ASP.net monitor all cells where the cell value has been changed

Hi

I have a GridView which has 30 template textbox columns, 4 of which are shown below.
They go from D1 through to  D30
I want detect all cells or text boxes where the value has changes when I click a button
How do I monitor and maintain a record of all cells that are changed?

   <asp:TemplateField HeaderText="D1" Visible="false" ItemStyle-Wrap="true" >
                           <ItemTemplate>
                                <asp:Textbox ID="D1" runat="server"  Visible="true" Width="20"></asp:Textbox>
                           </ItemTemplate>
                      </asp:TemplateField>
                      <asp:TemplateField HeaderText="D2" Visible="false" ItemStyle-Wrap="true" >
                           <ItemTemplate>
                                <asp:Textbox ID="D2" runat="server"  Visible="true" Width="20" ></asp:Textbox>
                           </ItemTemplate>
                      </asp:TemplateField>
                     <asp:TemplateField HeaderText="D3" Visible="false" ItemStyle-Wrap="true">
                           <ItemTemplate>
                                <asp:Textbox ID="D3" runat="server"  Visible="true" Width="20" ></asp:Textbox>
                           </ItemTemplate>
                      </asp:TemplateField>
                      <asp:TemplateField HeaderText="D4" Visible="false" ItemStyle-Wrap="true" >
                           <ItemTemplate>
                                <asp:Textbox ID="D4" runat="server"  Visible="true" Width="20" ></asp:Textbox>
                           </ItemTemplate>
                      </asp:TemplateField>

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