Private Sub tyAI_Open(ByRef Cancel As Boolean) Handles tyAI.Open
    Cancel = True
End Sub

private void mcAI_Open(out bool Cancel)
{
    Cancel = true;
}

mcAI.Open +=
    new Microsoft.Office.Interop.Outlook.ItemEvents_10_OpenEventHandler(mcAI_Open);

private void mcAI_Open(ref bool Cancel)
{
    Cancel = true;
}