对三层的基本知识已经有了一定的了解和掌握，可是要想真正去灵活运用它还须要我们去实践，仅仅有经过亲自己主动手操作实现后才干理解得更加透彻。

首先我们须要建一张数据表，以我的为例，建一张Users表，加入UserName、Password、Email等字段。

然后就開始在VS上进行操作，先建一些主要的项目，例如以下表：

实体类是贯穿于三层之间，进行数据传递：

Public Class UserInfoPublic ID As IntegerPublic UserName As StringPublic Password As StringPublic Email As StringProperty UID As IntegerGetReturn IDEnd GetSet(value As Integer)ID = valueEnd SetEnd PropertyProperty UUserName As StringGetReturn UserNameEnd GetSet(value As String)UserName = valueEnd SetEnd PropertyProperty UPassword As StringGetReturn PasswordEnd GetSet(value As String)Password = valueEnd SetEnd PropertyProperty UEmail As StringGetReturn EmailEnd GetSet(value As String)Email = valueEnd SetEnd Property
End Class

DAL层是对数据库进行操作：

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Data
Imports System.Data.SqlClient
Imports Add.ModelPublic Class UserDAOPublic Function ConnectString() As String     '连接数据库ConnectString = "server=.;UID=sa;PWD=123456;database=Login"End FunctionPublic Function AddUser(ByVal userinfo As UserInfo) As BooleanDim con As SqlClient.SqlConnectionDim cmd As SqlClient.SqlCommandDim intResult As Integercon = New SqlClient.SqlConnection(ConnectString())cmd = con.CreateCommand()cmd.CommandText = "insert into Users(UserName,Password,Email) values('" & userinfo.UserName & "','" & userinfo.Password & "','" & userinfo.Email & "')"cmd.CommandType = CommandType.Textcon.Open()intResult = cmd.ExecuteNonQuery()If intResult < 0 ThenReturn TrueElseReturn FalseEnd Ifcon.Close()con = NothingEnd FunctionPublic Function IsExist(ByVal userinfo As Model.UserInfo) As BooleanDim con As SqlClient.SqlConnection = New SqlClient.SqlConnection(ConnectString)Dim cmd As SqlClient.SqlCommandDim dalReader As SqlClient.SqlDataReadercmd = con.CreateCommandcmd.CommandText = "select * from Users where UserName='" & userinfo.UserName & "'"cmd.CommandType = CommandType.Textcon.Open()dalReader = cmd.ExecuteReader()'推断数据是否已经存在If dalReader.HasRows = True ThenReturn TrueElseReturn FalseEnd Ifcon.Close()con = NothingEnd Function
End Class

BLL层进行逻辑推断，而且实现UI和DAL层的数据交互：

Imports Add.DAL
Imports Add.Model
Public Class ManagerPublic Function Adduser(ByVal user As UserInfo) As BooleanDim dao As New UserDAODim userinfo As New UserInfoIf dao.IsExist(user) = True ThenReturn FalseElsedao.AddUser(user)Return TrueEnd IfEnd Function
End Class

UI层是用户操作的界面，为用户提供输入的界面和返回信息：

Imports Add.BLL
Imports Add.Model
Public Class Form1Private Sub btnOK_Click(sender As Object, e As EventArgs) Handles btnOK.ClickDim AddStu As New Manager  '实例化B层Dim UStudent As New UserInfo   '实例化实体类If Trim(txtUserName.Text) = "" ThenMessageBox.Show("请输入username")Exit SubEnd IfIf Trim(txtPassword.Text) = "" ThenMessageBox.Show("请输入password")Exit SubEnd IfIf Trim(txtPassword2.Text) = "" ThenMessageBox.Show("请确认password")Exit SubEnd IfIf Trim(txtPassword.Text) <> Trim(txtPassword2.Text) ThenMessageBox.Show("两次password不同样，请又一次输入")Exit SubEnd If'将输入的数据存储到实体类，进行传值UStudent.UserName = txtUserName.TextUStudent.Password = txtPassword.TextUStudent.Email = txtEmail.TextIf AddStu.Adduser(UStudent) Then  '用户不存在MessageBox.Show("加入成功")Else   '用户已经存在MessageBox.Show("该用户已注冊，请又一次加入")End IfEnd SubPrivate Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.ClickEndEnd Sub
End Class

运行程序：

总结：

通过这个小样例，让自己对三层的理解更加深刻，也知道了该怎样运用它。原先停留在表面上的理解，至此也升华到了操作上。尽管先前有非常多不理解的地方，可是通过实践，自己得到了理解。所以，当我们遇到问题的时候，不要只去理解，要通过实践去做，唯独真正做过之后才干真正理解。