out List<string>未赋值的问题

Question

        List<String> comnameList = new List<string>();
        List<String> comipList = new List<string>();
        List<String> comcomList = new List<string>();public void GetList(out List<string>comnameList, out  List<string>comipList, out List<string>comcomList)
        {
           con.ConnectionString = @"User id=sa;pwd=.;database=db; ;server=(local)";
            con.Open();
            cmd.Connection = con;
            cmd.CommandText = "select comname, ip, community from comindex";
            SqlDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                comnameList.Add(String.Format("{0}", reader[0]));
                comipList.Add(String.Format("{0}", reader[1]));
                comcomList.Add(String.Format("{0}", reader[2]));
            }
            reader.Close();
            con.Close();
        }出现错误：使用了未赋值的 out 参数“comcomList”
          使用了未赋值的 out 参数“comipList”
          使用了未赋值的 out 参数“comcomList”
是为何呢？c#List<string>SQLout

Answer 1

定义方法：
  public List<CompanyEmployee> GetEmployees(string sql, out List<string> listKeys)
        {
            listKeys = new List<string>();
            listKeys.Clear();
            Context.CompanyEmployee.Name = "a";
            var query = Context.CompanyEmployee.Where(sql).ToList();
            foreach (var item in query)
            {
                listKeys.Add(item.EmpId);
            }
            return query;
        }
调用：
 List<string> listTemp = null;
GetEmployees(sql, out listTemp);

Answer 2

因为while (reader.Read())条件有可能不满足，
这个时候返回的list是空的，而out要求返还前要赋值的

Answer 3

        public void GetList(out List<string> comnameList, out  List<string> comipList, out List<string> comcomList)
        {
            //加上这几个试试呢、、、
            comnameList = new List<string>();
            comipList = new List<string>();
            comcomList = new List<string>();
                       con.ConnectionString = @"User id=sa;pwd=.;database=db; ;server=(local)";
            con.Open();
            cmd.Connection = con;
            cmd.CommandText = "select comname, ip, community from comindex";
            SqlDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                comnameList.Add(String.Format("{0}", reader[0]));
                comipList.Add(String.Format("{0}", reader[1]));
                comcomList.Add(String.Format("{0}", reader[2]));
            }
            reader.Close();
            con.Close();
        }

Answer 4

对Out而言，必须在方法中对其完成初始化。