;
	InvokeHelper(0xc5, DISPATCH_PROPERTYGET, VT_DISPATCH, (void*)&result, parms,
		&Cell1);
	return result;
}
LPDISPATCH CXLWorkbook::GetWorksheets(short nXLWorksheetNo)
{
	VARIANT vWorkSheetNo;
	VariantInit(&vWorkSheetNo);
	V_VT(&vWorkSheetNo) = VT_I2;
	V_I2(&vWorkSheetNo) = nXLWorksheetNo;
	static BYTE parms[] =
		VTS_VARIANT;
	LPDISPATCH result;
	InvokeHelper(0x1ee, DISPATCH_PROPERTYGET, VT_DISPATCH, (void*)&result, parms, &vWorkSheetNo );
	return result;
}

MyDll.h
-------

namespace MyDll
{
   class MyOwnDll
   {
       //function here
       static _declspec(dllexport) void PrintMsg();
   };
}

MyDll.cpp
---------
namespace MyDll
{
    void MyOwnDll::PrintMsg()
    {
        ::MessageBox(0,"","",0);
    }
}

MyProjDlg.cpp
-------------
{
     ...............
     ...............

     void MyProjDlg::MyMsg() //function added to a button
     {
         MyOwnDll mod;     //myTapi, mod ==> Undeclared Identifier
         mod.PrintMsg();  //mod ==> Undeclared Identifier
     }
}