Index was outside the bounds of the array.

今天寫了這麼一個函數：

 private string DateFormatConversion(string str)
        {
                string year = "";
                char[] pstr = str.ToCharArray();
                for (int k = 0; k < 4; k++)
                {
                    year += pstr[k];
                }
                return year;
        }