string sql = "SELECT DISTINCT Year(tdate) FROM saletransaction ORDER BY Year(tdate) DESC";
 cmd = new sqlCommand(sql,con);                
 dr = cmd.ExecuteReader();
 DateTime dt;
 while (dr.Read())
 {
     if (dr.HasRows == true)
     {
         dt = Convert.ToDateTime(dr["tdate"].ToString()); //tdate is the name of the column (getting an error at this line. )
         comboBox1.Items.Add(dt.Year.ToString());
     }
 }