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.

F

Answer 1

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.

Answer 2

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<Item>();
        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 => new {i.Id, i.Name})
            .Distinct().Select(x => 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; }
}

Answer 3

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

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

2. 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();
   

Answer 4

This is how I was able to group by with Linq. Hope it helps.

var query = collection.GroupBy(x => x.title).Select(y => y.FirstOrDefault());

Answer 5

var distinctItems = items.Distinct();

To match on only some of the properties, create a custom equality comparer, e.g.:

class DistinctItemComparer : IEqualityComparer<Item> {

    public bool Equals(Item x, Item y) {
        return x.Id == y.Id &&
            x.Name == y.Name &&
            x.Code == y.Code &&
            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());