How to get value of GridView BoundField Visible = False

Question

<asp:GridView ID="GridViewInvoice" runat="server" ShowHeaderWhenEmpty="true" DataKeyNames="DOID" AutoGenerateColumns="False" CssClass="mGrid" HeaderStyle-CssClass="th" PagerStyle-CssClass="pgr"

Width="100%" BorderStyle="Solid" style="overflow:auto" OnRowDataBound="GridViewInvoice_RowDataBound" OnRowEditing="GridViewInvoice_RowEditing" OnRowDeleting="GridViewInvoice_RowDeleting" >

<AlternatingRowStyle BackColor="#CCCCFF" />

<Columns>

<asp:BoundField DataField="Tax1" Visible="false" HeaderText="Tax1" SortExpression="Tax1" HeaderStyle-Width="7%" ItemStyle-CssClass="colHidden" HeaderStyle-CssClass="colHidden" ItemStyle-HorizontalAlign="Right" ReadOnly="false" />

</Columns>