This error might occur if you have selected your selection list (dropdownlist) multiple times. You can add multiple=”true” e.g.
in your selection list so as to enable multiple item select.

If you don’t want your selection list to be multiple select enabled then you have to check whether you have selected the selection list multiple times either in your design (aspx file) or in code (aspx.cs file).

convert string to datetime
string datestring = “18 March 2008″;
DateTime parseddate = Convert.ToDateTime(datestring);
Response.Write(parseddate);
convert datetime to string

Change page title and meta tags dynamically/programmatically in C#.net
to change page title
Page.Title = “New Page Title”;