c# - 如何使用 linq 和 EntityFramework 获取对象的补充列表

Question

我有两种类型，一种是另一种的缩减:

    public class A
    {
        public int ID { get; set; }

        public string Name { get; set; }
    }

    public class B
    {
        public int ID { get; set; }

        public string Name { get; set; }

        public string Description { get; set; }
    }

我有一个 List<A>称为 As这是由客户传递给我的，我有一个带有 DbSet<B> 的 EntityFramework DbContext称为 Bs .这两种类型可以在 ID 上进行匹配。

我要得到的是所有的As不在 Bs 中.你将如何在 Linq 中编写它？我正在尝试使用联接，但我似乎无法理解它。

在 T-SQL 中我会做这样的事情:

SELECT A.*
FROM A LEFT JOIN B
    ON A.ID = B.ID
WHERE B.ID is NULL

Answer 1

你可以这样做:

List<A> As = ... //This is a list in memory

//Get all ids of As
var A_ids = As.Select(x => x.ID).ToList();

//Get all ids that are in both As (memory) and TableB (database)
//We put them in a HashSet for performance reasons
var ids_in_tableB_also =
   new HashSet<int>(
       db.TableB
       .Where(x => A_ids.Contains(x.ID))
       .Select(x => x.ID));

//Get the A objects that are in `As` but of which IDs are not in ids_in_tableB_also
//This operation would have been slower if we haven't used the HashSet above
var rest_of_As = As.Where(x => !ids_in_tableB_also.Contains(x.ID)).ToList();