Using the CCheckListBox and Getting Check State Notification

• Download source - 12.4 KB

Introduction

I like the CCheckListBox class provided with MFC, however, its use isn’t obvious and the wizard assistance stops at the CListBox class. I’ll describe how to insert it easily in your project. (There may be easier ways to do it, but this is how I do it and it works.)

I’ll also show how you can add event notification so that you can find out when the check box state changes.

Creating the CCheckListBox Member

• Create a new MFC Application or Dialog Application.
• In the resource editor, add a "regular" list box to the dialog.
  • Right click the listbox properties, then the Styles tab
  • Ensure the Owner Draw = Fixed
  • Ensure Has Strings = checked
• Holding the CTRL key, double click on the listbox in the resource dialog.

The wizard will only give you the option to create it as a CListBox, choose that, we will change it in code.

In the header code, change the wizard generated code from:

// Dialog Data
     //{{AFX_DATA(CCheckListBoxCBNDlg)
     enum { IDD = IDD_CHECKLISTBOXCBN_DIALOG };
     CListBox  m_ctlCheckList;
     //}}AFX_DATA

to:

// Dialog Data
     //{{AFX_DATA(CCheckListBoxCBNDlg)
     enum { IDD = IDD_CHECKLISTBOXCBN_DIALOG };
     //}}AFX_DATA
CCheckListBox m_ctlCheckList;

In the body, change the following generated code from:

void CCheckListBoxCBNDlg::DoDataExchange(CDataExchange* pDX)
{
     CDialog::DoDataExchange(pDX);
     //{{AFX_DATA_MAP(CCheckListBoxCBNDlg)
     DDX_Control(pDX, IDC_LIST1, m_ctlCheckList);
     //}}AFX_DATA_MAP
}

to:

void CCheckListBoxCBNDlg::DoDataExchange(CDataExchange* pDX)
{
     CDialog::DoDataExchange(pDX);
     //{{AFX_DATA_MAP(CCheckListBoxCBNDlg)
     //}}AFX_DATA_MAP
     DDX_Control(pDX, IDC_LIST1, m_ctlCheckList);
}

Adding Items to the CCheckListBox

Now, you can add stuff to the checklist in your OnInitDialog member, like:

     m_ctlCheckList.ResetContent();
//   m_ctlCheckList.SetCheckStyle( BS_AUTO3STATE );
     m_ctlCheckList.SetCheckStyle( BS_3STATE );
     m_ctlCheckList.AddString("Fumble");
     m_ctlCheckList.SetCheck( 0, 0 );
     m_ctlCheckList.AddString("Bumble");
     m_ctlCheckList.SetCheck( 1, 1 );
     m_ctlCheckList.AddString("Gumble");
     m_ctlCheckList.SetCheck( 2, 2 );

Note that the MSDN documentation is a little flimsy when it comes to the description of BS_AUTO3STATE and BS_3STATE. If you use BS_3STATE, then you will not get check box notifications and the states are locked (changeable in code only). If you use BS_AUTO3STATE, then you will get notifications of state changes, and the check boxes will manage themselves. You will just have to experiment with them to see which one gives you the effect you want.

Determining Check Box State Changes

You can still use the wizard for the check list control you’ve created, but you’ll see that the list is limited to CListBox specific items:

I wanted a handler to know when a check box state changed (not a selection change). To accomplish this, manually add an event handler in the header as shown below. Note that if the user clicks on a check box, you will get two events for the click, first, OnCheckchangeList1, followed by OnSelchangeList1.

Caution: This is important if you depend on the current selection to change the checkbox state in a structure, i.e., the call to GetCurSel will be the new selection in the OnCheck call, even though OnSelchange hasn’t been called.

//{{AFX_MSG(CCheckListBoxCBNDlg)
virtual BOOL OnInitDialog();
afx_msg void OnSysCommand(UINT nID, LPARAM lParam);
afx_msg void OnPaint();
afx_msg HCURSOR OnQueryDragIcon();
afx_msg void OnSelchangeList1();
afx_msg void OnCheckchangeList1();
//}}AFX_MSG

In the body, add the event handler to the message map:

BEGIN_MESSAGE_MAP(CCheckListBoxCBNDlg, CDialog)
     //{{AFX_MSG_MAP(CCheckListBoxCBNDlg)
     ON_WM_SYSCOMMAND()
     ON_WM_PAINT()
     ON_WM_QUERYDRAGICON()
     ON_LBN_SELCHANGE(IDC_LIST1, OnSelchangeList1)
     //}}AFX_MSG_MAP
     ON_CLBN_CHKCHANGE(IDC_LIST1, OnCheckchangeList1)
END_MESSAGE_MAP()

And add your implementation of the handler.

void CCheckListBoxCBNDlg::OnCheckchangeList1() 
{
     // TODO: Add your control notification handler code here
     TRACE( "CCheckListBoxCBNDlg::OnCheckchangeList1\n" );     
}

Conclusion

At this point, you have a check list box that you can easily extend. Several other CodeProject articles show multi check list box classes and list view report views with check boxes. This is the simplest implementation of the MFC CCheckListBox.

Some people do not like the CCheckListBox because it leads to some ambiguity, but it really depends on the context it’s used in. For example, does checking the item turn the thing on or does the thing get enabled. When does it get turned on, when I check it or when I press OK/Apply in the dialog. Use this control with caution.

The code in this article has minimal error checking, and is meant for illustrative purposes only.

Project has been tested on VC++ 6.0 SP5, Win 2K SP4.

License

This article has no explicit license attached to it, but may contain usage terms in the article text or the download files themselves. If in doubt, please contact the author via the discussion board below. A list of licenses authors might use can be found here.