Question

I am using distinct which says

Returns distinct elements from a sequence by using the default
equality comparer to compare values.

Yet when I run this code, I get multiple same id’s

        var ls = ls2.Distinct().OrderByDescending(s => s.id);
        foreach (var v in ls)
        {
            Console.WriteLine(v.id);
        }

I implemented these in my class yet this still doesnt work

class Post : IComparable<Post>, IEqualityComparer<Post>, IComparer<Post>

This is how I implemented it

                int IComparable<Post>.CompareTo(Post other)
                {
                    return (int)(id - other.id);
                }

                bool IEqualityComparer<Post>.Equals(Post x, Post y)
                {
                    return x.id == y.id;
                }

                int IEqualityComparer<Post>.GetHashCode(Post obj)
                {
                    throw new NotImplementedException();
                }

                int IComparer<Post>.Compare(Post x, Post y)
                {
                    return (int)(x.id - y.id);
                }

Answer 1

You need to properly implement GetHashCode() in your comparer – in your case you can just return the hash code of the id:

int IEqualityComparer<Post>.GetHashCode(Post obj)
{
   return obj.id.GetHashCode();
}

Also as pointed out by @dash in a comment you need to implement IEquatable<T> in Post if you choose to go that route (option 1).

A comparer should be implemented in a separate class that you can then pass in in one of the Distinct() overloads (option 2), i.e. in your case could be class MyPostComparer:

var ls = ls2.Distinct(new MyPostComparer()).OrderByDescending(s => s.id);

A third option would be to use the DistinctBy() method of the MoreLinq project.