Set sqldatasource at run time

The below asp code sets an sqldatasource.  
I want the the program to look at C:\InetPub\wwwroot\web.config and set the connection string based upon what it finds there.  In other words the parameter ConnectionStrings:DS2DSSQLConnStr won't be set until the program has checked C:\InetPub\wwwroot\web.config

What's the best way to do this?

<asp:SqlDataSource ID="sdsSchemas" runat="server" ConnectionString="<%$ ConnectionStrings:DS2DSSQLConnStr %>"
        SelectCommand="RO_SelectEquityCSchema" SelectCommandType="StoredProcedure" 
        UpdateCommand="RO_UpdateCSchema" UpdateCommandType="StoredProcedure">
        <SelectParameters>
            <asp:SessionParameter Name="rtrExchangeCode" SessionField="RTRExch" />
            <asp:SessionParameter Name="rtrInstrumentTypeCode" SessionField="RTRitc" />            
        </SelectParameters>        
        <UpdateParameters>
            <asp:Parameter Name="FileNameCode" Type="String" />
            <asp:Parameter Name="Required" Type="String" />
            <asp:Parameter Name="rc" Type="int16" Direction="output" />
        </UpdateParameters>
    </asp:SqlDataSource>

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