OrderedDictionary in .NET 3.5 and .NET 4.0 throw exception

gistfile1.cs

//這段程式碼，在4.0裡面會出錯，在3.5不會出錯。

    class Program
    {
        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");
        }
    }