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嗨，我是编程新手，目前正在从事一个项目，该项目允许用户使用文本框将其注册数据输入本地数据库。该代码的工作原理是将项目添加到数据库中，并且在我按下“Show_users”按钮后，它会将它们显示在 listBox_users 列表框中。

我的问题是，当我从 listBox_users 中选择一个名称时，它应该在上部 textBox'es 中显示有关所选用户的数据，我曾经使用我为 listBox_users 创建的事件首先输入数据，但我得到一个“无法从已关闭的数据库中读取数据”的错误。

namespace Userform
{
public partial class Form1: Form
{
    SqlCeDataReader rdr;

    public Form1()
    {
        InitializeComponent();
    }

    // Some code between...

    private void button_ShowUsers_Click(object sender, EventArgs e) //code that shows users in listBox
    {

        var dt = new DataTable();
        string connectionString2 = @"Data Source=MyDatabase;Password=xxxxxx;";

        using (var cn = new SqlCeConnection(connectionString2))
        using (var cmd = new SqlCeCommand("Select * From Users", cn))
        {
            cn.Open();

            using (var reader = cmd.ExecuteReader())
            {
                dt.Load(reader);
                var results = (from row in dt.AsEnumerable()
                               select new
                               {
                                   //UserID = row.Field<int>("ID"),
                                   FirstName = row.Field<string>("Firsname"),
                                   LastName = row.Field<string>("Lastname"),
                                   FullName = row.Field<string>("Firstname") + " " + row.Field<string>("Lastname")
                               }).ToList();

                listBox_users.DataSource = results;
                listBox_users.DisplayMember = "FullName";

                rdr = cmd.ExecuteReader();
                }
            }
        }

    //I made an event for the listBox_users:

    private void listBox_users_SelectedIndexChanged(object sender, EventArgs e) 
    //event code that should show listbox selected data in the textBoxes
    {            
        if (listBox_inimesed.SelectedItem != null && rdr != null)
        {
            try
            {
                if (rdr.Read())
                {

                    textBox1_firstname.Text = rdr.GetString(1);
                    textBox2_lastname.Text = rdr.GetString(2);
                    textBox3_email.Text = rdr.GetString(3);
                    textBox4_address.Text = rdr.GetString(4);
                    dateTimePicker1.Value = rdr.GetDateTime(5);
                    richTextBox_info.Text = rdr.GetString(6);
                }
                else MessageBox.Show("Object not found");
            }
            finally
            {
                rdr.Close();
            }
        }
    }
}
4

1 回答 1

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您正在使用的阅读器所依赖的连接已在 button_ShowUsers_Click 事件中关闭。

尝试跨事件共享连接和 DataReader 不是一个好习惯。这将导致未正确处理的打开连接。更好的做法是在每个事件方法中创建 Connection、Command 和 DataReader。通过使用“使用”语句，它们将在每种方法中正确关闭和处理。您还可以删除类级别变量“rdr”。

namespace Userform
{
    public partial class Form1 : Form
    {
        const string connectionString2 = @"Data Source=MyDatabase;Password=xxxxxx;";

        public Form1()
        {
            InitializeComponent();
        }

        // Some code between...

        private void button_ShowUsers_Click(object sender, EventArgs e) //code that shows users in listBox
        {
            var dt = new DataTable();

            using (var cn = new SqlCeConnection(connectionString2))
            using (var cmd = new SqlCeCommand("Select * From Users", cn))
            {
                cn.Open();

                using (var reader = cmd.ExecuteReader())
                {
                    dt.Load(reader);
                    var results = (from row in dt.AsEnumerable()
                                   select new
                                   {
                                       //UserID = row.Field<int>("ID"),
                                       FirstName = row.Field<string>("Firsname"),
                                       LastName = row.Field<string>("Lastname"),
                                       FullName = row.Field<string>("Firstname") + " " + row.Field<string>("Lastname")
                                   }).ToList();

                    listBox_users.DataSource = results;
                    listBox_users.DisplayMember = "FullName";
                }
            }
        }

    //I made an event for the listBox_users:

        private void listBox_users_SelectedIndexChanged(object sender, EventArgs e)
        //event code that should show listbox selected data in the textBoxes
        {
            if (listBox_inimesed.SelectedItem != null)
            {
                using (var cn = new SqlCeConnection(connectionString2))
                using (var cmd = new SqlCeCommand("Select * From Users", cn))
                {
                    cn.Open();

                    using (var reader = cmd.ExecuteReader())
                    {
                        if (reader.Read())
                        {
                            textBox1_firstname.Text = reader.GetString(1);
                            textBox2_lastname.Text = reader.GetString(2);
                            textBox3_email.Text = reader.GetString(3);
                            textBox4_address.Text = reader.GetString(4);
                            dateTimePicker1.Value = reader.GetDateTime(5);
                            richTextBox_info.Text = reader.GetString(6);
                        }
                        else MessageBox.Show("Object not found");
                    }
                }
            }
        }
    }
}
于 2013-04-18T15:01:43.853 回答