Textbox value does not return on gridview linkbutton click

Dear Experts,

Can anybody help me to sort out the issue?

Issue Description : I need to get the textbox entered value in gridview at the time of linkbutton click event.

Design Page :

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="prefLine"
                                                    AllowPaging="True" AllowSorting="True"
                                                    OnPageIndexChanging="GridView1_PageIndexChanging" Width="100%" BackColor="White" BorderColor="#999999" EnableModelValidation="True" GridLines="Vertical">
                                                    <Columns>
                                                        <asp:TemplateField HeaderText="Edit" >
                                                            <ItemTemplate>
                                                                <asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl='<%#String.Format("frmSls.aspx?id={0}&det={1}", HttpUtility.UrlEncode(Eval("nSlsID").ToString()), HttpUtility.UrlEncode(Eval("nSlsDetID").ToString())) %>'
                                                                    Text="Edit" CommandName="select">
                                                                </asp:HyperLink>
                                                            </ItemTemplate>
                                                        </asp:TemplateField>
                                                        <asp:BoundField DataField="nSlsDetID" HeaderText="Location">
                                                            <HeaderStyle HorizontalAlign="Center" />
                                                            <ItemStyle HorizontalAlign="Center" />
                                                        </asp:BoundField>
                                                        <asp:TemplateField HeaderText="Position">
                                                            <ItemTemplate>
                                                                <asp:Label ID="lblRowNumber" Text='<%# Container.DataItemIndex + 1 %>' runat="server" />
                                                            </ItemTemplate>
                                                        </asp:TemplateField>
                                                        <asp:TemplateField HeaderText="Swap">
                                                            <ItemTemplate>
                                                                <telerik:RadNumericTextBox ID="txtSwapTo" Runat="server" Width="30px" NumberFormat-DecimalDigits="0" />
                                                            </ItemTemplate>
                                                        </asp:TemplateField>
                                                        <asp:TemplateField HeaderText="">
                                                            <ItemTemplate>
                                                                <asp:LinkButton ID="lnkSwap" CssClass="button" CommandArgument="swap" runat="server" Text="&#8225;" OnClick="ChangePreference" Font-Bold="True" ForeColor="Maroon" Font-Size="Medium">
                                                                    <img src="../images/swap-arrows.gif" height="16" width="16" alt=""/>
                                                                </asp:LinkButton>
                                                            </ItemTemplate>
                                                        </asp:TemplateField>
                                                    </Columns>
                                                    <FooterStyle BackColor="#CCCCCC" ForeColor="Black" />
                                                    <PagerStyle BackColor="#999999" ForeColor="Black" HorizontalAlign="Center" />
                                                    <SelectedRowStyle BackColor="#008A8C" Font-Bold="True" ForeColor="White" />
                                                    <HeaderStyle BackColor="#000084" Font-Bold="True" ForeColor="White" BorderColor="Black" BorderStyle="Solid" VerticalAlign="Middle" />
                                                    <AlternatingRowStyle BackColor="#DCDCDC" />
                                                    <PagerSettings Position="TopAndBottom" />
                                                </asp:GridView>

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Code Behind :

Protected Sub ChangePreference(sender As Object, e As EventArgs)
        Dim commandArgument As String = TryCast(sender, LinkButton).CommandArgument

        Dim rowIndex As Double = TryCast(TryCast(sender, LinkButton).NamingContainer, GridViewRow).RowIndex
        Dim rowtxt As GridViewRow = GridView1.Rows(rowIndex)
        Dim locationId As Double = Convert.ToDouble(GridView1.Rows(rowIndex).Cells(1).Text)
        Dim preference As Double = 0 'Convert.ToDouble(GridView1.DataKeys(rowIndex).Value)
        Dim txtswap As RadNumericTextBox = TryCast(rowtxt.FindControl("txtSwapTo"), RadNumericTextBox)
        preference = Val(txtswap.Text)
        Me.UpdatePreference(locationId, preference)

        fillDetaisTable(conn)
        Response.Redirect(Request.Url.AbsoluteUri)
    End Sub

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Here 'preference = Val(txtswap.Text)' I get the value as '0(Zero)'.

Actually I should get textbox entered value.

Can anyone help me please?

Regards,
D Patel