Question

我的程序运行，它最初成功连接到数据库。我可以运行查询，一切正常。但是，我似乎无法断开连接并重新连接到数据库。输出说：“System.Data.dll中出现'System.InvalidOperationException'类型的第一次机会异常”这是我的连接方法：

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.Threading;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    class TCNInteraction
    {
        static string connectionString = "Server=localhost\\SQLEXPRESS;Database=TCN;Trusted_Connection=true";
        Button theButton;
        TextBox theTextBox;
        bool currentlyConnectedToADatabase = false;
        SqlConnection cnn = new SqlConnection(connectionString);
        /*
         *Uses the already existing SQL connection string and attempts to establish a connection 
         */
        public bool connectToDatabase(Button inputButton, TextBox inputDatabaseBox)
        {
            theButton = inputButton;
            try
            {
                Console.WriteLine("Attempting to open connection...");
                if (cnn.State != ConnectionState.Open)
                {
                    Console.WriteLine("About to open the connection");
                    cnn.Open();
                }
                Console.Write("I opened the connection!");
                currentlyConnectedToADatabase = true;
                displayRowsOfDatabase(inputDatabaseBox);
                theButton.Text = "Connected";
                return true;

            }
            catch (Exception ex)
            {
                MessageBox.Show("Cannot open connection!");
                return false;
            }
            finally
            {
                cnn.Close();
            }

        }

        private void displayRowsOfDatabase(TextBox inputDatabaseBox)
        {

            theTextBox = inputDatabaseBox;
            using (cnn)
            {

                SqlCommand command = new SqlCommand("SELECT SomeNumber,SomeText, AnotherNumber FROM tcn.Demo;", cnn);
                SqlDataReader reader = command.ExecuteReader();


                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        string printString = int.Parse(reader["SomeNumber"].ToString()) + " " + reader["SomeText"].ToString() + " " + int.Parse(reader["AnotherNumber"].ToString()) + Environment.NewLine;
                        theTextBox.Text += printString;
                    }
                }
                else
                {
                    Console.WriteLine("No rows found.");
                }
                reader.Close();
            }
            cnn.Close();
        }


        public void disconnectFromDatabase()
        {
            MessageBox.Show("Disconnected from Database");
            cnn.Close();
            theButton.Text = "Connect to Database";
            //theTextBox.Text = "";
            currentlyConnectedToADatabase = false;
        }
    }
}

Answer 1

在finally中加入try/catch块以关闭连接。另外，我建议使用bool变量来确定连接是否成功。

public bool connectToDatabase(Button inputButton, TextBox inputDatabaseBox)
{
    bool status = false;

    theButton = inputButton;
    try
    {
        Console.WriteLine("Attempting to open connection...");

        //THIS IS WHERE I GET STUCK THE SECOND TIME
        if (cnn.State != ConnectionState.Open)
        {
            cnn.Open();
        }

        Console.Write("I opened the connection!");
        currentlyConnectedToADatabase = true;
        displayRowsOfDatabase(inputDatabaseBox);
        theButton.Text = "Connected";

        status = true;
    }
    catch (Exception ex)
    {
        MessageBox.Show("Cannot open connection!");
    }
    finally
    {
        cnn.Close();
    }

    return status;
}

Answer 2

我认为您希望if语句为

        //THIS IS WHERE I GET STUCK THE SECOND TIME
        if (cnn.State == ConnectionState.Open)
        {
            cnn.Close();
            cnn.Open();
        }

使用！=如果连接未打开，则只调用cnn.Close（）。

Answer 3

您的try/catch阻止没有关闭您的连接。负责关闭与finally块的连接。即使程序中发生任何异常，它也会关闭连接。请尝试以下...

public bool connectToDatabase(Button inputButton, TextBox inputDatabaseBox)
{
    theButton = inputButton;
    try
    {
            Console.WriteLine("Attempting to open connection...");

            //THIS IS WHERE I GET STUCK THE SECOND TIME
            if (cnn.State != ConnectionState.Open)
            {
                cnn.Open();
            }
            Console.Write("I opened the connection!");
            currentlyConnectedToADatabase = true;
            displayRowsOfDatabase(inputDatabaseBox);
            theButton.Text = "Connected";
            return true;

    }
    catch (Exception ex)
    {
        MessageBox.Show("Cannot open connection!");
    }
    finally
    {
        cnn.Close();
    }

}
public void disconnectFromDatabase()
    {
        MessageBox.Show("Disconnected from Database");
        if(cnn.State == ConnectionState.Open)
        {
            cnn.Close();
        }
        theButton.Text = "Connect to Database";
        //theTextBox.Text = "";
        currentlyConnectedToADatabase = false;
    }

Answer 4

您似乎尝试关闭抛出异常的已关闭连接：

if (cnn.State == ConnectionState.Open)
{
   cnn.Close();
}
cnn.Open();

如果连接已经打开，请尝试上面关闭连接，然后将其打开

我可以打开一次SqlConnection，但我不能再这样做了

4 个答案: