Event to detect item added to ComboBox

Question

I am creating a custom control that inherits from ComboBox. I need to detect when a Item is added to the ComboBox to perform my own checks. It doesn\'t matter if it\'s C# or Vb.

Answer 1

I think the best approach would be to listen for the native ComboBox messages:

• CB_ADDSTRING
• CB_INSERTSTRING
• CB_DELETESTRING
• CB_RESETCONTENT

Don't be fooled by the word STRING, they are all fired whenever you add, insert or delete an item. So when the list is cleared.

Public Class UIComboBox
    Inherits ComboBox

    Private Sub NotifyAdded(index As Integer)
    End Sub

    Private Sub NotifyCleared()
    End Sub

    Private Sub NotifyInserted(index As Integer)
    End Sub

    Private Sub NotifyRemoved(index As Integer)
    End Sub

    Protected Overrides Sub WndProc(ByRef m As Message)
        Select Case m.Msg
            Case CB_ADDSTRING
                MyBase.WndProc(m)
                Dim index As Integer = (Me.Items.Count - 1)
                Me.NotifyAdded(index)
                Exit Select
            Case CB_DELETESTRING
                MyBase.WndProc(m)
                Dim index As Integer = m.WParam.ToInt32()
                Me.NotifyRemoved(index)
                Exit Select
            Case CB_INSERTSTRING
                MyBase.WndProc(m)
                Dim index As Integer = m.WParam.ToInt32()
                Me.NotifyAdded(If((index > -1), index, (Me.Items.Count - 1)))
                Exit Select
            Case CB_RESETCONTENT
                MyBase.WndProc(m)
                Me.NotifyCleared()
                Exit Select
            Case Else
                MyBase.WndProc(m)
                Exit Select
        End Select
    End Sub

    Private Const CB_ADDSTRING As Integer = &H143
    Private Const CB_DELETESTRING As Integer = &H144
    Private Const CB_INSERTSTRING As Integer = 330
    Private Const CB_RESETCONTENT As Integer = &H14B

End Class