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We can add, remove and insert the items from the item source collection of the Gantt Control as in the below code example
(gantt.ItemsSource as ObservableCollection<TaskDetails>).Add(new TaskDetails
StartDate = (gantt.ItemsSource as ObservableCollection<TaskDetails>)[count-1].FinishDate,
Duration = new TimeSpan(2, 0, 0, 0)
Also, we have modified the sample for your reference. You can download the sample from the following location.
Note: InbuildTaskCollection property is provided to access the task items when item source is imported from the xml which makes it easier to access the task items.