Update Datatable from DataGrid

[gi11ies]

I'm wondering if anyone has any tips on updating a value in a DataTable from a textbox in a DataGrid.

I have my text box value populated in the DataGrid using:

<itemtemplate>
<asp:textbox id="txtQuantity" Width="30" Text='<%#Container.DataItem("Quantity")%>' runat="server"/>
</itemtemplate>

I have a button that when pressed, I would like it to call a routine to update all the rows in the DataTable and DataGrid with the new values entered into the DataGrid, but just cant get it to work.

The Code I have been using, but it fails to work is as follows:

Sub btnUpdate (sender as System.Object, e as System.EventArgs )
objDT = Session("Cart")
Dim dgItem as DataGridItem
For Each objDR In objDT.Rows
objDR("Quantity") = dgItem.FindControl("txtQuantity")
Next

dg.DataBind()
objDT = dg.DataSource
Session("Cart") = objDT
End Sub

Any tips or suggestions on how to do this would be great.

Gillies