为什么我从这个声明中得到 Null。 C# 中的查询语法

发布于 2024-09-03 04:21:12 字数 814 浏览 3 评论 0 原文

这不起作用。将 Null 返回到 dept_list。

        var dept_list = ((from map in DtMapGuestDepartment.AsEnumerable()
                         where map.Field<Nullable<long>>("Guest_Id") == 174
                         select map.Field<Nullable<long>>("Department_id")).Distinct())as IEnumerable<DataRow>;
       DataTable dt = dept_list.CopyToDataTable(); //dept_list comes null here

这按预期工作。

        var dept_list = from map in DtMapGuestDepartment.AsEnumerable()
                         where map.Field<Nullable<long>>("Guest_Id") == 174
                         select map;
       DataTable dt = dept_list.CopyToDataTable(); //when used like this runs correct.

我在这里犯了什么错误。？

原文

This is not working. Returns Null to dept_list.

        var dept_list = ((from map in DtMapGuestDepartment.AsEnumerable()
                         where map.Field<Nullable<long>>("Guest_Id") == 174
                         select map.Field<Nullable<long>>("Department_id")).Distinct())as IEnumerable<DataRow>;
       DataTable dt = dept_list.CopyToDataTable(); //dept_list comes null here

This works as desired.

        var dept_list = from map in DtMapGuestDepartment.AsEnumerable()
                         where map.Field<Nullable<long>>("Guest_Id") == 174
                         select map;
       DataTable dt = dept_list.CopyToDataTable(); //when used like this runs correct.

What mistake is being done by me here. ?

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稀香 2024-09-10 04:21:12

您的第一个查询返回可枚举的值（部门 ID），而不是可枚举的数据行（如第二个查询中所示）。

由于 IEnumerable> 不是 IEnumerable 的子类型，因此 as 运算符返回 null。

（作为旁注，使用普通转换而不是 as 会给你一个 InvalidCastException，它比仅仅返回 null 更有助于查找错误。）

编辑：如果你真的需要一个 DataTable最后，我想你将不得不手动构建它（未经测试）：

var dept_list = ((from map in DtMapGuestDepartment.AsEnumerable()
                     where map.Field<Nullable<long>>("Guest_Id") == 174
                     select map.Field<Nullable<long>>("Department_id")).Distinct())

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Department_id", typeof(long?)));
foreach (long? dept in dept_list) {
    dt.Rows.Add(dept);
}

Your first query is returning an enumerable of values (of the department IDs) instead of an enumerable of data rows (as in the second query).

Since IEnumerable<Nullable<long>> is not a subtype of IEnumerable<DataRow>, the as operator returns null.

(As a side note, using a normal cast instead of as would have given you an InvalidCastException, which is more helpful in finding errors than just returning null.)

EDIT: If you really need a DataTable in the end, I guess you will have to construct it manually (untested):

var dept_list = ((from map in DtMapGuestDepartment.AsEnumerable()
                     where map.Field<Nullable<long>>("Guest_Id") == 174
                     select map.Field<Nullable<long>>("Department_id")).Distinct())

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Department_id", typeof(long?)));
foreach (long? dept in dept_list) {
    dt.Rows.Add(dept);
}

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