PropertySheet DLLs Followup (DanRollins)

Have you checked whether CreateFunc contains a valid address before calling it? E.g.

FARPROC CreateFunc;

CreateFunc = GetProcAddress(handletoDLL, "ExportedName");

if ( NULL != CreateFunc) {

  aPage = (CPropertyPage*)CreateFunc();

} else {

  // error
}

BTW, you should use

extern "C" __declspec(dllexport) void* CreatePage()

to avoid exporting the function with a C++ mangled name...

You need to call
   AFX_MANAGE_STATE(AfxGetStaticModuleState());
in every case where control is passed from the EXE to the DLL.  That big block of comments provided at the top of the AppWizard-generated file goes into excruciating detail in this regard.

-- Dan

Also, when that error occurs, bring up the Call Stack to learn where the code went awry.  You can also diagnose many problems by single-stepping with the debugger.  You can trace right into the DLL.  The trick is putting a breakpoint on the

    aPage = (CPropertyPage*)CreateFunc();

line.

-- Dan

I did a debug, and it points out this line: m_Page = new CPage(); and says the expression doesn't evaluate.

note: i added the AFX_MANAGE_STATE line at the top of that function

jkr: I have checked and it does have the correct name. The function is being called.

>> the expression doesn't evaluate
Are you saying that the DLL cannot be compiled?  If so, then there is some chance that that is related to your problem.
-- Dan

heh, no the DLL compiles. It compiles just fine. When the error occurs, and i click the debug button, it points to that line. I check the output window and under the line the references "m_Page = new CPage();" It says that the expression doesn't evaluate.

also, if I set a breakpoint on this line: "aPage = (CPropertyPage*)CreateFunc();". I get this for the value aPage in the Debug output window:
Name: aPage
Value: 0x00000000 {CPropertyPage hWnd=???}

That line has a + sign in front of it. When I click it this drops down underneath:

Name: CDialog
Value: CXX0030: Error: expression cannot be evaluated

There are also a few more dropdowns that list different names but they pretty much all say "expression cannot be evaluated".

the variable aPage will have no value until the line:

   aPage = (CPropertyPage*)CreateFunc();

has been executed.  You need to breakpoint that line, and then single-step.  That could put you right into the DLL code where you can single-step the CreateFunc() function.

-- Dan

well what i said earlier was that this line (which is in the CreateFunc function) is causing the problem:
m_Page = new CPage();
It tells me that this expression can not be evaluated (this is the line in the CreateFunc function).
If I mouse over the part that says "CPage()" it displays this tooltip: CPage = {CPage hWnd=???}

Could this be the problem? If not, i must not be understanding what you are saying. I debug my application letting it execute the code line by line until it hits an error, it then stops and points out that line to me. What else am i supposed to do?

right click on that line.  
choose Insert/Remove breakpoint
run the program
when it stops, it will display that line
now choose menu item Debug/Step Into

Now you can execute each program line, one line at a time.  In this case, that will involve stepping into the code that is in the DLL (the exported function, CreateFunc() code ).

-- Dan

yes i tried that however, after going through about 10,000 lines of code related to AFX_MANAGE_STATE, i just stepped out until I got down to the mPage = net CPage() line. Then it shows this error:
First-chance exception in MainApplication.exe (THEDLL.DLL): 0xC0000005: Access Violation.

I don't see how assaigning a new CPropertyPage derived object to a pointer would cause an Access Violation.

Show your code leading up to the

    mPage= new CPage();

line in the DLL.  How have you defined mPage?

As soon as you are in the DLL, you can put a breakpoint in the constructor for your CPage.  I'd have to guess that something in there is going wrong.

hmm, doesn't it seem easier to just export the m_Page pointer? And then just load it and call AddPage from the main program instead of messing with this function? It would probably eliminate a lot of possible errors. If this would be a better choice, how would I do what I did with the function to the pointer? (the GetProcAddress Part, in other words, how can i load the pointer into the main program so that I can call AddPage with it as a parameter?)

When you use a .LIB to interface the EXE to the DLL, it is very easy to export class objects and data variable.  As I understand your question, you will be using LoadLibrary, so the only way to access the DLL is via GetProcAddress, which does not return object or data -- only addresses of functions -- so at is not an option.

What's left is to simulate the ActiveX concept of a 'class factory' ... the DLL provides a function that creates and initializes the object and passes back a pointer to that object.

If you want to use 'normal DLL Linking' (using a LIB file and requiring the presence of all potentially needed DLLs at runtime) then that will simplify your task considerably.

-- Dan

BEGIN[DLL CODE, DLL CODE, DLL CODE, DLL CODE]

Note the CPage I have been referring to has been renamed to CStatPage. Here's all the relavent code:
***********************************************************
// StatPage.cpp : implementation file
//

#include "stdafx.h"
#include "dllapp.h"
#include "StatPage.h"

#ifdef _DEBUG
#define new DEBUG_NEW
#undef THIS_FILE
static char THIS_FILE[] = __FILE__;
#endif

/////////////////////////////////////////////////////////////////////////////
// CStatPage property page

IMPLEMENT_DYNCREATE(CStatPage, CPropertyPage)

CStatPage::CStatPage() : CPropertyPage(CStatPage::IDD)
{
     //{{AFX_DATA_INIT(CStatPage)
          // NOTE: the ClassWizard will add member initialization here
     //}}AFX_DATA_INIT
}

CStatPage::~CStatPage()
{
}

void CStatPage::DoDataExchange(CDataExchange* pDX)
{
     CPropertyPage::DoDataExchange(pDX);
     //{{AFX_DATA_MAP(CStatPage)
          // NOTE: the ClassWizard will add DDX and DDV calls here
     //}}AFX_DATA_MAP
}


BEGIN_MESSAGE_MAP(CStatPage, CPropertyPage)
     //{{AFX_MSG_MAP(CStatPage)
          // NOTE: the ClassWizard will add message map macros here
     //}}AFX_MSG_MAP
END_MESSAGE_MAP()

/////////////////////////////////////////////////////////////////////////////
// CStatPage message handlers

***********************************************************

// StatPage.h : header file
//

/////////////////////////////////////////////////////////////////////////////
// CStatPage dialog

class CStatPage : public CPropertyPage
{
     DECLARE_DYNCREATE(CStatPage)

// Construction
public:
     CStatPage();
     ~CStatPage();

// Dialog Data
     //{{AFX_DATA(CStatPage)
     enum { IDD = IDD_STATDLG };
          // NOTE - ClassWizard will add data members here.
          //    DO NOT EDIT what you see in these blocks of generated code !
     //}}AFX_DATA


// Overrides
     // ClassWizard generate virtual function overrides
     //{{AFX_VIRTUAL(CStatPage)
     protected:
     virtual void DoDataExchange(CDataExchange* pDX);    // DDX/DDV support
     //}}AFX_VIRTUAL

// Implementation
protected:
     // Generated message map functions
     //{{AFX_MSG(CStatPage)
          // NOTE: the ClassWizard will add member functions here
     //}}AFX_MSG
     DECLARE_MESSAGE_MAP()

};

***********************************************************

// MAIN DLL HEADER FILE (IMPORTANT SECTIONS)
CStatPage* m_Page;
__declspec(dllexport) void* CreatePage();

***********************************************************

// MAIN DLL SOURCE FILE (IMPORTANT SECTIONS)
__declspec(dllexport) void* CreatePage()
{
     AFX_MANAGE_STATE(AfxGetStaticModuleState());
     m_Page = new CStatPage;
     return (void*)m_Page;
}

***********************************************************
END[DLL CODE, DLL CODE, DLL CODE, DLL CODE]

HERE COMES THE MAIN PROGRAM SOURCE CODE

***********************************************************

// MAIN PROGRAM (EXE) HEADER FILE (IMPORTANT SECTIONS)
CPropertyPage *m_Page;
CMainProgramDlg dlgacc;

***********************************************************

// MAIN PROGRAM (EXE) SOURCE FILE (IMPORTANT SECTIONS)
HMODULE hMod = LoadLibrary( dllPath );
FARPROC CreateFunc;
CreateFunc = GetProcAddress(hMod, "CreatePage");
if(CreateFunc == NULL)
{
     MessageBox("DLL NOT LOADED");
}
else
{
     m_Page = (CPropertyPage*)CreateFunc();
     dlgacc.m_sheet.AddPage(m_Page);
}

***********************************************************

The rest of the important code in the main app is just the code to setup the property sheet but i know that code is correct.

BTW - m_sheet was Mysheet but i changed it to a more normal variable name. m_sheet is the name of the property sheet

well that worked. It didn't seem any different than what I was doing. I must have made some stupid type errors. Oh well. Thanks! You've been a big help!

If I did this right you should get 500 expert points (if the system hasn't changed)

hmm, the increase points didn't work. oh well, i'll find a way to give you some more points later.

I'm glad I could help!

To award additional points, just make a new Question in the C++ TA.  Set it's title to
          Points for DanRollins
and place this in the question text:

For your help with: https://www.experts-exchange.com/cplusprog/Q.20299685.html

-- Dan