如何合并两个列表

Question

I want to merge two list of the following class

public class ContactsByUser
{
    public int UserId { get; set; }
    public string Username { get; set; }
    public int CountContacts { get; set; }
    public int CountBehaviorInterview { get; set; }

    public override bool Equals(object obj)
    {
        return this.UserId == ((ContactsByUser)obj).UserId;
    }

    public override int GetHashCode()
    {
        return UserId.GetHashCode();
    }
}

Now I have 2 lists containing the following

    List<ContactsByUser> contacts = new List<ContactsByUser>();
contacts.Add( new ContactsByUser { UserId = 1, Username = user1, CountContacts = 42, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 2, Username = user2, CountContacts = 25, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 3, Username = user3, CountContacts = 7, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 4, Username = user4, CountContacts = 10, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 5, Username = user5, CountContacts = 23, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 6, Username = user6, CountContacts = 60, CountBehaviorInterview = 0});
contacts.Add( new ContactsByUser { UserId = 7, Username = user7, CountContacts = 3, CountBehaviorInterview = 0});

List<ContactsByUser> interviews = new List<ContactsByUser>();
interviews .Add( new ContactsByUser { UserId = 1, Username = user1, CountContacts = 0, CountBehaviorInterview = 33});
interviews .Add( new ContactsByUser { UserId = 2, Username = user2, CountContacts = 0, CountBehaviorInterview = 21});
interviews .Add( new ContactsByUser { UserId = 3, Username = user3, CountContacts = 0, CountBehaviorInterview = 8});
interviews .Add( new ContactsByUser { UserId = 4, Username = user4, CountContacts = 0, CountBehaviorInterview = 17});

What I would like to have when I merge these 2 lists is the following result

{ UserId = 1, Username = user1, CountContacts = 42, CountBehaviorInterview = 33});
{ UserId = 2, Username = user2, CountContacts = 25, CountBehaviorInterview = 21});
{ UserId = 3, Username = user3, CountContacts = 7, CountBehaviorInterview = 8});
{ UserId = 4, Username = user4, CountContacts = 10, CountBehaviorInterview = 17});
{ UserId = 5, Username = user5, CountContacts = 23, CountBehaviorInterview = 0});
{ UserId = 6, Username = user6, CountContacts = 60, CountBehaviorInterview = 0});
{ UserId = 7, Username = user7, CountContacts = 3, CountBehaviorInterview = 0});

On the first list, the CountBehaviorInterview property will always be 0, and on the second list the CountContacts will always be 0.

I have tried using union but all it does is return a list with the 2 lists inside. Is there a way merge these the way i need?

Answer 1

If you're fine with changing the objects in the contacts list, you could use a simple Join with foreach :

foreach(var joined in contacts.Join(interviews, 
                                    o => o.UserId, 
                                    i => i.UserId, 
                                    (originalContact, i) => new {originalContact, i.CountBehaviorInterview}))
{
    joined.originalContact.CountBehaviorInterview = joined.CountBehaviorInterview;
}

---

If you want to create new instances of ContactsByUser , then you're looking for a left outer join, something like this:

var result = from i in contacts
             join o in interviews on i.UserId equals o.UserId into j
             from o in j.DefaultIfEmpty()
             select  new ContactsByUser
                     {
                         UserId = i.UserId, 
                         Username = i.Username, 
                         CountContacts = i.CountContacts, 
                         CountBehaviorInterview = j.Any() ? j.Single().CountBehaviorInterview : 0
                      };

or, using a different method (may be slower, depending on your data, but probably doesn't matter):

var result = from i in contacts
             from o in interviews.Where(k => k.UserId == i.UserId).DefaultIfEmpty()
             select  new ContactsByUser
                     {
                         UserId = i.UserId, 
                         Username = i.Username, 
                         CountContacts = i.CountContacts, 
                         CountBehaviorInterview = o != null ? o.CountBehaviorInterview : 0
                      };

Answer 2

If you don't want to create a new instance of ContactsByUser you could:

var interviewsByUserId = interviews.ToDictionary(i=>i.UserId, i=>i);
contacts.ForEach(c => 
{
    ContactsByUser interview;
    if (!interviewsByUserId.TryGetValue(c.UserId, out interview))
        return;

    c.CountBehaviorInterview = interview.CountBehaviorInterview;
});

Answer 3

Here's how I would tackle this:

var lookup = interviews.ToLookup(i => i.UserId, i => i.CountBehaviorInterview);

var result =
    contacts
        .Select(c => new ContactsByUser()
        {
            UserId = c.UserId,
            Username = c.Username,
            CountContacts = c.CountContacts,
            CountBehaviorInterview = lookup[c.UserId].Sum(),
        })
        .ToList();

This nicely handles missing and also duplicate items in the interviews list.

I get this result: