Problem: Data validation in MFC is a bit difficult. We only get
a couple of DDV functions that left a lot to be desired. For example, there is
no easy way to make sure a “Dropdown Combo Box” is not empty.
Solution: After some searching in MSDN documents and
googling, I cannot find a good clean solution. But, I am inspired by the MFC
source code and found the following solution.
In the "DoDataExchange" function of your dialog, you only need to add a few
lines of code (highlighted below) to validate an empty combo box.
Following this logic, you can easily add more sophiscate validation logic, and
make sure the focus will be set to the control that you want user to change.
void CMyDlg::DoDataExchange(CDataExchange* pDX)
{
CMyDlg::DoDataExchange(pDX);
//{{AFX_DATA_MAP(CTaskMTTPage)
DDX_CBString(pDX, IDC_COMBOFILENAME, m_strFilename);
//}}AFX_DATA_MAP
//customized data verification
if(pDX->m_bSaveAndValidate)
{
//we will only check data during validation
if(m_strFilename.IsEmpty())
{
CString errorMessage = "The file name field cannot be empty.";
AfxMessageBox(errorMessage, MB_ICONEXCLAMATION);
errorMessage.Empty();
pDX->PrepareCtrl(IDC_COMBOFILENAME); //make sure pDX will focus to this control
pDX->Fail();
}
}
}
Extra: In the process of searching for the solution mentioned
above, I found this interesting problem: VC7 and VC98 have slightly different
definition of public members of CDataExchange class (notice the highlighted
code segment below). Why should Microsoft change definition of public member of
their library? Isn't public member supposed to be some sort of binding
contract?
Another interesting discovery is that Visual Studio intell-sense won't show "m_hWndLastControl"
although it is public member of the class. Did I miss something here?
Why do I care about this obscure member of CDataExchange? Well, I
am trying to find a way to make sure the dialog will set focus on the correct
control after an error condition occurs. My first intent was to manipulate the
m_hWndLastControlm_idLastControl member, thus lead
to the discovery. Eventually, I found out calling function PrepareCtrl
is a much better solution.
Definition of class
CDataExchange in VC98 include:
// CDataExchange - for data exchange and validation
class CDataExchange
{
// Attributes
public:
BOOL m_bSaveAndValidate; // TRUE => save and validate data
CWnd* m_pDlgWnd; // container usually a dialog
// Operations (for implementors of DDX and DDV procs)
HWND PrepareCtrl(int nIDC); // return HWND of control
HWND PrepareEditCtrl(int nIDC); // return HWND of control
void Fail(); // will throw exception
#ifndef _AFX_NO_OCC_SUPPORT
CWnd* PrepareOleCtrl(int nIDC); // for OLE controls in dialog
#endif
// Implementation
CDataExchange(CWnd* pDlgWnd, BOOL bSaveAndValidate);
HWND m_hWndLastControl; // last control used (for validation)
BOOL m_bEditLastControl; // last control was an edit item
};
Definition of class
CDataExchange in VC7 include:
// CDataExchange - for data exchange and validation
class CDataExchange
{
// Attributes
public:
BOOL m_bSaveAndValidate; // TRUE => save and validate data
CWnd* m_pDlgWnd; // container usually a dialog
// Operations (for implementors of DDX and DDV procs)
HWND PrepareCtrl(int nIDC);
HWND PrepareEditCtrl(int nIDC);
void Fail(); // will throw exception
CDataExchange(CWnd* pDlgWnd, BOOL bSaveAndValidate);
#ifndef _AFX_NO_OCC_SUPPORT
COleControlSite* PrepareOleCtrl(int nIDC); // for OLE controls in dialog
#endif
// Implementation
UINT m_idLastControl; // last control used (for validation)
BOOL m_bEditLastControl; // last control was an edit item
};
