（1 ）Convert.ToDateTime(string)
string格式有要求，必须是yyyy-MM-dd hh:mm:ss
（2）：Convert.ToDateTime(string, IFormatProvider)
DateTime dt;
DateTimeFormatInfo dtFormat = new System.GlobalizationDateTimeFormatInfo();
dtFormat.ShortDatePattern = "yyyy/MM/dd";
dt = Convert.ToDateTime("2012/11/26", dtFormat);
（3）：DateTime.ParseExact()
string dateString = "20121126";
DateTime dt = DateTime.ParseExact(dateString, "yyyyMMdd", System.Globalization.CultureInfo.CurrentCulture);
或者
DateTime dt = DateTime.ParseExact(dateString, "yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture);
