public void link(particle a, particle b){ if (a == null) return; if (b == null) return; a.link (b); pair_st s = new pair_st (a, b); if (!pDict.ContainsKey (s)) { pDict.Add (s, new List<pair_part> ()); } var par = new pair_part(a.id,b.id); pDict [s].Add (par); }