"The connection is already Open (state = Open)"

Question

我是C#开发的新手，在尝试创建一个简单的应用程序时，我遇到了一个问题。我正在尝试根据第一个组合框的 SelectedIndexChanged 属性填充一个组合框。Visual Studio在第二个cn中声明。打开（在私有无效cboCities_SelectedIndexChanged中），连接已经打开。但是，从我的代码来看，我相信我已经关闭了它？我做错了什么？任何和所有的帮助都受到高度赞赏。谢谢！

public partial class Form1 : Form
{
    private SqlCeConnection cn = new SqlCeConnection(@"Data Source = Program FilesParkSurveyParkSurvey.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

您应该更改代码以在每个方法中声明一个新连接，并将其包装在 using 语句中。然后它将自动关闭并处置。