How to populate ListBox with stored procedure results

SqlConnection conn = new SqlConnection(connectionString);
try
{
   conn.Open();
   SqlCommand cmd = new SqlCommand("pr_MySelectProcedure", conn);
   cmd.CommandType = CommandType.StoredProcedure;
   cmd.Connection = conn;
   string tempVal = string.Empty;
   using (SqlDataReader reader = cmd.ExecuteReader())
   {
      // if (reader.HasRows == true) ... passed this test, so continue with other code
      do
      {
         // passes test: if (reader.FieldCount > 0)
         tempVal = reader[0].ToString(); // exception error here: no data present
         // also fails with: tempVal = reader["firstcolumnname"].ToString();
      } while (reader.Read());
   }
}
catch ...