Is there a “not equal” in a linq join

问题

I am trying accomplish the LINQ query below but I need a "not equal" instead of equal, so that filteredEmployees has all employees from groupA minus groupB.

List<Employee> groupA = getEmployeeA();
List<Employee> groupB = getEmployeeB();        

var filteredEmployees = from a in groupA 
                        join b in groupB on a.Name equals b.Name
                        select a;

回答1:

You don't need a join for that:

var filteredEmployees = groupA.Except(groupB);

Note that this will be a sequence of unique employees - so if there are any duplicates in groupA, they will only appear once in filteredEmployees. Of course, it also assumes you've got a reasonable equality comparer¹. If you need to go specifically on name, you can use ExceptBy from MoreLINQ:

var filteredEmployees = groupA.ExceptBy(groupB, employee => employee.Name);

Or without going into a third party library:

var groupBNames = new HashSet<string>(groupB.Select(x => x.Name));
var filteredEmployees = groupA.Where(x => !groupBNames.Contains(x.Name));

¹ As pointed out in the comments, you can pass in an IEqualityComparer<T> as an argument to Except. I have a ProjectionEqualityComparer class in MiscUtil which makes it easy to build a comparer of the kind you need:

// I can't remember the exact method name, but it's like this :)
var comparer = ProjectionEqualityComparer<Employee>.Create(x => x.Name);
var filteredEmployees = groupA.Except(groupB, comparer);

回答2:

No, a "not equal" operator would get you all combinations of groupA and groupB except the ones where the items were the same.

Using the Except method gets you what you want:

var filteredEmployees = groupA.Except(groupB);

回答3:

In Entity Framework 6, I've gotten better results using

var filteredEmployees = groupA.Where(a => !groupB.Select(b => b.Name).Contains(a.Name));

回答4:

use of this code for decreased server cost :

The code works very fast compared to other code

        var noExistList = (from n in groupA 
                                join o in groupA  on n.Id equals o.Id into p
                                where p.Count() == 0
                                select n).ToList();

note: groupA is a new list for add.

来源：https://stackoverflow.com/questions/3762869/is-there-a-not-equal-in-a-linq-join

标签

linq

join

equals