CListCtrl with LVS_EX_CHECKBOXES style

Question

I'm using CListCtrl with LVS_EX_CHECKBOXES style. And I need to have at least two of the checkboxes were set checked at any time.

How can I do this?

Answer 1

First you need to trap the LVN_ITEMCHANGING notification, which is most easily done by deriving your own class from CListCtrl (for example, called CMyListCtrl) and then adding a message map entry like the following:

BEGIN_MESSAGE_MAP(CMyListCtrl, CListCtrl)
    ON_NOTIFY_REFLECT(LVN_ITEMCHANGING, &CMyListCtrl::OnLvnItemchanging)
END_MESSAGE_MAP()

Then, you write the message handler like so:

void CMyListCtrl::OnLvnItemchanging(NMHDR *pNMHDR, LRESULT *pResult)
{
    // an item has changed
    LPNMLISTVIEW pNMLV = reinterpret_cast<LPNMLISTVIEW>(pNMHDR);

    // by default, allow change
    *pResult = 0;

    // see if item was checked or unchecked
    if ((pNMLV->uNewState & 0x2000) == 0x2000)
    {
        // item was checked - do anything you like here
    }
    else if ((pNMLV->uNewState & 0x1000) == 0x1000)
    {
        // item was unchecked - see how many selections we have
        if (/* pseudocode */ number of selected items < 2)
        {
            // disallow change
            *pResult = 1;
        }
    }
}

The condition is pseudo-code so you can decide how to keep track of the number of selections - maybe keep a count by adding code to the above method, or put a loop in there to get the check state of each item and make a tally.

I think this should give you enough to get moving, so please update your question if you get stuck further.