intersect two lists with different objects

Question

I have a list of ObjA and ObjB as follows:

List<ObjA> List1;
List<ObjB> List2;

Both ObjA and ObjB has a common field which is User and I want to intersect them based on User.Id.

class ObjA
{ 
  User user;
  .... other properties
}

class ObjB
{ 
  User user;
  .... other properties
}

class User
{
    int Id;
     .... other props
}

How can i intersect these two lists on User.Id with linq?

As a result I want only the list of Users.

score 40 · Accepted Answer · answered Jul 01 '12 at 19:39

40

The general idea is

var commonUsers = list1.Select(a => a.User).Intersect(list2.Select(b => b.User));

However, by itself this assumes that User implements IEquatable<User>, which does not seem to be the case here. So you either need to add this implementation or use the Intersect overload that accepts a custom IEqualityComparer<User>.

answered Jul 01 '12 at 19:39

Jon

428,835
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And then use `commonUsers` to filter both lists. – Oded Jul 01 '12 at 19:40
1

@Oded: That doesn't seem to be what he's after. – Jon Jul 01 '12 at 19:43
1

If you do want the results from one (or both) lists, and not just commonUsers, a join works well. See http://stackoverflow.com/a/5468803/329301 – Daniel Mar 19 '14 at 20:42

score 12 · Answer 2 · answered Nov 02 '16 at 01:18

1.look at this simple code

  var result = (from objA in objAList
                join objB in objBList on objA.user.Id equals objB.user.Id
                select objA/*or objB*/).ToList();

2.complete code

 class QueryJoin
{
    static void Main(string[] args)
    {
        //create users
        User user1 = new User { Id = 1, Name = "anuo1" };
        User user2 = new User { Id = 2, Name = "anuo2" };
        User user3 = new User { Id = 3, Name = "anuo3" };
        User user4 = new User { Id = 4, Name = "anuo4" };
        User user5 = new User { Id = 5, Name = "anuo5" };
        //create objAList
        List<ObjA> objAList = new List<ObjA>();
        objAList.Add(new ObjA { user = user1 });
        objAList.Add(new ObjA { user = user2 });
        objAList.Add(new ObjA { user = user3 });
        //create objBList
        List<ObjB> objBList = new List<ObjB>();
        objBList.Add(new ObjB { user = user3 });
        objBList.Add(new ObjB { user = user4 });
        objBList.Add(new ObjB { user = user5 });

        //intersect
        var result = (from objA in objAList
                      join objB in objBList on objA.user.Id equals objB.user.Id
                      select objA/*or objB*/).ToList();

    }

}

class ObjA
{
    public User user { get; set; }
}

class ObjB
{
    public User user { get; set; }
}

class User
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Raphaël Althaus · Answer 3 · 2012-07-01T19:52:01.353

without need of IEqualityComparer or IEquatable (which would be better anyway)

var commonUsers = list1
                  .Select(l1 => l1.User)
                  .Where(u => list1
                       .Select(l => l.User.Id)
                       .Intersect(list2
                          .Select(l2 => l2.Id))
                       .Contains(u.Id));

or

var commonUsers = list1.Select(l1 => l1.User)
                      .Where(u=> list2.Select(l2 => l2.User.Id)
                                        .Contains(u.Id));

score 2 · Answer 4 · answered Jul 01 '12 at 19:41

The standard way is to utilize an IEqualityComparer object. The default one uses the standard equality comparison. Create a class that implements the IEqualityComparer interface and performs the comparison you want. Then you can call an overload of IEnumerable.Intersect that accepts an instance of your custom compare class

score 0 · Answer 5 · answered Oct 21 '21 at 08:17

0

You can use the extension method Linq

var result = List1.Join(List2, e1 => e1.ID, e2 => e2.ID, (e1, e2)=> e1);

For more information , you can visite https://www.tutorialsteacher.com/linq/linq-joining-operator-join

answered Oct 21 '21 at 08:17

Adam

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intersect two lists with different objects

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