Error C2065 when using two MFC projects - c++

Question

I have a Dialog with a CListCtrl with some data, below is a button. My second project is a dialog with a tri-state checkbox. Now I want to couple both projects, so when selecting a list item and clicking the button the tristate dialog appers showing something.
Well, I added the second project to another one and changed its type to .dll, but receive a build error
error C2065: 'IDD_MFCAPP2_DIALOG' : undeclared identifier
IDD_MFCAPP2_DIALOG is the id of he checkbox dialog. When adding the project both id's had the same value, so I changed this in resource.h manually to a different one, but it did not solve the problem. The projects work very well separately. What do I miss?
UPDATE:
I have reduced the project to a minimum, the problem is the same
MFCApp1Dlg.h
class CMFCApp1Dlg : public CDialogEx
{
// Construction
public:
CMFCApp1Dlg(CWnd* pParent = NULL); // standard constructor
// Dialog Data
enum { IDD = IDD_MFCAPP1_DIALOG };
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
// Implementation
protected:
HICON m_hIcon;
// Generated message map functions
virtual BOOL OnInitDialog();
afx_msg void OnPaint();
afx_msg HCURSOR OnQueryDragIcon();
DECLARE_MESSAGE_MAP()
public:
afx_msg void OnBnClickedButtonChange();
CButton m_BtnChange;
};
Its cpp file only contains the standard things. Just the button handler calls other app
void CMFCApp1Dlg::OnBnClickedButtonChange()
{
CMFCApp2Dlg dlg;
if (dlg.DoModal())
{
// do something...
}
}
MFCApp2Dlg.h looks the same
class CMFCApp2Dlg : public CDialogEx
{
// Construction
public:
CMFCApp2Dlg(CWnd* pParent = NULL); // standard constructor
// Dialog Data
enum { IDD = IDD_MFCAPP2_DIALOG };
protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support
// Implementation
protected:
HICON m_hIcon;
// Generated message map functions
virtual BOOL OnInitDialog();
afx_msg void OnPaint();
afx_msg HCURSOR OnQueryDragIcon();
DECLARE_MESSAGE_MAP()
};
The .rc file are also not spectaculary
IDD_MFCAPP1_DIALOG DIALOGEX 0, 0, 315, 151
STYLE DS_SETFONT | DS_FIXEDSYS | WS_POPUP | WS_VISIBLE | WS_CAPTION | WS_THICKFRAME
EXSTYLE WS_EX_APPWINDOW
CAPTION "MFCApp1"
FONT 8, "MS Shell Dlg", 0, 0, 0x1
BEGIN
DEFPUSHBUTTON "OK",IDOK,76,119,50,14
PUSHBUTTON "Cancel",IDCANCEL,157,117,50,14
PUSHBUTTON "Push me!",IDC_BUTTON_CHANGE,121,44,50,14
END
and
IDD_MFCAPP2_DIALOG DIALOGEX 0, 0, 203, 101
STYLE DS_SETFONT | DS_FIXEDSYS | WS_POPUP | WS_VISIBLE | WS_CAPTION | WS_THICKFRAME
EXSTYLE WS_EX_APPWINDOW
CAPTION "MFCApp2"
FONT 8, "MS Shell Dlg", 0, 0, 0x1
BEGIN
DEFPUSHBUTTON "OK",IDOK,46,64,50,14
PUSHBUTTON "Cancel",IDCANCEL,109,65,50,14
CONTROL "Check1",IDC_CHECK1,"Button",BS_AUTO3STATE | WS_TABSTOP,51,17,39,10
CONTROL "Check2",IDC_CHECK2,"Button",BS_AUTO3STATE | WS_TABSTOP,51,39,39,10
END
Any suggestions would be helpful.
UPDATE2:
Resource.h of App1
#define IDD_MFCAPP1_DIALOG 102
#define IDR_MAINFRAME 128
#define IDC_BUTTON_CHANGE 1000
// Next default values for new objects
//
#ifdef APSTUDIO_INVOKED
#ifndef APSTUDIO_READONLY_SYMBOLS
#define _APS_NEXT_RESOURCE_VALUE 129
#define _APS_NEXT_COMMAND_VALUE 32771
#define _APS_NEXT_CONTROL_VALUE 1001
#define _APS_NEXT_SYMED_VALUE 101
#endif
#endif
and Resource.h of App2 respectively
#define IDD_MFCAPP2_DIALOG 202
#define IDR_MAINFRAME 228
#define IDC_CHECK1 2000
#define IDC_CHECK2 2001
// Next default values for new objects
//
#ifdef APSTUDIO_INVOKED
#ifndef APSTUDIO_READONLY_SYMBOLS
#define _APS_NEXT_RESOURCE_VALUE 229
#define _APS_NEXT_COMMAND_VALUE 32771
#define _APS_NEXT_CONTROL_VALUE 2002
#define _APS_NEXT_SYMED_VALUE 201
#endif
#endif

Answer 1

The message just tells you that the ID is an unknown identifier. So just not known. So usually:
Make sure that you included the correct resource.h file AND make sure that the ID is defined in the resource.h