对象引用未设置为数据契约中对象的实例

我试图通过数据合约发送从数据库获取的值。因为它是一个列表，所以我在数据契约中定义了一个列表。但是我提到了错误。数据契约如下:

[DataContract]
    public class a
    {
        [DataMember]
        public string SessionId { get; set; }
        [DataMember]
        public string StartDate { get; set; }
        [DataMember]
        public string EndDate { get; set; }
        [DataMember]
        public string p1 { get; set; }
        [DataMember]
        public string p2 { get; set; }
        [DataMember]
        public string p3 { get; set; }
        [DataMember]
        public string p4 { get; set; }
        [DataMember]
        public string p5 { get; set; }
        [DataMember]
        public string p6 { get; set; }
    }
    [DataContract]
    public class b : ReturnValuesBase
    {
        [DataMember]
        public List<a> abc;
    }

在我的数据层中，函数定义如下:

public b Search(string SessionId, string StartDate, string EndDate, string a, string b)
    {
        b abc = new b();
        try
        {
            SqlConnection CnStr = new SqlConnection();
            CnStr = new SqlConnection(ConnString);
            CnStr.Open();
            SqlCommand cmd = new SqlCommand("[dbo].[abc]", CnStr);
            SqlParameter prmSQL = new SqlParameter();
            cmd.CommandType = CommandType.StoredProcedure;
            DateTime SDate = Convert.ToDateTime(StartDate);
            DateTime EDate = Convert.ToDateTime(EndDate);
            prmSQL = cmd.Parameters.Add(new SqlParameter("@a", SqlDbType.VarChar, 50));
            if (CriticalOptions == "")
            {
                prmSQL.Value = DBNull.Value;
            }
            else
            {
                prmSQL.Value = a;
            }
            prmSQL = cmd.Parameters.Add(new SqlParameter("@StartDate", SqlDbType.DateTime));
            prmSQL.Value = SDate;
            prmSQL = cmd.Parameters.Add(new SqlParameter("@EndDate", SqlDbType.DateTime));
            prmSQL.Value = EDate;
            prmSQL = cmd.Parameters.Add(new SqlParameter("@Ma", SqlDbType.VarChar,50));
            prmSQL.Value = a;
            SqlDataReader DataReader = cmd.ExecuteReader();
            DataSet DS = new DataSet();
            SqlDataAdapter DataAdapter = new SqlDataAdapter();
            DataAdapter.SelectCommand = cmd;
            CnStr.Close();
            DataAdapter.Fill(DS);
            for (int i = 0; i < DS.Tables[1].Rows.Count; i++)
            {
                DemoSearchList Demo_List = new DemoSearchList();
                Demo_List.a=(Convert.ToString(DS.Tables[1].Rows[i]["a"]));
                Demo_List.b=(Convert.ToString(DS.Tables[1].Rows[i]["b"]));
                Demo_List.c=(Convert.ToString(DS.Tables[1].Rows[i]["c"]));
                Demo_List.d=(Convert.ToString(DS.Tables[1].Rows[i]["d"]));
                Demo_List.e=(Convert.ToString(DS.Tables[1].Rows[i]["e"]));
                Demo_List.f = SessionId;
                Demo_List.g = StartDate;
                Demo_List.h = EndDate;
                a.abc.Add(Demo_List);
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
        return DemoSearchList;
    }

我得到的错误在这一行:

DemoSearchList.DemoSearchLists.Add(Demo_List);

谁来帮帮我