I can't pretend that I quite understand what it is you're trying to do, but I THINK that something like this is what you need:
SqlCommand cmd = new SqlCommand("select qno from QUIZ1 where applicationname = @appname", con);
cmd.Parameters.AddWithValue("@appname", "MFDL");
con.Open();
SqlDataReader sdr = cmd.ExecuteReader();
List<string> numbersList = new List<string>();
while (sdr.Read())
{
numbersList.Add(Convert.ToString(sdr["qno"]));
}
string[] numbersArray = numbersList.ToArray();
Mind you, if I were you, I'd work with the resultset as a genereic list instead of an array, but if that's what you really want, then it's up to you...