Treat the array as a LIFO stack:

do while !isnull(this.ToDoObjects[1])
    lnTopOfStack = alen(this.ToDoObjects)
    loCurrent = this.ToDoObjects[lnTopOfStack]
    loTopOfStack = lnTopOfStack - 1
    if lnTopOfStack = 0
        this.todo[1] = null
    else
        dimension this.ToDoObjects[lnTopOfStack]
    endif
    * do whatever with loCurrent
enddo

If you don't want to constantly redimension the array, create and maintain a corresponding ToDoObjectCount property.