Syntax issue in finding the textbox controls used in a GridViews emptytemplate control

Syntax issue in finding the textbox controls used in a GridViews
emptytemplate control

I'm trying to insert data into an xml file via an emptydatatemplate
control that consist of two textbox controls and a link control:
...the design:
<emptydatatemplate>
No Data Found. <br />
<asp:TextBox ID="txtAltprodID" runat="server"></asp:TextBox><br />
<asp:TextBox ID="txtAltprodNum" runat="server"></asp:TextBox><br />
<asp:LinkButton ID="lnkpro" runat="server" OnClick="writeProductInfo"
Text="Add Product Number"></asp:LinkButton>
</emptydatatemplate>
The input submitted by the textbox control are to be placed within the
product table of the following XML file:
<PersonalData>
<products>
<product>
<productID>2</productID>
<productNumber>123744</productNumber>
</product>
<product>
<productID>7</productID>
<productNumber>123456</productNumber>
</product>
</products>
<customers>
<customer>
<LastName>Doe</LastName>
<FirstName>Sam</FirstName>
<Address>1234 Crescent RD</Address>
<City>San Antonio</City>
<State>Texas</State>
<ZipCode>67894</ZipCode>
</customer>
</customers>
</PersonalData>
I Created the following event handler for the Add Product info link:
Public Sub writeProductInfo(ByVal sender As System.Object, ByVal e As
System.EventArgs)
Dim altprodID As String =
DirectCast(gvProductNumber.EmptyDataTemplate.FindControl("txtAltprodID"),
TextBox).Text
Dim altprodNum As String =
DirectCast(gvProductNumber.EmptyDataTemplate.FindControl("txtAltprodNum"),
TextBox).Text
BindGridView()
Dim oDs As DataSet = gvProductNumber.DataSource
Dim oDr As DataRow = oDs.Tables(0).NewRow
oDr("productID") = altprodID
oDr("productNumber") = altprodNum
oDs.Tables(0).Rows.Add(oDr)
oDs.WriteXml(Request.PhysicalApplicationPath +
"~/xml/PersonDataVI.xml")
BindGridView()
End Sub
...however, there is an issue in my syntax:
Dim altprodID As String =
DirectCast(gvProductNumber.EmptyDataTemplate.FindControl("txtAltprodID"),
TextBox).Text
Dim altprodNum As String =
DirectCast(gvProductNumber.EmptyDataTemplate.FindControl("txtAltprodNum"),
TextBox).Text
...could I get some help in how I'm to go about in setting up the two
lines above to read the ID's of the textbox controls from the
emptydatatemplate? Thank You