Question

我是编程新手，目前正在开发一个允许用户输入的项目使用textBoxes将数据注册到本地数据库。代码的作用是将项目添加到数据库中按下“Show_users”按钮后，它会在listBox_users列表框中显示它们。

我的问题是，当我从listBox_users中选择一个名称时，它应该在我用来输入的上部文本框中显示有关所选用户的数据使用我为listBox_users创建的事件，首先是数据，但我得到一个错误，“无法读取已经关闭的数据库中的数据”。

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();
            }
        }
    }
}

Answer 1

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被关闭并正确处理。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");
                    }
                }
            }
        }
    }
}

如何使用C＃中本地数据库中的数据在textBox中显示列表框项

1 个答案: