TaskChangingEventArgs

Each New... property is the value the task would take, or null when that value is not changing. The task itself has not changed yet: compare Task's properties with the new values to see what the reader did.

Set CancelEventArgs.Cancel to true to refuse the change; the bar, cell or row goes back to how it was and nothing is written to the task. Otherwise the chart calls Apply and every chart sharing the task redraws.

A move of a task whose length comes from its planned start and finish, rather than a ProjectTask.Duration, carries the length it was scheduled with as NewDuration too, so the task keeps its length when it moves.

gantt.TaskChanging += (sender, e) =>
{
    // Work that has been signed off stays where it is.
    if (e.Task.Status == "Done" && (e.NewStart.HasValue || e.NewDuration.HasValue))
    {
        e.Cancel = true;
        return;
    }

    // Nothing is planned to take longer than a quarter.
    if (e.NewDuration > 65)
        e.Cancel = true;
};