Listbox Notification message

I am trying to prevent items fr开发者_JS百科om being selected in listbox that matches specified condition. After some MSDN Study I came to know that LBN_SELCHANGE is sent to it's parent window via WM_COMMAND so I tried to hook LBN_SELCHANGE message sent by the list box on OnNotifyMessage as below.

 public class SimpleListBox:ListBox
{
    public SimpleListBox()
    {
        SetStyle(ControlStyles.EnableNotifyMessage, true);
    }
    private const int LBN_SELCANCEL = 0x3;
    private const int LBN_SELCHANGE = 0x1;
    protected override void OnNotifyMessage(Message m)
    {
        switch (m.Msg)
        {
            ////http://msdn.microsoft.com/en-us/library/bb775161(VS.85).aspx
            case (int)WindowsMessages.WM_COMMAND: //0x111
                if (((int)m.WParam).LoWord() == LBN_SELCHANGE)
                {
                    int i = 0;
                }
                break;
        }
        base.OnNotifyMessage(m);
    }

}

But This seems not working, please guide me if I have missed something. Thanks in advance.

---

Yes, that cannot work. LBN_SELCHANGE is sent after the deed was done, the item is already selected. You could only unselect it.

You can already do this without trapping the Windows message. Here's a silly example, it only allows selecting even numbered items:

  public partial class Form1 : Form {
    public Form1() {
      InitializeComponent();
      listBox1.SelectionMode = SelectionMode.MultiSimple;
      listBox1.SelectedIndexChanged += new EventHandler(listBox1_SelectedIndexChanged);
    }

    void listBox1_SelectedIndexChanged(object sender, EventArgs e) {
      for (int ix = listBox1.SelectedIndices.Count - 1; ix >= 0; --ix) {
        int index = listBox1.SelectedIndices[ix];
        if (index % 2 != 0) listBox1.SelectedIndices.Remove(index);
      }
    }
  }

An inevitable problem is that the selection blinks when it is selected by the user and unselected by your program. You should look at the CheckedListBox control if that's not desirable.