上篇论述了三层的基本概念，作用，关系，现在展示下系统中用户登陆过程中简单应用三层结构设计思想。

      vb.NET的操作如下

       首先建立以下windows应用程序以及类库

  首先建立实体类

Public Class userInfo
    Private _username As String
    Public Property UserName As String
        Get
            Return _username

        End Get
        Set(ByVal value As String)
            _username = value

        End Set
    End Property
    Private _pwd As String
    Public Property Pwd As String
        Get
            Return _pwd

        End Get
        Set(value As String)
            _pwd = value
        End Set
    End Property
End Class

Public Class Form1

    Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
        Me.Close()
    End Sub

    Private Sub btnOK_Click(sender As Object, e As EventArgs) Handles btnOK.Click
        Try
            Dim euser2 As New Model.userInfo
            Dim euser3 As New Model.userInfo

            euser2.UserName = txtUserName.Text.Trim '将U层数据传递给实体
            euser2.Pwd = txtPassword.Text.Trim

            Dim mgr As New BLL.LoginMannager '将数据传递给B层
            euser3 = mgr.UserLogin(euser2)
        Catch ex As Exception
            MessageBox.Show(ex.Message.ToString())

        End Try
    End Sub
End Class

  DAL层：前提引用实体类

Imports System.Data
Imports System.Data.SqlClient
Imports Model

Public Class user
    '创建数据库链接
    Public conn As New SqlConnection("server=LXY;database=login;user Id=sa;password=123456")
    Public Function SelectUser(ByVal User As Model.userInfo) As Model.userInfo
        '传递实体uesrinfo而不是参数id.username 等，这样方便对实体中参数进行调用
        Dim reader As SqlDataReader
        Dim eUser As New Model.userInfo
        Dim sql As String = "select UserName,PassWord FROM User_info WHERE UserName=@username and PassWord=@password"
        Dim cmd As New SqlCommand(sql, conn)

        cmd.CommandText = sql
        cmd.CommandType = CommandType.Text
        cmd.Parameters.Add(New SqlParameter("@username", User.UserName))
        cmd.Parameters.Add(New SqlParameter("@password", User.Pwd))
        conn.Open()
        reader = cmd.ExecuteReader
        While reader.Read()
            eUser.UserName = reader.GetString(0)
            eUser.Pwd = reader.GetString(1)
        End While
        Return eUser
        conn.Close()


    End Function
End Class

Public Class LoginMannager
    Public Function UserLogin(ByVal User As Model.userInfo) As Model.userInfo
        Dim uDao As New DAL.user
        Dim eUser1 As New Model.userInfo

        eUser1 = uDao.SelectUser(User) '将数据传给D层

        If IsNothing(eUser1.UserName) Then
            Throw New Exception("登录失败，请验证请验证登陆名和密码！")
        Else
            MsgBox("登陆成功！")

            Return eUser1
        End If

    End Function
End Class