Private Sub btnLogin_Click(sender As Object, e As EventArgs) Handles btnLogin.Click
        CheckLogin()
    End Sub
    Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
        Application.Exit()
    End Sub
Private Sub CheckLogin()
        errorProvider.Clear()
        If txtBoxUserName.Text = "" Then
            txtBoxUserName.Focus()
            errorProvider.SetError(txtBoxUserName, "请输入用户名")
            Exit Sub
        End If
        If txtBoxPassword.Text = "" Then
            txtBoxPassword.Focus()
            errorProvider.SetError(txtBoxPassword, "请输入密码")
            Exit Sub
        End If
        Dim queryString As String = "select count(*) from UserEnroll_Info where Username=@Username and UserPassword=@Password"
        Dim con As New SqlConnection(connectionString)
        Try
            con.Open()
            '连接成功
        Catch ex As Exception
            '连接失败
            errorProvider.SetError(btnLogin, "连接失败")
            Exit Sub
        End Try
        Dim cmd As New SqlCommand(queryString, con)
        cmd.Parameters.Add("@Username", SqlDbType.VarChar)
        cmd.Parameters("@Username").Value = txtBoxUserName.Text
        cmd.Parameters.Add("@Password", SqlDbType.VarChar)
        cmd.Parameters("@Password").Value = txtBoxPassword.Text
        Try
            Dim ret As Integer = Convert.ToInt32(cmd.ExecuteScalar)
            If ret Then
                '登录成功
                username = txtBoxUserName.Text
                Dim mainWnd As New MainWnd
                mainWnd.Show()
                Finalize()
            Else
                '登录失败
                errorProvider.SetError(btnLogin, "用户名或者密码错误")
            End If
        Catch ex As Exception
            '数据库操作失败
            errorProvider.SetError(btnLogin, "数据库操作失败")
            con.Close()
        End Try
        con.Close()
    End Sub

CREATE TABLE [dbo].[BookSeller_BookInfo](
  [REC_ID] [bigint] IDENTITY(1,1) NOT NULL,
  [ISBN] [nvarchar](20) NOT NULL,
  [BookName] [nvarchar](200) NOT NULL,
  [Author] [nvarchar](50) NOT NULL,
  [Publisher_Date] [smalldatetime] NOT NULL,
  [Publisher] [nvarchar](50) NOT NULL,
  [Class_Name] [nvarchar](50) NOT NULL,
  [Book_Price] [money] NOT NULL,
  [Book_Num] [int] NOT NULL,
  [Provider] [nvarchar](50) NOT NULL
)

CREATE TABLE [dbo].[Library_BookInfo](
  [REC_ID] [bigint] IDENTITY(1,1) NOT NULL,
  [ISBN] [nvarchar](20) NOT NULL,
  [BookName] [nvarchar](200) NOT NULL,
  [Author] [nvarchar](50) NOT NULL,
  [Publisher_Date] [smalldatetime] NOT NULL,
  [Publisher] [nvarchar](50) NOT NULL,
  [Class_Name] [nvarchar](50) NOT NULL,
  [Book_Price] [money] NOT NULL,
  [Book_Num] [int] NOT NULL,
  [Provider] [nvarchar](50) NOT NULL
)

CREATE TABLE [dbo].[UserEnroll_Info](
  [REC_ID] [int] IDENTITY(1,1) NOT NULL,
  [UserName] [nvarchar](50) NOT NULL,
  [UserPassword] [nvarchar](50) NOT NULL,
  [UserMemo] [nvarchar](200) NOT NULL
)

Private Sub mnuUser_Click(sender As Object, e As EventArgs) Handles mnuUser.Click
        Dim userManageWnd As New UserManageWnd
        userManageWnd.ShowDialog() '打开用户管理窗

connectionString = "Data Source=(local);Initial Catalog=图书现场采购系统;Integrated Security=True"

Public txtSQL As String '存放SQL语句
    Public DBSet As DataSet '查询得到的记录集
    Public ErrorMsg As String '存放错误信息
    Public Function ExecuteSQL(ByVal strSQL As String, ByRef errMsg As String) As Integer
        '函数执行SQL的INSERT、DELETE、UPDATE和SELECT语句
        '对于INSERT、DELETE、UPDATE语句，ExecuteSQL返回更新的记录数：-1表示程序异常；0表示更新失败；大于0表示操作成功，更新的记录数
        '对于SELECT语句：DBSet为返回的数据集；ExecuteSQL为返回的查询记录数。
        Dim cnn As SqlClient.SqlConnection
        Dim cmd As New SqlClient.SqlCommand()
        Dim adpt As SqlClient.SqlDataAdapter
        Dim rst As New DataSet()
        Dim SplitSQL() As String
        errMsg = ""
        Try
            SplitSQL = Split(strSQL)
            cnn = New SqlClient.SqlConnection(connectionString)
            If InStr("INSERT,DELETE,UPDATE", UCase(SplitSQL(0))) Then
                cmd.Connection = cnn
                cmd.Connection.Open()
                cmd.CommandText = strSQL
                ExecuteSQL = cmd.ExecuteNonQuery() '返回更新数据记录条数
            Else
                adpt = New SqlClient.SqlDataAdapter(strSQL, cnn)
                adpt.Fill(rst)
                ExecuteSQL = rst.Tables(0).Rows.Count '返回查询记录条数
                DBSet = rst
            End If
        Catch ex As Exception
            errMsg = ex.Message
            ExecuteSQL = -1  '表示执行SQL失败
        Finally
            rst = Nothing
            cnn = Nothing
        End Try
    End Function

Sub main()
    Dim LoginWnd As New LoginWnd
connectionString = "Data Source=(local);Initial Catalog=图书现场采购系统;Integrated Security=True"
    LoginWnd.ShowDialog()
End Sub