.net framework 3.5 ok, 4.0 會出錯

gistfile1.cs

        static void Main(string[] args)
        {
            var o = new OrderedDictionary();
            for (int i = 0; i < 3; i++)
            {
                o.Add(i, string.Format("{0}_{1}", i.ToString(), "yahoo"));
            }

            foreach (DictionaryEntry item in o)
            {
                o[item.Key] = DateTime.Now.ToString();
            }

            Console.WriteLine("no exception");
        }