gistfile1.cs

public static string ToIDList<T, U>(List<U> lookuplist) where U : ILookup<T> where T : struct
        {
            List<U> lookupidlist = lookuplist.FindAll(delegate(U lookup)
            {
                return lookup.ID.HasValue;
            });
            string idlist = String.Empty;
            if (lookupidlist != null)
            {
                string[] ids = new string[lookupidlist.Count];
                for (int x = 0; x < lookupidlist.Count; x++)
                {
                    ids[x] = lookupidlist[x].ID.ToString();
                }
                idlist = String.Join(",", Unique(ids));
            }
            return idlist;
        }