How to grand total in Dynamic Gridview when column was auto generated

Question

i want grand total on footer textbox plz suggest me best code.......

 

<asp:GridView ID="GridView1" OnRowCreated="GridView1_RowCreated" OnRowDataBound="GridView1_RowDataBound" runat="server" AutoGenerateColumns="False" Width="480px" Height="169px" ShowFooter="True" CellPadding="4" ForeColor="#333333" GridLines="None" OnSelectedIndexChanged="GridView1_SelectedIndexChanged">

<AlternatingRowStyle BackColor="White" ForeColor="#284775" />

<Columns>

<asp:TemplateField HeaderText="SrNO">

<ItemTemplate>

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

</ItemTemplate>

</asp:TemplateField>

<asp:TemplateField HeaderText="ProductName">

<ItemTemplate>

<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownlist1_SelectedIndexChanged2" AutoPostBack="true" ></asp:DropDownList>

</ItemTemplate>

</asp:TemplateField>

<asp:TemplateField HeaderText="Quntity">

<ItemTemplate>

<asp:TextBox ID="TextBox3" OnTextChanged="TextBox4_TextChanged" AutoPostBack="true" runat="server"></asp:TextBox>

</ItemTemplate>

</asp:TemplateField>

<asp:TemplateField HeaderText="Rate">

<ItemTemplate>

<asp:TextBox ID="TextBox4" OnTextChanged="TextBox4_TextChanged" AutoPostBack="true" runat="server" ></asp:TextBox>

</ItemTemplate>

<FooterTemplate>

<asp:Label ID="lbl1" Text="GrandTotal" runat="server"></asp:Label>

</FooterTemplate>

</asp:TemplateField>

<asp:TemplateField HeaderText="TotalAmount">

<ItemTemplate>

<asp:TextBox ID="TextBox5" OnTextChanged="TextBox4_TextChanged" AutoPostBack="true" runat="server"></asp:TextBox>

</ItemTemplate>

<FooterStyle Horizontal />

<FooterTemplate>

<asp:TextBox ID="TextBox6" runat="server" AutoPostBack="true" OnTextChanged="TextBox6_TextChanged" ></asp:TextBox>

</FooterTemplate>

</asp:TemplateField>

<asp:TemplateField>

<FooterStyle Horizontal />

<FooterTemplate>

<asp:Button ID="add" runat="server" Text="Add Row" OnClick="add_Click" />

</FooterTemplate>

</asp:TemplateField>