Validate the date in sql server

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C#

Database as follows

studentid crbillno DOM

133 124290 20-01-2014


In run mode screen as follows

Batch Date 28-06-2014


i have one button called Submit.

When i click the submit i want to validate dropdownlist date must be less than DOB date(DOM).


for that my code as follows

DateTime coursedate;
     coursedate = Convert.ToDateTime(DdlBthDates.SelectedItem.Text.ToString());
     Sql = "select DOM from course where stud_id = '" + Session["stud_id"] + "' and CR_BILL_NO  =  '" + Session["cr_bill_no"] + "'";
     coursedr = drr.ExecRdr(Sql);
     if (coursedate < Convert.ToDateTime(coursedr[0].ToString().Trim()))
     {
         Label23.Text = "date must be ";
         return;
     }
     coursedr.Close();

when i run the code shows error as follows

Invalid attempt to read no data present.

Regards,
Narasiman P.