C#数据库问题

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

namespace Lab6Q5
{
    public partial class Form1 : Form
    {
        OleDbConnection mDB = new OleDbConnection();

        public Form1()
        {
            InitializeComponent();
        }

        private void UpdateDataGrid(string sqlStr)
        {
            OleDbConnection connStr = new OleDbConnection(
            @"Provider=Microsoft.ACE.OLEDB.12.0;
            Data Source=C:\adpDB\Library2.accdb");

            DataTable dt = new DataTable();

            OleDbDataAdapter da =
            new OleDbDataAdapter(sqlStr, connStr);

            da.Dispose();
        }

        private void btnLoan_Click(object sender, EventArgs e)
        {
         int loanNumber = int.Parse(txtloanNo.Text);
         int patronNumber = int.Parse(txtpatronNo.Text);
         string ISBN = txtISBN.Text;

         string sqlLoan;
         OleDbCommand cmd;

      sqlLoan = "INSERT INTO Loan ("
        + "LoanNo, "
        + "LoanDate, "
        + "PatronNo, "

        + ") VALUES ("

        + toSql(txtloanNo.Text) + ", "
        + toSql(txtpatronNo.Text) + ", "
        + toSql(dateTimePicker1.Value.ToShortDateString())

        + ")";
      try
      {
          cmd = new OleDbCommand(sqlLoan, mDB);
          cmd.ExecuteNonQuery();
          MessageBox.Show("It is inserted successfully");
      }
      catch (Exception ex)
      {
          MessageBox.Show(ex.Message);
      }
      UpdateDataGrid(sqlLoan);

        string sqlLoanBook;

        sqlLoanBook = "INSERT INTO LoanBook ("
          + "LoanNo, "
          + "ISBN, "
          + "DueDate, "

          + ") VALUES ("

          + toSql(txtloanNo.Text) + ", "
          + toSql(txtISBN.Text) + ", "
         + toSql(dateTimePicker1.Value.ToShortDateString())

          + ")";
        try
        {
            cmd = new OleDbCommand(sqlLoanBook, mDB);
            cmd.ExecuteNonQuery();
            MessageBox.Show("It is inserted successfully");
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
        UpdateDataGrid(sqlLoanBook);

        string sqlBook;

        sqlBook = "UPDATE BOOK SET "
        + "ISBN = " + toSql(txtISBN.Text) + ", "
        + "Title = " + ", "
        + "Author = " + ", "
        + "Publisher = " + ", "
        + "Subject_Code = " + ", "
        + "Shelf_Location = " + ", "
        + "Fiction = " + ", "
        + "Price = " + ", "
        + "Bstatus = " + ", ";

        UpdateDataGrid(sqlBook);

      try {
        cmd = new OleDbCommand(sqlBook, mDB);
        cmd.ExecuteNonQuery();
        MessageBox.Show("It is inserted successfully");
      }
      catch (Exception ex) {
        MessageBox.Show(ex.Message);
      }
    }
          private string toSql(bool boolValue)
    {
      return boolValue.ToString();
    }

    private string toSql(DateTime dateTimeValue)
    {
      return "#" + dateTimeValue.ToString("mm-dd-yyyy") + "#";
      //return "'" + dateTimeValue.ToString("mm-dd-yyyy") + "'";
    }

    private string toSql(decimal decimalValue)
    {
      return decimalValue.ToString();
    }

    private string toSql(double doubleValue)
    {
      return doubleValue.ToString();
    }

    private string toSql(int intValue)
    {
      return intValue.ToString();
    }

    private string toSql(string stringValue)
    {
      return "'" + stringValue.Replace("'", "''") + "'";
    }
        }
    }

這個是我寫的程序
是用作图书管的借书程序。一位顾客在这种情况下，只能借出一本书。图书管理员输入书本编号，顾客编号及书本的ISBN。当图书管理员点击按钮时，借书细节将被保存到数据库中的表格及书本的记录将被更新。

但當我按了LOAN按鍵後
為何會出現這個消息框
資料未能被更新

求大大們幫助啊!!!! --------------------编程问答-------------------- 连接关闭了，而你还要访问数据库，调试代码，保持连接正常 --------------------编程问答-------------------- connection.Open();
//code
connection.Close(); --------------------编程问答--------------------

引用 2 楼 ohsorry 的回复:

connection.Open();
//code
connection.Close();

請問這句該放在那裏？ --------------------编程问答-------------------- 连接都没有打开，怎更新数据库？ --------------------编程问答-------------------- 消息显示的很明确,在执行非查询操作时,当然要保持你程序和数据库的通信啊,你关掉了怎么和数据库信息交互啊.
你可以在执行非查询之后关掉和释放和数据库的连接嘛. --------------------编程问答--------------------

引用 3 楼 d19921226 的回复:

引用 2 楼 ohsorry 的回复:connection.Open();
//code
connection.Close();

請問這句該放在那裏？

意思就是，在进行增删该查之前要打开数据库，操作完了也要记着关闭。
看你的代码要在每一句cmd.ExecuteNonQuery();之前加上sqlLoan.Open();之后加上sqlLoan.Close(); --------------------编程问答-------------------- 如果你不确定放在那里你就直接在定义sqlconnection之后就open,然后在程序的最后在close

补充：.NET技术 ,  C#