Assigning Task.Duration to an already added Task in VBA Microsoft Project

Question

I'm currently working on a VBA macro importing product-requirements into Microsoft Project.

I use the following code for adding/updating a task:

Function AddTask(strText As String, lngDuration As Long, taskParent As Task)
    Dim oldTask As Task
    Set oldTask = taskParent.OutlineChildren(strText)
    If oldTask Is Nothing Then
        Dim newTask As Task
        Set newTask = taskParent.OutlineChildren.Add(Name:=strText, Before:=LastIndexOf(taskParent) + 1)
        newTask.OutlineLevel = taskParent.OutlineLevel + 1
        newTask.Duration = lngDuration
        Set AddTask = newTask
    Else
        oldTask.Duration = lngDuration
        Set AddTask = oldTask
    End If
End Function

This works perfectly for a new task, but unfortunately I get a weird error when trying to update the Duration property on an old task.

Run-Time Error '1101'

Argument value is not valid

I really don't understand what the difference between

newTask.Duration = lngDuration

and

oldTask.Duration = lngDuration

What's going on here?
Please help!

Answer 1

Found it myself!

The problem was that sometimes tasks would have been added to the old task. So it is now a parent task containing a couple of child tasks. By definition the duration is now the sum of all the child durations. Thus the duration on the parent task can't be changed manually as its updated automatically.

So a simple check if the old tasks contains any OutlineChildren and then skipping it solves my problem.

If oldTask.OutlineChildren.Count = 0 Then
    oldTask.Duration = lngDuration
End If

Thanks