ComboBox drop down list hiding

I am creating a combo box with check boxes. Every thing is working fine but I am facing a problem.

When I open combo box list and check any value list box disappear. So if I have to check five values in combo box. I have to click 10 times.

I want to open the combo box and check all 5 items and then click out side the combo box and list

disappear!

I tried this by reading windows messages. I think that I shall hear for window message and when

combo box list disappear message arrived I skip this just by calling the return.

Here is my code:

protected override void WndProc(ref Message message)
    {
     const int CB_SHOWDROPDOWN=0x014F;
     if (message.Msg == CB_SHOWDROPDOWN ) 
     {
      开发者_如何学C if(message.WParam == (IntPtr)0)
       {
       SendMessage(Program.poForm.cmbShop.Handle, CB_SHOWDROPDOWN, (IntPtr)1,
         IntPtr.Zero);
       } 
       return;
     }
      base.WndProc(ref message);
    }

This is not working! Is there any work around to solve this problem?

Have you looked at this code project example of extending combo boxes with check boxes.

Is it absolutely necessary that you use a combo box? Because there is a checkedlistbos control in .NET.