c# - 使用 SQL datareader 在 c# 中提取 1 到多个自定义对象

Question

我遇到了 SQL 阅读器类的问题。

我在数据库上有一个名为 Airport 的自定义对象。但我无法正确使用数据读取器。当我尝试将所有机场提取到列表中时（请参见下面的方法 2），它似乎在将对象添加到列表之前跳过了 while(_reader.Read()) 循环。

有什么建议么？

要提取对象，我使用 3 种方法：

1. 要查找特定对象：

   public Airport FindAirportByCode(string _airportCode)
   {
       con.Open();
       string query = "SELECT * from Airport WHERE airportCode = '" + _airportCode + "'";
       SqlCommand cmd = new SqlCommand(query, con);
       SqlDataReader _reader = cmd.ExecuteReader();
   
       Airport retAirport = BuildAirport(_reader);
       _reader.Close();
       con.Close();
       return retAirport;
   }
   

2. 将所有机场放入列表

   public List<Airport> SelectAll()
   {
       con.Open();
       List<Airport> airports = new List<Airport>();
       string query = "SELECT * from Airport";
       SqlCommand cmd = new SqlCommand(query, con);
   
       SqlDataReader _reader = cmd.ExecuteReader();
   
       while (_reader.Read())
       {
           Airport temAirport = new Airport();
           temAirport = BuildAirport(_reader);
           airports.Add(temAirport); // It seems to skip this step and only add the last Airport from BuildAirport to the list.
       }
   
       _reader.Close();
       con.Close();
       return airports;
   }
   

3. 将其构建到 C# 中的对象中

       private Airport BuildAirport(SqlDataReader _reader)
   {
   
       Airport temAirport = new Airport();
       while (_reader.Read())
       {
           temAirport.airportCode = (string) _reader["airportCode"];
           temAirport.airportName = (string) _reader["airportName"];
           temAirport.country = (string) _reader["country"];
           temAirport.city = (string) _reader["city"];
       }
       _reader.Close();
   
   
   
       return temAirport;
   }
   

Answer 1

在第三步中，您reader再次枚举，这就是为什么当您想要检索所有机场时只获得一个机场。

private Airport BuildAirport(SqlDataReader _reader)
{

    Airport temAirport = new Airport();
    temAirport.airportCode = (string) _reader["airportCode"];
    temAirport.airportName = (string) _reader["airportName"];
    temAirport.country = (string) _reader["country"];
    temAirport.city = (string) _reader["city"];
    return temAirport;
}

现在我们必须改变你的第一种方法，因为改变BuildAirport会破坏它。我们现在必须读一行FindAirportByCode。

public Airport FindAirportByCode(string _airportCode)
{
    con.Open();
    string query = "SELECT * from Airport WHERE airportCode = @airportCode";
    SqlCommand cmd = new SqlCommand(query, con);
    cmd.Parameters.AddWithValue("@airportCode", _airportCode);

    SqlDataReader _reader = cmd.ExecuteReader();

    Airport retAirport = null;
    if (_reader.Read())
    {
         retAirport = BuildAirport(_reader);
    }

    _reader.Close();
    con.Close();
    return retAirport;
}

使用参数化查询以确保安全。