（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);