Validation textbox at editble mode?

Hi,
when I clicked edit button, the Quantity Field displayed value into a textbox at selected row of datagrid.
the question is: how can I validate this textbox?

thanks.You have to use a TemplateColumn. Then, just add the validation control to the TemplateColumn's EditItemTemplate. Assume the field name is SSN, you'd have something like:

<asp:TemplateColumn>
 <ItemTemplate>
 <%# DataBinder.Eval(Container.DataItem, "SSN") %>
 </ItemTemplate>
 <EditItemTemplate>
 <asp:TextBox runat="server" id="SSNValue" Text='<%# DataBinder.Eval(Container.DataItem, "SSN") %>'></asp:TextBox>
 <asp:RequiredFieldValidator runat="server" ControlToValidate="SSNValue" ErrorMessage="..." />
 </EditItemTemplate>
</asp:TemplateColumn>

That should about do it!