Question

我是C＃的新手，并且在这里发表过第一篇文章。我正在写一个Windows窗体应用程序，用作数据库输入工具。为此，我要连接到访问数据库以读取列表并捕获新输入

我得到的错误是：

System.InvalidOperationException：'ExecuteReader：连接属性尚未初始化。'

在

OleDbDataReader reader = cmd.ExecuteReader();

导致错误的表格代码为：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.OleDb;

namespace Metrics_Data
{
    public partial class Login : Form
    {
        Welcome conndata = new Welcome();
        public Login()
        {

               InitializeComponent();  

        }

        private void btn_exit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void Login_Load(object sender, EventArgs e)
        {

        }

        private void btn_login_Click(object sender, EventArgs e)
        {
            //Verify Password and UserID
            string Password = null;
            OleDbCommand cmd = new OleDbCommand("SELECT * from [Users] WHERE UserID = @UserID",conndata.myconn);
            cmd.Parameters.Add("@UserID", OleDbType.VarChar).Value = txt_userid;            
            OleDbDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
            Password = reader["User_Password"].ToString();
            }
            reader.Close();
            cmd.Dispose();

            //Load Next Form
            UserHome userHomeform = new Metrics_Data.UserHome();
            this.Hide();
            userHomeform.ShowDialog();
        }
    }
}

从欢迎屏幕连接的代码为：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.OleDb;
using Microsoft.Win32;

namespace Metrics_Data
{
    public partial class Welcome : Form
    {
        public OleDbConnection myconn = null;
        public Welcome()
        {
            try
            {
                InitializeComponent();
                OleDbConnection myconn = new OleDbConnection(@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=\\148.96.211.237\GroupShares\Quality\Electronic Checksheets\MD.accdb;jet OLEDB:Database Password=""Warranty""");

                myconn.Open();
                lbl_Connstatus.Text = "Connected to DB";
                p_dbconn.Visible = true;
            }
            catch(Exception)
            {
                MessageBox.Show("error connecting to DB");
                btn_cfg.Visible = false;
                btn_home.Visible = false;
                btn_help.Visible = true;

            }
        }

        class Variables
        {
            public static dynamic LoginType;
            public static dynamic UserID;
            public static dynamic Admin;
            public static dynamic FirstUse;
            public static dynamic Zone;
            public static dynamic Shift;
            public static dynamic Group;
            public static dynamic CDate;
            public static dynamic GateLevel;
            public static dynamic CountermeasureID;
        }

            private void Welcome_Load(object sender, EventArgs e)
        {

        }

        private void btn_help_Click(object sender, EventArgs e)
        {
            //If program is unable to connect with database, initiate sequence where computer's installed access database engine's are displayed in popups, then tell user that program cannot connect
            //popups are displayed to help user. If no database engine popups are displayed, see the following URL for troubleshooting information:
            //https://support.microsoft.com/en-us/help/2874601/can-t-use-the-access-odbc-driver-or-oledb-provider-outside-office-clic
            //A possible fix to this problem is to install access runtime 2013. The URL below has the link to where it's stored locally:
            //\\10.35.193.112\Open\AccessRuntime2013 
            //Update: This also resolves concerns for computers with MS Office 2016, in which the user will receive a popup indicating they have an OLEDB database engine, but the program still will
            //not work. IT may be required to give users temporary administrative rights to install AccessRuntime2013.
            string AccessDBAsValue = string.Empty;
            RegistryKey rkACDBKey = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Classes");
            if (rkACDBKey != null)
            {
                foreach (string subKeyName in rkACDBKey.GetSubKeyNames())
                {
                    if (subKeyName.Contains("Microsoft.ACE.OLEDB"))
                    {
                        MessageBox.Show(Convert.ToString(subKeyName));
                    }
                }
            }
            MessageBox.Show("The program was unable to connect to the database.");
        }

        private void btn_exit_Click(object sender, EventArgs e)
        {

            Environment.Exit(1);
        }

        private void btn_home_Click(object sender, EventArgs e)
        {
            Login loginform = new Metrics_Data.Login();
            Variables.LoginType = "Data Entry";
            loginform.ShowDialog();
        }

关于我在这里做错什么的任何想法？？？我已经验证了数据库的路径，并且初始连接时的连接字符串没有给出任何错误，在Visual Studio的错误空间中也没有任何错误。

非常感谢！

Answer 1

正如注释中指出的那样，您的构造函数是隐藏具有本地变量的表单属性myconn，因此，当您引用form属性时，它将为null。

但是，如果我不建议您不要将连接存储在form属性中，那我将被撤职。 ADO.NET连接是池化的，通常创建起来很便宜，因此只需在需要时创建一个连接，并在处理完要为其创建的查询后就将其处置（使用using）。

您可以将连接字符串存储在一个位置，但是共享连接对象通常不是一个好主意。

不使用共享连接也会使连接变得更简单，因为您不必担心检查连接是否打开，也不必担心打开两次，等等。

C＃访问数据库连接问题

1 个答案: