Asp.net with VB.NET adding rows into data table in run time

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ASP.NET

hi all
i am trying to add rows into data table but after add one row then click save the other row disappear because the post back
how can i add new rows but with stay of row before it
thnx
this is my code:

Dim dcItemID = New DataColumn("ItemID", GetType(Int32))
            Dim dcUnit = New DataColumn("UnitID", GetType(Int32))
            Dim ItemPrice = New DataColumn("ItemPrice", GetType(Int32))
            Dim sellprice = New DataColumn("SellPrice", GetType(Int32))


            dt.Columns.Add(dcItemID)
            dt.Columns.Add(dcUnit)
            dt.Columns.Add(ItemPrice)
            dt.Columns.Add(sellprice)

            dt.Rows.Add(Convert.ToInt32(rcItemName.SelectedValue), Convert.ToInt32(rcUnitID.SelectedValue), tbItemPrice.Text, tbSellprice.Text)

What I have tried:

i tried every thing that found in internet plz help