public class GPExecuteToolEventArgs : System.EventArgs 

{
  ArcGIS.Desktop.Core.Events.GPExecuteToolEvent.Subscribe(e =>
        {
          // Handle the event
          string id = e.ID;           // Same as history ID
          if (e.IsStarting == false)  // Execute completed
            _ = e.GPResult.ReturnValue;
          System.Windows.MessageBox.Show("event triggered.");
        });
  Geoprocessing.ExecuteToolAsync("management.GetCount", Geoprocessing.MakeValueArray(@"c:\shape_file.shp"));
}