How to initialize an empty value to a datetime variable?

Question

I am trying to check if the DateTime variables "starttime" and "endtime" have null values and then trying to initialize to an empty value as below, but I'm running into below compilation errors. What is the best way to achieve this?

string htmllink = "";
DateTime? starttime = null;
DateTime? endtime = null;
htmllink = (dbNullCheck.isColumnNull(rdr, "html_link")) ? "" : rdr.GetString(3);
starttime = (dbNullCheck.isColumnNull(rdr, "start_time")) ? "" : rdr.GetString(4);
endtime = (dbNullCheck.isColumnNull(rdr, "end_time")) ? "" : rdr.GetString(5);

results.htmllist.Add(new gethtmllist() { resulthtmllink = htmllink, 
    duration = (starttime - endtime).ToString() });

Error:

Error 2 Cannot implicitly convert type 'string' to 'System.DateTime?'

UPDATE:-

                string htmllink = "";
                htmllink = (dbNullCheck.isColumnNull(rdr, "html_link")) ? "" : rdr.GetString(3);                    
                DateTime? starttime = (dbNullCheck.isColumnNull(rdr, "start_time")) ? new DateTime() : rdr.GetDateTime(4);
                DateTime?  endtime = (dbNullCheck.isColumnNull(rdr, "end_time")) ? new DateTime() : rdr.GetDateTime(5);

               results.htmllist.Add(new gethtmllist()
                {
                    resulthtmllink = htmllink,
                    duration = starttime.HasValue && endtime.HasValue ? (endtime.Value - starttime.Value).ToString() : ""
                });

Answer 1

I try an string.Empty to test:starttime = string.IsNullOrEmpty(str) ? (DateTime?)null : DateTime.Now;

It is successed. So your code could be:

starttime = (dbNullCheck.isColumnNull(rdr, "start_time")) ? (DateTime?)null : rdr.GetDateTime(4);
    endtime = (dbNullCheck.isColumnNull(rdr, "end_time")) ? (DateTime?)null: rdr.GetDateTime(5);

Update 1: .Net Fiddle example for string.Empty: Link

Answer 2

You can't set dates to a string

starttime = (dbNullCheck.isColumnNull(rdr, "start_time")) ? "" : rdr.GetString(4);
endtime = (dbNullCheck.isColumnNull(rdr, "end_time")) ? "" : rdr.GetString(5);

should be

starttime = (dbNullCheck.isColumnNull(rdr, "start_time")) ? new DateTime() : rdr.GetDateTime(4);
endtime = (dbNullCheck.isColumnNull(rdr, "end_time")) ? new DateTime() : rdr.GetDateTime(5);