I have many databases in web.config connectionstring plus I have multiple platforms like UAT system and PRD system.  In the AppSettings I coded:  

<add key="Platform" value="UAT"/>

and in ConnectionStrings I have for example:

<add name="PRD_OAM" connectionString=....

<add name="UAT_OAM" connectionString=....

What I would like to do on an aspx web page code:

<asp:TemplateField HeaderText="OAM<br />Service Line" 

                              ItemStyle-HorizontalAlign="Center"  SortExpression="Service_Line"> 

               <EditItemTemplate>  

                               <asp:DropDownList ID="ddlEditService_Lines" runat="server"  

                                              DataSourceID="sqlEditGet_Service_Lines" 

                                              AppendDataBoundItems="True"  

                                              DataTextField="Service_Line" 

                                              DataValueField="Service_Line_ID"  

                                              BackColor="LemonChiffon" 

                                              CssClass="control">  

                                              <asp:ListItem Text="No Service Line" Value="0"></asp:ListItem>

                               </asp:DropDownList>

                              <asp:SqlDataSource ID="sqlEditGet_Service_Lines" runat="server"  

--- Problem ---->                   ConnectionString="'<%$appSettings: Platform %>'+'_OAM'" 

                                              SelectCommand="sp_OAM_Get_Service_Lines" 

                                             SelectCommandType="StoredProcedure"> 

                              </asp:SqlDataSource>

               </EditItemTemplate>  

               <ItemTemplate> <%# Eval("Service_Line")%> </ItemTemplate> 

</asp:TemplateField>

Prior to building & publishing the code I change the Platform value to swing the code from one system to another before publishing to the system.

Where Im struggling is with web pages that use SqlDataSource & Connectionstring within the EditItemTemplate.  In all other cases Ive resolved any issues.  Can this be done?  If not, what about in the code behind?  There I run into the problem of the connection string is not initialized in the OnRowEdit routine.

Im working with VS 2010 and Net 3.5 using C#.

Thanks for any help you can provide!

Try asking in a Visual Studio forum. This one is for Visio

https://social.msdn.microsoft.com/Forums/vstudio/en-us/home?category=visualstudio