通过查资料，敲例子，对三层也有了一定的理解，下面是我通过对三层的学习之后，敲的一段机房收费系统中登录界面的例子。希望大家多多提出宝贵意见！
界面：
实体类-Model（和数据库表对应）
[vb]
Public Class User
Private name As String
Private password As String
Private Userid As String
Private level As String
Private account As String
'以下为可读写属性
Public Property user_name() As String
Get
Return name
End Get
Set(ByVal value As String)
name = value
End Set
End Property
Public Property user_id() As String
Get
Return Userid
End Get
Set(ByVal value As String)
Userid = value
End Set
End Property
Public Property user_pwd() As String
Get
Return password
End Get
Set(ByVal value As String)
password = value
End Set
End Property
Public Property user_level() As String
Get
Return level
End Get
Set(ByVal value As String)
level = value
End Set
End Property
Public Property user_account() As String
Get
Return account
End Get
Set(ByVal value As String)
account = value
End Set
End Property
End Class
UI层-表现层 （界面的操作）
[vb]
Public Class Form1
Private Sub butCancel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butCancel.Click
End
End Sub
Private Sub butOK_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butOK.Click
'定义一个实体类对象，将文本框中的内容赋给实体类对象
Dim thisUser As Model.User = New Model.User
thisUser.user_id = txtUserID.Text.ToString()
thisUser.user_pwd = txtUserPWD.Text.ToString()
'定义一个业务逻辑层对象，根据业务逻辑层返回的结果，通知用户信息
Dim uB As BLL.UserBLL = New BLL.UserBLL
If uB.query_user(thisUser) Then
MsgBox(True)
Else
MsgBox(False)
End If
End Sub
End Class
BLL层-业务逻辑层 （将数据层的数据返回，传递数据）
[vb]
Public Class UserBLL
'定义一个D层的对象
Dim db As DAL.UserDB = New DAL.UserDB
'此函数的作用是返回D层的执行结果
Public Function query_user(ByVal model As Model.User) As Boolean
Return db.Query_User(model)
End Function
End Class
DAL层-数据访问层 （数据库连接，查询语句）
[vb]
Imports System.Data.SqlClient
Public Class UserDB
'数据库连接语句
Dim str As String = "Data Source=localhost;Initial Catalog=Charge;Persist Security Info=True;User ID=sa;Password=123456"
Dim conn As SqlClient.SqlConnection
Public Sub New() '构造函数 建立连接，打开数据库
conn = New SqlClient.SqlConnection
conn.ConnectionString = str
conn.Open()
End Sub
'查询数据库中的用户是否存在
Public Function Query_User(ByVal tstUser As Model.User) As Boolean
'数据库查询语句
Dim sqlstr As String
sqlstr = "Select * From user_info Where user_id='" & tstUser.user_id & "' And user_PWD = '" & tstUser.user_pwd & "'"
'查询
Dim sqlcmd As SqlClient.SqlCommand = New SqlClient.SqlCommand(sqlstr, conn)
Dim myReader As SqlDataReader '定义一个reader用来读数据
myReader = sqlcmd.ExecuteReader() '运行你的查询，结果到myReader
If myReader.Read() Then '如果查到了数据
Return True
Else
Return False
End If
End Function
End Class
总结：
学习了三层架构，将一个系统分成DAL,BLL,UI层，实现了客户端和数据库之间的解耦，当我需要改动某一项功能时，只需改动本层的结构，这样大大的提高了系统的可维护性和可扩展性。
当然与我们之前用vb做的机房收费系统的登陆功能相比，当时不管是操作数据库的还是响应用户的需求等，所有的实现均在放在一起实现，分不清层次，用到什么就打开什么，对于系统的可理解性和维护性都不符合编码要求。
