Distinct in LINQ-C#

Question

I have the below list and query expression to get the distinct list.

 List<LinqTest> myList = new List<LinqTest>();
 myList.Add(new LinqTest() { id = 1, value = "a" });
 myList.Add(new LinqTest() { id = 1, value = "b" });
 myList.Add(new LinqTest() { id = 2, value = "c" });
 myList.Select(m => new { m.id}).Distinct().ToList();

I have referred the below SO link linkfor the code. Even after using distinct() i'm getting 3 records with the duplicate values. what could be the reason?

Answer 1

The 3 records that you see at debug time is your existing list. I think all that you've missed is an assignment.

List<LinqTest> myList = new List<LinqTest>();
myList.Add(new LinqTest() { id = 1, value = "a" });
myList.Add(new LinqTest() { id = 1, value = "b" });
myList.Add(new LinqTest() { id = 2, value = "c" });
// not sure why new {m.id} was used in this context
List<int> distinctList = myList.Select(m => m.id).Distinct().ToList();

Answer 2

In your case the Distinct() method uses the default equality comparer of anonimous type object. Therefore the result is not expected. If you want to get unique id values use

List<int> ListOfIDs = myList.Select(m => m.id).Distinct().ToList();

If you want to get distinct objects of a custom type, then you need to implement IEquatable generic interface and provide your own GetHashCode and Equals methods for the type. See MSDN for details.