<asp:TemplateColumn  HeaderText ="Attributo">                    
                     <FooterTemplate>
                     <asp:DropDownList ID="add_att" Columns="5" Runat="server"  Width="150px" >                           
                       </asp:DropDownList>
                               
                    </FooterTemplate>     
                    <ItemTemplate>
                      <%# DataBinder.Eval(Container.DataItem,"AttDes") %>
                    </ItemTemplate>             
                    <EditItemTemplate>
                      <asp:DropDownList ID="mod_att" Columns="5" Runat="server"  Width="150px" OnSelectedIndexChanged="changeValueAttribute_OnSelectedIndexChanged" AutoPostBack="true">                          
                       </asp:DropDownList>
                    </EditItemTemplate>
                    </asp:TemplateColumn>


<asp:TemplateColumn  HeaderText ="Valore Attributo">                    
                     <FooterTemplate>
                     <asp:DropDownList ID="add_valatt" Columns="5" Runat="server"  Width="150px" >                           
                       </asp:DropDownList>
                               
                    </FooterTemplate>     
                    <ItemTemplate>
                      <%# DataBinder.Eval(Container.DataItem,"VleDes") %>
                    </ItemTemplate>             
                    <EditItemTemplate>
                      <asp:DropDownList ID="mod_valatt" Columns="5" Runat="server"  Width="150px" >                          
                       </asp:DropDownList>
                    </EditItemTemplate>
                    </asp:TemplateColumn>


-------------------------------
protected void changeValueAttribute_OnSelectedIndexChanged(object sender, EventArgs e)
    { 
        DropDownList ddl = ((DropDownList)sender);
        string id = ((DropDownList)sender).SelectedValue.ToString();

        //LoadValueAttribute(ddl, null, id);
        LoadAttribute(ddl, id);
        
    }