update MS Project in VB.net

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VB

Hello,

i have bellow code in my application for updating MS project,
first i set a date (A_date) in actual date then i want to update project based on it but updateproject doesn't work!!!
what should i do?

Dim pjApplication As New Microsoft.Office.Interop.MSProject.Application

Dim iFileIndex As Int16

pjApplication.FileOpen(m_path) 
pjApplication.Visible = False
pjApplication.AppMinimize()

iFileIndex = pjApplication.ActiveProject.Index

' i have code there to set actual start
'For Each mTask In mTasks
'    If pjApplication.ActiveProject.Tasks(indx).Predecessors <> "" Then
'        pjApplication.ActiveProject.Tasks(indx).ActualStart = A_date
'        pjApplication.ActiveProject.Tasks(indx).PercentComplete = 5           
'    End If
'next

pjApplication.UpdateProject(true, A_date, _  
Microsoft.Office.Interop.MSProject.PjProjectUpdate.pjReschedule)
                                   pjApplication.FileExit(Microsoft.Office.Interop.MSProject.PjSaveType.pjSave)