如何在EF Core中使用LINQ将原始数据类型列表与实体连接起来？

Question

In a .NET Standard 2.0 C# class library project, I am using EF Core . Using LINQ, I want to join a list of integers with a list of users to get the results like follows:

 //List of Ids(can contain 10,000+ entries)
 var lstNums = new List<int>(){1,2,3,4 };

 var lstOutput = (from u in _context.Users
                  join id in lstNums on u.Id equals id 
                  select u).ToList();

This query was working fine in EntityFramwork 6.2 in Database-First approach. Recently I migrated to EF Core Code-First and this query stopped working. I tested with 3 records in lstNums but still it's not working.

The Contains() method works fine in this case but the problem is I don't know how entries lstNums will contain. I am not using Contains() since it will exceed the IN clause max-length in some cases.

Any solution to this problem is highly appreciated.

Answer 1

Joins to in-memory collections are not supported well by all EF versions. EF6 supports the for primitive type collection like in your example, while EF Core doesn't. This might change in some future version (they are working extensively on query translation, and currently even rewriting it for v3.0).

The safer approach for such type of in-memory collections though is to use Contains instead of join, because it's know to work in all EF versions, eg instead of

join id in lstNums on u.Id equals id

use

where lstNums.Contains(u.Id)