Remove duplicates in the list using linq

Question

I have a class Items with properties  Id  Name  Code  Price    The List of Items is populated with duplicated items    For ex     1         Item1       IT00001         100 2         Item2       IT00002         200 3         Item3       IT00003         150 1         Item1       IT00001         100 3         Item3       IT00003         150   How to remove the duplicates in the list using linq

User · Answer

You have three option here for removing duplicate item in your List:

Use a a custom equality comparer and then use Distinct(new DistinctItemComparer()) as @Christian Hayter mentioned.

Use GroupBy, but please note in GroupBy you should Group by all of the columns because if you just group by Id it doesn't remove duplicate items always. For example consider the following example:

List<Item> a = new List<Item>
{
    new Item {Id = 1, Name = "Item1", Code = "IT00001", Price = 100},
    new Item {Id = 2, Name = "Item2", Code = "IT00002", Price = 200},
    new Item {Id = 3, Name = "Item3", Code = "IT00003", Price = 150},
    new Item {Id = 1, Name = "Item1", Code = "IT00001", Price = 100},
    new Item {Id = 3, Name = "Item3", Code = "IT00003", Price = 150},
    new Item {Id = 3, Name = "Item3", Code = "IT00004", Price = 250}
};
var distinctItems = a.GroupBy(x => x.Id).Select(y => y.First());

The result for this grouping will be:

{Id = 1, Name = "Item1", Code = "IT00001", Price = 100}
{Id = 2, Name = "Item2", Code = "IT00002", Price = 200}
{Id = 3, Name = "Item3", Code = "IT00003", Price = 150}

Which is incorrect because it considers {Id = 3, Name = "Item3", Code = "IT00004", Price = 250} as duplicate. So the correct query would be:

var distinctItems = a.GroupBy(c => new { c.Id , c.Name , c.Code , c.Price})
                     .Select(c => c.First()).ToList();

3.Override Equal and GetHashCode in item class:

public class Item
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Code { get; set; }
    public int Price { get; set; }

    public override bool Equals(object obj)
    {
        if (!(obj is Item))
            return false;
        Item p = (Item)obj;
        return (p.Id == Id && p.Name == Name && p.Code == Code && p.Price == Price);
    }
    public override int GetHashCode()
    {
        return String.Format("{0}|{1}|{2}|{3}", Id, Name, Code, Price).GetHashCode();
    }
}

Then you can use it like this:

var distinctItems = a.Distinct();

User · Answer

An universal extension method   public static class EnumerableExtensions       public static IEnumerable lt T gt  DistinctBy lt T  TKey gt  this IEnumerable lt T gt  enumerable  Func lt T  TKey gt  keySelector                return enumerable GroupBy keySelector  Select grp   gt  grp First               Example of usage   var lstDst   lst DistinctBy item   gt  item Key

User · Answer

var distinctItems   items GroupBy x   gt  x Id  Select y   gt  y First

User · Answer

Try this extension method out  Hopefully this could help   public static class DistinctHelper       public static IEnumerable lt TSource gt  DistinctBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector                var identifiedKeys   new HashSet lt TKey gt             return source Where element   gt  identifiedKeys Add keySelector element               Usage   var outputList   sourceList DistinctBy x   gt  x TargetProperty

User · Answer

Use Distinct   but keep in mind that it uses the default equality comparer to compare values  so if you want anything beyond that you need to implement your own comparer    Please see http   msdn microsoft com en-us library bb348436 aspx for an example

User · Answer

If there is something that is throwing off your Distinct query  you might want to look at MoreLinq and use the DistinctBy operator and select distinct objects by id   var distinct   items DistinctBy  i   gt  i Id

User · Answer

List lt Employee gt  employees   new List lt Employee gt          new Employee Id  1 Name  AAAAA         new Employee Id  2 Name  BBBBB         new Employee Id  3 Name  AAAAA         new Employee Id  4 Name  CCCCC         new Employee Id  5 Name  AAAAA       List lt Employee gt  duplicateEmployees   employees Except employees GroupBy i   gt  i Name                                                Select ss   gt  ss FirstOrDefault                                                  ToList

User · Answer

When you don t want to write IEqualityComparer you can try something like following     class Program        private static void Main string   args                 var items   new List lt Item gt             items Add new Item  Id   1  Name    Item1             items Add new Item  Id   2  Name    Item2             items Add new Item  Id   3  Name    Item3                Duplicate item         items Add new Item  Id   4  Name    Item4               Duplicate item         items Add new Item  Id   2  Name    Item2              items Add new Item  Id   3  Name    Item3              var res   items Select i   gt  new  i Id  i Name                Distinct   Select x   gt  new Item  Id   x Id  Name   x Name   ToList                now res contains distinct records              public class Item       public int Id   get  set         public string Name   get  set

User · Answer

Another workaround  not beautiful buy workable   I have an XML file with an element called  MEMDES  with two attribute as  GRADE  and  SPD  to record the RAM module information  There are lot of dupelicate items in SPD   So here is the code I use to remove the dupelicated items           IEnumerable lt XElement gt  MList               from RAMList in PREF Descendants  MEMDES               where  string RAMList Attribute  GRADE       DDR4              select RAMList           List lt string gt  sellist   new List lt string gt              foreach  var MEMList in MList                        sellist Add  string MEMList Attribute  SPD   Value                      foreach  string slist in sellist Distinct                          comboBox1 Items Add slist

User · Answer

This is how I was able to group by with Linq  Hope it helps   var query   collection GroupBy x   gt  x title  Select y   gt  y FirstOrDefault

User · Answer

var distinctItems   items Distinct      To match on only some of the properties  create a custom equality comparer  e g    class DistinctItemComparer   IEqualityComparer lt Item gt         public bool Equals Item x  Item y            return x Id    y Id  amp  amp              x Name    y Name  amp  amp              x Code    y Code  amp  amp              x Price    y Price             public int GetHashCode Item obj            return obj Id GetHashCode                 obj Name GetHashCode                 obj Code GetHashCode                 obj Price GetHashCode              Then use it like this   var distinctItems   items Distinct new DistinctItemComparer

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