How to add Required Field Validator for TextBox inside GridView ItemTemplate

HTML

    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="Item" HeaderText="Item" />
            <asp:TemplateField HeaderText="Price">
                <ItemTemplate>
                    <asp:TextBox ID="txtPrice" runat="server" Text='<%# Eval("Price") %>'></asp:TextBox><br />
                    <asp:RequiredFieldValidator ID="rfvPrice" ControlToValidate="txtPrice" runat="server"
                        ErrorMessage="Required" ForeColor="Red"></asp:RequiredFieldValidator>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Quantity">
                <ItemTemplate>
                    <asp:TextBox ID="txtQuantity" runat="server" Text='<%# Eval("Quantity") %>'></asp:TextBox><br />
                    <asp:RequiredFieldValidator ID="rfvQuantity" ControlToValidate="txtQuantity" runat="server"
                        ErrorMessage="Required" ForeColor="Red"></asp:RequiredFieldValidator>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <br />
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" />

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Item"), new DataColumn("Price"), new DataColumn("Quantity") });
        dt.Rows.Add("Shirt");
        dt.Rows.Add("Football", 299, 10);
        dt.Rows.Add("Shirt", 545, 15);
        dt.Rows.Add("Disc", 99, 20);
        dt.Rows.Add("Watch", 200, 45);
        dt.Rows.Add("Clock", 670, 97);
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}