How to save List of object in to session variable

Unable to cast object of type EMONBL.PersonList to type System.Data.DataSet'.

using (PersonList pList = new PersonList())
            {
                Session["name"] = (PersonList)pList.GetRoleTypePersonDetails();

                DataTable dt = new DataTable();
                DataSet ds=new DataSet();
                try
                {
                    if (Session["name"] != null)
                    {
                        ds = (DataSet)Session["name"];
                        DataView dv = new DataView(ds.Tables[0]);
                        string s = "User";
                        dv.RowFilter = "name=" + s + "";
                        DataTable dt1 = new DataTable();
                        dt1 = dv.ToTable();
                        DropDownList1.DataSource = dt1;
                        DropDownList1.DataTextField = "lastName";
                        DropDownList1.DataValueField = "ID";
                        DropDownList1.DataBind();
                    }