Question

I got the following structure:

 One user can have many Groups and a Group can have many users.

Now I want to get all users that are in the same group like the user to filter by.

For example,
The user “Theo” is in Group one and two. I want all users that are in Group one OR Group two.

How can I achive this with LINQ or generally c#?

This code doesn’t work:

var res = (IEnumerable<User>)Users;
foreach (var item in user.Groups) {
    res = res.Where(usr => usr.Groups.Contains(item));
}
return res.ToList();

Users is a list I got from another method and user is a parameter of type User.

Answer 1

I assume:

• The User class has a property Groups of type List<Group>
• The Group class has a property Users of type List<User>

Use the LINQ SelectMany method:

var usersInSameGroups = user.Groups.SelectMany(group => group.Users).ToList();

Or in query-syntax:

var usersInSameGroups = (from g in user.Groups
                         from userInGroup in g.Users
                         select userInGroup).ToList();

UPDATE

return (from user2 in Users
        where user2.Groups.Intersect(user.Groups).Any()  // Keeps only user2 if it has a common group with user
        select user2).ToList();