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public Outlook.Explorer oExpl;
public Outlook.MAPIFolder oMapi;
public Office.CommandBarButton oCBB;
public Outlook.AppointmentItem Appt;

void barButton_Click(Microsoft.Office.Core.CommandBarButton commandBarButton, ref bool CancelDefault)
        {
            oExpl = Application.ActiveExplorer();
            if (oExpl != null)
            { 
                oMapi = oExpl.CurrentFolder;
                if (oMapi.DefaultItemType == OlItemType.olAppointmentItem)
                {
                    oCBB = (Microsoft.Office.Core.CommandBarButton)oExpl.CommandBars.FindControl(Type.Missing, 1106, Type.Missing, Type.Missing);
                    if (oCBB != null)
                    {
                        try
                        {
                            oCBB.Execute();
                            Appt = (Outlook.AppointmentItem)Application.ActiveInspector().CurrentItem;
                        }
                        catch (System.Exception ex)
                        {
                            System.Windows.Forms.MessageBox.Show(ex.ToString());
                        }
                    }
                }
            }
}