每当别人问我关于三层的知识的时候我总是推荐自己的《三层架构之抽象工厂加反射》这篇博客。这篇博客是自己当时学习三层的时候写的一个简单的登录例子。这个例子通过登录这个用例简单的介绍了三层，然后过度的三层+简单工厂再过渡到三层+抽象工厂。不管说从方法命名或者代码风格上来说自己都不敢说是最好的，但是体现三层我感觉还是绰绰有余的。然而最近在给别人讲述三层时候被他人指出这篇博客的代码有些许错误，虽然别人指出的方式有些令自己不快，甚至气愤！但是毕竟还有人关注自己，还有人看自己的博客，还有人给自己指出不足，从这个方面来看这还应该算是好事。有错误就改！有错误才能进步嘛。也许这就是米老师总是说的教学相长吧，这次终于体会到了，而且体会很深刻！！！

教学相长

虽有嘉肴，弗食不知其旨也；虽有至道，弗学不知其善也。是故学然后知不足，教然后知困。知不足，然后能自反也；知困，然后自强也。故曰：教学相长也。

----摘自《礼记•学记》

真的是“知不足”了，真的是想“自强”了，为了验证代码的正确性决定将这个登录的小例子又从最简单的三层开始做起逐步增加难度（降低耦合度），重新实现了一遍。再一次用了用VB.NET，再次回到了傻瓜式的VS环境中。

代码如下：

简单三层

界面层

Public Class Login Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click Dim LUser As New Entity.User Dim BCheck As New Bll.BllLogin LUser.User_Id = txtUserId.Text LUser.User_Pwd = txtPwd.Text If BCheck.Check(LUser) = True Then MsgBox("登录成功！") Else MsgBox("登录失败！") End If End Sub Private Sub btnCancle_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCancle.Click End End Sub End Class

BLL层

Public Class BllLogin Function Check(ByVal User As Entity.User) As Boolean Dim DaUser As New DAL.DalUserInfo Dim BlUser As New Entity.User BlUser.User_Id = User.User_Id BlUser = DaUser.Check(BlUser) If BlUser.User_Pwd = User.User_Pwd Then Return True Else Return False End If End Function End Class

DAL层

Imports System.Data.SqlClient Public Class DalUserInfo Dim ConnStr As String = "Data Source=192.168.24.186;Initial Catalog=Student;User ID=sa;Pwd=123456" Dim conn As SqlConnection = New SqlConnection(ConnStr) Function Check(ByVal User As Entity.User) As Entity.User Dim sql As String = "select * from UserInfo where User_ID='" & User.User_Id + "'" Dim cmd As SqlCommand = New SqlCommand(sql, conn) Dim read As SqlDataReader Try conn.Open() read = cmd.ExecuteReader read.Read() User.User_Id = read.Item("User_ID") User.User_Pwd = read.Item("User_Pwd") Return User Catch ex As Exception User.User_Pwd = "" Return User End Try End Function End Class

实体层

Public Class User Private userId As String Private userPwd As String 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 userPwd End Get Set(ByVal value As String) userPwd = value End Set End Property End Class

三层+简单工厂

界面层代码不变

Bll层

Public Class BllLogin Function Check(ByVal User As Entity.User) As Boolean Dim DalFactory As New DAL.DFactory Dim BlUser As New Entity.User BlUser.User_Id = User.User_Id BlUser = DalFactory.CreateUserInfo.Check(BlUser) If BlUser.User_Pwd = User.User_Pwd Then Return True Else Return False End If End Function End Class

工厂

Imports [Interface] Public Class DFactory 'Dim DataBase As String = "Access" Dim DataBase As String = "Sql" Function CreateUserInfo() As IUserInfo Dim DB As IUserInfo Select Case DataBase Case "Sql" DB = New DalUserInfo 'Case "Access" ' DB = New D_UserInfoAccess End Select Return DB End Function End Class

接口

Public Interface IUserInfo Function Check(ByVal IUser As Entity.User) As Entity.User End Interface

DAL层

Imports System.Data.SqlClient Public Class DalUserInfo : Implements [Interface].IUserInfo Dim ConnStr As String = "Data Source=192.168.24.186;Initial Catalog=Student;User ID=sa;Pwd=123456" Dim conn As SqlConnection = New SqlConnection(ConnStr) Public Function Check(ByVal IUser As Entity.User) As Entity.User Implements [Interface].IUserInfo.Check Dim sql As String = "select * from UserInfo where User_ID='" & IUser.User_Id + "'" Dim cmd As SqlCommand = New SqlCommand(sql, conn) Dim read As SqlDataReader Try conn.Open() read = cmd.ExecuteReader read.Read() IUser.User_Id = read.Item("User_ID") IUser.User_Pwd = read.Item("User_Pwd") Return IUser Catch ex As Exception IUser.User_Pwd = "" Return IUser End Try End Function End Class

三层+抽象工厂

界面层代码不变

Bll层代码不变

接口代码不变

工厂代码

Imports [Interface] Imports System.Reflection Public Class DFactory 'Dim DataBase As String = "Access" 'Dim DataBase As String = "Sql" Dim strDB As String = System.Configuration.ConfigurationSettings.AppSettings("DBString") Function CreateUserInfo() As IUserInfo Return CType(Assembly.Load("DAL").CreateInstance("DAL.DalUserInfo" & strDB), IUserInfo) End Function End Class

DAL层

Imports System.Data.SqlClient Public Class DalUserInfoSql : Implements [Interface].IUserInfo 'Dim ConnStr As String = "Data Source=192.168.24.186;Initial Catalog=Student;User ID=sa;Pwd=123456" Dim strConnStr As String = System.Configuration.ConfigurationSettings.AppSettings("ConnStr") Dim conn As SqlConnection = New SqlConnection(strConnStr) Public Function Check(ByVal IUser As Entity.User) As Entity.User Implements [Interface].IUserInfo.Check Dim sql As String = "select * from UserInfo where User_ID='" & IUser.User_Id + "'" Dim cmd As SqlCommand = New SqlCommand(sql, conn) Dim read As SqlDataReader Try conn.Open() read = cmd.ExecuteReader read.Read() IUser.User_Id = read.Item("User_ID") IUser.User_Pwd = read.Item("User_Pwd") Return IUser Catch ex As Exception IUser.User_Pwd = "" Return IUser End Try End Function End Class

配置文件

<?xml version="1.0" encoding="utf-8" ?> <configuration> <startup> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0,Profile=Client" /> </startup> <appSettings> <add key="ConnStr" value ="Data Source=192.168.24.186;Initial Catalog=Student;User ID=sa;Pwd=123456"></add> <add key="DBString" value ="Sql"></add> </appSettings> </configuration>

对于知识的认知真的是每次都有不同的感受，以前的目标只是实现，无论怎么样只要实现就好，有很多地方都是不理解的，现在再去看那些代码感觉理解起来轻松了很多。敲代码的过程中感触最大的就是java和net平台的区别。通过这段时间做DRP的经历明显感觉net简直是傻瓜化的操作，而java操作起来就要烦人的多，也许现在还是对java的IDE不太熟悉吧。