How to find the last descendant of an activity/task?

You can use the GanttControl.GetIndexOfLastDescendant method for this purpose as follows:

// Get the current selected activity view:

IActivityView selItem = this.ganttControl.GanttTable.SelectedItem as IActivityView;

// Find the index of the last descendant

int lastActivityIndex = this.ganttControl.GetIndexOfLastDescendant(selItem);

// Get hold of the last descendant

IActivity lastDescendant = this.ganttControl.ActivityViews[lastDescendant].Activity;