DistinctForKey.cs

public static IEnumerable<T> Distinct(this IEnumerable<T> set, Func<T,U> propertySelector)
{
    HashSet<U> distinct = new HashSet<U>();
    foreach(var a in set)
    {
         var key = propertySelector.Invoke(a);
         if(!distinct.Contains(key))
         {
              distinct.Add(key);
              yield return a;
         }
    }
    yield break;
}