string was not recognized as a valid datetime

Question

[FormatException: String was not recognized as a valid DateTime.]
System.DateTimeParse.Parse(String s, DateTimeFormatInfo dtfi, DateTimeStyles styles) +
5052855 System.Convert.ToDateTime(String value)

declaration global

 public string frdate { get; set; }
 public string todate { get; set; }

i want fix this error " String was not recognized as a valid DateTime."

DateTime frdate = Convert.ToDateTime(obj.frdate);
DateTime todate = Convert.ToDateTime(obj.todate);