Public Class cListBoxLimit
    Inherits ListBox
    Dim maxcount As Integer = 10
    Protected Overrides Sub OnMouseClick(ByVal e As MouseEventArgs)
        Dim index As Integer = Me.IndexFromPoint(e.X, e.Y)
        If Me.SelectedIndices.Count > maxcount Then
            Me.SetSelected(index, False)
        End If
    End Sub
End Class