public sealed class TaskEndedEvent : ArcGIS.Core.Events.CompositePresentationEvent<TaskEndedEventArgs> 

public void TaskEvents()
{
  TaskStartedEvent.Subscribe(OnTaskStarted);
  TaskEndedEvent.Subscribe(OnTaskCompletedOrCancelled);
}

private void OnTaskStarted(TaskStartedEventArgs args)
{
  string userName = args.UserID;    // ArcGIS Online signed in userName.  If not signed in to ArcGIS Online then returns the name of the user logged in to the Windows OS.
  string projectName = args.ProjectName;

  Guid taskItemGuid = args.TaskItemGuid;
  string taskItemName = args.TaskItemName;
  string taskItemVersion = args.TaskItemVersion;

  Guid taskGuid = args.TaskGuid;
  string taskName = args.TaskName;

  DateTime startTime = args.StartTime;
}

private void OnTaskCompletedOrCancelled(TaskEndedEventArgs args)
{
  string userName = args.UserID;    // ArcGIS Online signed in userName.  If not signed in to ArcGIS Online then returns the name of the user logged in to the Windows OS.
  string projectName = args.ProjectName;

  Guid taskItemGuid = args.TaskItemGuid;
  string taskItemName = args.TaskItemName;
  string taskItemVersion = args.TaskItemVersion;

  Guid taskGuid = args.TaskGuid;
  string taskName = args.TaskName;

  DateTime startTime = args.StartTime;
  DateTime endTime = args.EndTime;
  double duration = args.Duration;

  bool completed = args.Completed;    // completed or cancelled
}