expression tree instead of using LINQ
public static IEnumerable<T> TakeTopDistinct<T>(this IEnumerable<T> source, int count)
{
    if (source == null) throw new ArgumentNullException("source");
    if (count < 0) throw new ArgumentOutOfRangeException("count");
    if (count == 0) yield break;

    var comparer = Comparer<T>.Default;
    LinkedList<T> selected = new LinkedList<T>();

    foreach(var value in source)
    {
        if(selected.Count < count // need to fill
            || comparer.Compare(selected.Last.Value, value) < 0 // better candidate
            )
        {
            var tmp = selected.First;
            bool add = true;
            while (tmp != null)
            {
                var delta = comparer.Compare(tmp.Value, value);
                if (delta == 0)
                {
                    add = false; // not distinct
                    break;
                }
                else if (delta < 0)
                {
                    selected.AddBefore(tmp, value);
                    add = false;
                    if(selected.Count > count) selected.RemoveLast();
                    break;
                }
                tmp = tmp.Next;
            }
            if (add && selected.Count < count) selected.AddLast(value);
        }
    }
    foreach (var value in selected) yield return value;
}

List<T> GetFirstNElements (IList<T> list, int n) where T :IComparable <T>{
    list.Sort();
    List<T> returnList = new List<T>();
    for(int i = 0; i<n; i++){
        returnList.Add(list[i]);
    }
    return returnList;
}