如果可以更改contacts列表中的对象，可以在foreach中使用简单的Join:

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;
}

如果要创建ContactsByUser的新实例，那么您正在寻找左外部联接，如下所示:

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
                      };

或者，使用其他方法(可能会更慢，具体取决于您的数据，但可能没有关系):

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
                      };