“连接已经打开（状态=开）”

Question

我是新的C＃开发，并尝试创建一个简单的应用程序，我遇到了一个问题。我想基于第一个ComboBox的SelectedIndexChanged属性填充一个ComboBox。 Visual Studio在第二个cn.Open（在private void cboCities_SelectedIndexChanged）中声明连接已经打开。但是，从我的代码中我相信我已经关闭了它？我做错了什么？任何和所有的帮助是高度赞赏。谢谢！

public partial class Form1 : Form 
{ 
    private SqlCeConnection cn = new SqlCeConnection(@"Data Source = \Program Files\ParkSurvey\ParkSurvey.sdf; Persist Security Info = False; Password = *"); 

    public Form1() 
    { 
     InitializeComponent(); 
    } 

    private void Form1_Load(object sender, EventArgs e) 
    { 
     cn.Open(); 
     SqlCeCommand command = cn.CreateCommand(); 
     command.CommandText = "SELECT Name FROM Cities ORDER BY Name ASC"; 
     SqlCeDataAdapter da = new SqlCeDataAdapter(command); 
     DataSet ds = new DataSet(); 
     da.Fill(ds); 
     cn.Close(); 
     cboCities.ValueMember = "CityId"; 
     cboCities.DisplayMember = "Name"; 
     cboCities.DataSource = ds.Tables[0]; 
     cboCities.SelectedIndex = -1; 

    } 

    private void cboCities_SelectedIndexChanged(object sender, EventArgs e) 
    { 
     if (cboCities.SelectedIndex > -1) 
     { 
      cn.Open(); 
      string Cities = cboCities.SelectedValue.ToString(); 
      SqlCeCommand command = cn.CreateCommand(); 
      command.CommandText = "SELECT Name FROM Parks WHERE CityId ='" + Cities + "'"; 
      SqlCeDataAdapter da = new SqlCeDataAdapter(command); 
      DataSet ds = new DataSet(); 
      da.Fill(ds); 
      if (cn.State == ConnectionState.Open) 
      cn.Close(); 
      cboParks.ValueMember = "ParkId"; 
      cboParks.DisplayMember = "Name"; 
      cboParks.DataSource = ds.Tables[0]; 
      cboParks.SelectedIndex = -1; 

     } 
    } 

} 

Answer 1

你应该改变你的代码以声明每个方法一个新的连接，并在using语句包裹。然后它会自动关闭并处理。