How To Assign ConnectionString To A SqlDataSource Within The Namespace?

hi

i am trying to assign ConnectionString to a SqlDataSource   within the namespace but i am getting this error

plz advise

 
Object reference not set to an instance of an object. 

namespace myNameSpace
{   

    public partial class NRP : System.Web.UI.Page
    {
   
        SqlDataSource sds1;


        protected void Page_PreInit(object sender, EventArgs e)
        {
            sds1= new SqlDataSource();
            sds1.ID = "sds1";
            this.Controls.Add(sds1);

            sds1.ConnectionString = ConfigurationManager.ConnectionStrings["my_conn_str"].ToString();

        }
    }
}