调试易

我想把用sql语句插到的记录存到一个数据集中，然后通过判断这个数据集是否为空来决定下一步的操作。部分代码是这样的：string _str = "select * from plate_table where hashvalue='" + pwd + "'";
                DataSet dt = new DataSet();
                dt = DbOperate.getDataSet(_str);
                if (dt.Tables.Count==1 && dt.Tables[0].Rows.Count!=0)
                {
                    //string _sql = "update plate_table set theme='" + theme[k] + "',publishtime='" + publishTime[k] + "',lasttime='" + lastTime[k] + "',sponsor='" + sponsor[k] + "',replynum='" + replyNum[k] + "',readnum='" + readNum[k] + "' where hashvalue='" + pwd + "'";
                    string _sql="delete * from plate_table where hashvalue='"+pwd+"'";
                    DbOperate.execCommand(_sql);
                    string _sql1 = "insert into plate_table(theme,publishtime,lasttime,sponsor,replynum,readnum,hashvalue) values('" + theme[k] + "','" + publishTime[k] + "','" + lastTime[k] + "','" + sponsor[k] + "','" + replyNum[k] + "','" + readNum[k] + "','" + pwd + "')";
                    DbOperate.execCommand(_sql1);
                }
                else
                {
                    string _sql = "insert into plate_table(theme,publishtime,lasttime,sponsor,replynum,readnum,hashvalue) values('" + theme[k] + "','" + publishTime[k] + "','" + lastTime[k] + "','" + sponsor[k] + "','" + replyNum[k] + "','" + readNum[k] + "','" + pwd + "')";
                    DbOperate.execCommand(_sql);
                }
在类Dboperate中用这样一段代码：public static DataSet getDataSet(string _sql) {
            try {
                OleDbDataAdapter _orclDataAdaprer;
                DataSet _orclDataSet = new DataSet();                if (!object.Equals(_dataReader, null) && _dataReader.IsClosed == false) {
                    _dataReader.Close();
                    _dataReader = null;
                }
                _orclDataAdaprer = new OleDbDataAdapter(_sql, _orclConnection);                _orclDataAdaprer.Fill(_orclDataSet);
                return _orclDataSet.Copy();            } catch (Exception ex) {
                //MessageBox.Show(_sql + Environment.NewLine + e.Message);
                throw ex;
            }
        }在这句上dt = DbOperate.getDataSet(_str);中断，并且提示类型初始值设定项引发异常
请大家帮忙分析一下为什么会出现这种情况？请尽量说的明白一些。谢谢了。