ASP.NET

public class ExSqlDataSource : SqlDataSource
{
    public override string ConnectionString
    {
        get
        {
            return base.ConnectionString;
        }
        set
        {
            string conString = HttpContext.Current.Items["MyConnecionString"] as string;
            if (string.IsNullOrEmpty(conString))
                conString = value;
                   
            base.ConnectionString = conString;
        }
    }    
}

<cc1:ExSqlDataSource ID="SqlDataSource1" runat="server" 
ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
SelectCommand="..."></cc1:ExSqlDataSource>

public partial class Default_ : System.Web.UI.Page
{
    protected void Page_Init(object sender, EventArgs e)
    {
        HttpContext.Current.Items["MyConnecionString"] = "new connect string goes here";
    }
}