Distinct a list with objects by id

Question

I have a program where there is a topic (like a forum), people can react to that topic.

USER:

1. id
2. first name
3. last name

TOPIC:

1. id
2. subject

REACTION:

1. id
2. topic_id
3. content

Code:

List<USER> ListOfAllUsers = new List<USER>();
var AllReactions = from r in db.REACTIONs
                   where r.topic_id == _topic_id
                   select r;

foreach (var itemX in AllReactions)
{
    ListOfAllUsers.Add(itemX.USER);
}

//Distinct the list of duplicates
var ListOfUsers = ListOfAllUsers.Distinct().ToList();

Now, the "distinct" list still has some duplicates, how do I distinct the list based on the user id's? Or maybe there is another better way to do this. Thanks in advance.

Answer 1

You can use GroupBy to achieve that

var ListOfUsers = ListOfAllUsers.GroupBy(x => x.Id)
                                  .Select(g => g.First())
                                  .ToList();

Answer 2

Distinct has an overload that receives an instance of IEqualityComparer<T>, which is an object that contains the logic that allows LINQ to know which two objects are equal, and thus one should be eliminated.

You need to implement this (very simple) interface, something like this:

public class UserEqualityComparer : IEqualityComparer<User>
{
      public bool Equals(User x, User y)
      {
           return x.Id == y.Id;
      }

      public int GetHashCode (User obj)
      {
           return obj.Id.GetHashCode();
      }
}

And then pass an instance of UserEqualityComparer to Distinct():

var ListOfUsers = ListOfAllUsers.Distinct(new UserEqualityComparer()).ToList();

Answer 3

I suggest you to let database return distinct users for you:

    List<USER> ListOfAllUsers = 
         db.REACTIONs.Where(r => r.topic_id == _topic_id)
                     .Select(r => r.USER)
                     .Distinct()
                     .ToList();

That will be translated into single SQL query. Something like (assume your USER table has two columns - Id and Name):

SELECT 
    [Distinct1].[Id] AS [Id], 
    [Distinct1].[Name] AS [Name]
    FROM ( SELECT DISTINCT 
        [Extent2].[Id] AS [Id], 
        [Extent2].[Name] AS [Name]
        FROM  [dbo].[USER] AS [Extent1]
        INNER JOIN [dbo].[REACTION] AS [Extent2] 
            ON [Extent1].[Id] = [Extent2].[UserId]
        WHERE @id = [Extent1].[topic_id]
    )  AS [Distinct1]

Answer 4

MoreLinq (available on NuGet) has a DistincBy method that allow you to use a delegate as equality comparer.

So you only have to do something like this :

var ListOfUsers = ListOfAllUsers.DistinctBy(user => user.id).ToList();

Edit : MoreLinq Link

Answer 5

I tried in .Net core similar code. No need to define explicit comparator. Override Equals and GetHashcode in class. No need to GroupBy. By default Distinct() works even if other props are distinct.

public class User  
    {
        public int Id { get; set; }    
        public string FirstName { get; set; }            
        public string LastName { get; set; }

        public override bool Equals(object obj)
        {
            if (ReferenceEquals(this, obj)) return true;    
            if (obj is User && (obj as User).Id == this.Id) return true;    
            return false;          
        }

        public override int GetHashCode()
        {
            int hashProductCode = Id.GetHashCode();    
            return hashProductCode;
        }    
    }

User[] users =  {
                new User {Id=1, FirstName="John", LastName="Smith" },
                new User {Id=2, FirstName="Mary", LastName="Blood" },
                new User {Id=1, FirstName="Sergey", LastName="Ivanov" }
            };

            var usersDistinct = users.Distinct().ToArray();
            Console.WriteLine(usersDistinct.Count()); //2 =John + Mary