SQL语句在查询分析中能执行，但放在程序中出错？

select a.depname as 部门,c.AccountName as" +
            "科目名称,b.billid as 凭证号,b.money as 金额,b.week as 周数,b.date as 日期"
            + "from department as a inner join fare as b"
            + "on(a.id=b.depid) inner join account as c on (b.accountid=c.id)"在查询分析器中能执行，但在程序中出错， dt = ds4.Tables[0]; 未将对象引用为实例