c# - 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.

Answer 1

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>.