Question

我正试图将3个实体表（仅具有少量列的填充数据）连接到数据表中

DataTable dttable = new DataTable();

dttable.Columns.Add("billno", typeof(String));
dttable.Columns.Add("date", typeof(DateTime));
dttable.Columns.Add("time", typeof(String));
dttable.Columns.Add("sname", typeof(String));
dttable.Columns.Add("name", typeof(String));
dttable.Columns.Add("qty", typeof(Decimal));
dttable.Columns.Add("rate", typeof(Decimal));


var rows = from mobjbmast in Context.bmasts.AsEnumerable()
           join mobjbtran in Context.btrans
           on mobjbmast.billno equals mobjbtran.billno
           join mobjwaiter in Context.waiters
           on mobjbmast.scode equals mobjwaiter.code
           where mobjbmast.billno == mbillno
           select dttable.LoadDataRow(new object[]
           {
                 mobjbmast.billno,
                 mobjbmast.date,
                 mobjbmast.time,
                 mobjwaiter.name,
                 mobjbtran.name,
                 mobjbtran.qty,
                 mobjbtran.rate
           }, false);

期望上面的代码可以填充数据表。但是没有任何结果。调试以上代码后，调试器中将显示“行”变量

无法计算表达式。不支持该操作。未知错误：0x80070057'

所以我尝试了这个：

var rows = from mobjbmast in Context.bmasts.AsEnumerable()
           join mobjbtran in Context.btrans.AsEnumerable()
           on mobjbmast.billno equals mobjbtran.billno
           join mobjwaiter in Context.waiters.AsEnumerable()
           on mobjbmast.scode equals mobjwaiter.code
           where mobjbmast.billno == mbillno
           select new { billno = mobjbmast.billno, date = mobjbmast.date, time = mobjbmast.time, sname = mobjwaiter.name, name = mobjbtran.name, qty = mobjbtran.qty, rate = mobjbtran.rate };

方案没有变化。

通过ToList()

进行了上述尝试

var rows = from mobjbmast in Context.bmasts.ToList()
           join mobjbtran in Context.btrans.ToList()
           on mobjbmast.billno equals mobjbtran.billno
           join mobjwaiter in Context.waiters.ToList()
           on mobjbmast.scode equals mobjwaiter.code
           where mobjbmast.billno == mbillno
           select new { billno = mobjbmast.billno, date = mobjbmast.date, time = mobjbmast.time, sname = mobjwaiter.name, name = mobjbtran.name, qty = mobjbtran.qty, rate = mobjbtran.rate };

这次，错误消失了，但是在调试器中，我看到“枚举未产生结果”消息。

还尝试添加DefaultIfEmpty()。

我知道我可以将此输出加载到List<T>中。但是这里需要一个数据表，并且不需要一个类。

如何构造查询以返回适当的IEnumerable以转换为数据表？

Answer 1

最后，根据一些网站的建议，我像这样解决了它

DataTable dttable = new DataTable();

dttable.Columns.Add("billno", typeof(String));
dttable.Columns.Add("date", typeof(DateTime));
dttable.Columns.Add("time", typeof(String));
dttable.Columns.Add("sname", typeof(String));
dttable.Columns.Add("name", typeof(String));
dttable.Columns.Add("qty", typeof(Decimal));
dttable.Columns.Add("rate", typeof(Decimal));

var rows = from mobjbmast in Context.bmasts.AsEnumerable()
           join mobjbtran in odlsContext.btrans
           on mobjbmast.billno equals mobjbtran.billno
           join mobjwaiter in Context.waiters
           on mobjbmast.scode equals mobjwaiter.code
           where mobjbmast.billno == mbillno
           let billarray = new object[]
           {
                mobjbmast.billno,
                mobjbmast.date,
                mobjbmast.time,
                mobjwaiter.name,
                mobjbtran.name,
                mobjbtran.qty,
                mobjbtran.rate
           }
           select billarray;
foreach (var array in rows)
{
    dttable.Rows.Add(array);
}

如何解决错误-无法评估表达式。不支持该操作。未知错误：0x80070057

1 个答案: