使用SQL数据读取器在C＃中提取1到许多自定义对象

Question

I'm having issues with the SQL reader class.

I have a custom object named Airport on the database. But im having trouble using the datareader correctly. When i try to extract all Airports into a list(see method 2 below) it seems to jump of the while(_reader.Read()) loop before adding the object to the list.

Any suggestions?

To extract the object do i use 3 methods:

1. To find a specific object:

    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. To get all Airports into a list

    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. To build it into an object in C#

     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

In third step you are enumarating reader again and that is why you get only one airport when you want to retrieve all of them.

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;
}

And now we have to change your first method because changing BuildAirport breaks it. We have to read one row in FindAirportByCode now.

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;
}

Use parametrized queries for security.