三层：
用图做说明：
MVC：
用图做说明：
三层与MVC联系：
用图做说明：
三层与MVC区别：
第二：三层适用于任何一个项目，MVC有选择性的使用，且主要使用在WEB页面上
第三：三层基于业务的逻辑来分；MVC基于页面来分。
第四：三层目的重点是高内聚，低耦合，即解耦；MVC目的重点是实现WEB系统的职能划分，也是解耦。
第五：三层不一定就是三层，也有可能是二层，乃至多层；MVC却总是三层
三层实例：
上面是对三层和MVC简单的一个理解，由于现在侧重点不是MVC，而是三层，所以也就没有做有关MVC的例子，对于MVC怎样也不了解。
层次展示：
窗体界面：
Common类：
与数据相关，与工程无关。
Imports System.Data.sqlClient Public Class CDataBase '要连接的数据库连接字符串 Public Shared Function strConnection() As String strConnection = "data source=192.168.24.83;initial catalog=charge_sys;user id=sa;password=123456;" End Function Public Shared Function ExecQuery(ByVal strsql As String) As sqlClient.sqlDataReader Dim Coon As New sqlClient.sqlConnection Dim cmd As New sqlClient.sqlCommand Dim Reader As sqlClient.sqlDataReader '数据库连接 Coon = New sqlClient.sqlConnection(CDataBase.strConnection()) cmd.Connection = Coon cmd.CommandText = strsql cmd.CommandType = CommandType.Text Try Coon.Open() '查询 Reader = cmd.ExecuteReader() '读取字段 Catch ex As Exception MsgBox(ex.Message()) End Try Return Reader Coon.Close() End Function End Class
DAL层：
Imports System.Data.sqlClient Public Class DabaBase Function SeletUser(ByVal leavel As Modal.Muser) As List(Of Modal.Muser) Dim strsql As String Dim reader As sqlClient.sqlDataReader Dim common = New Common.CDataBase Dim UserLeavel As Modal.Muser Dim Dlist As New List(Of Modal.Muser) Dim Query As String Query = leavel.level '获取传过来的leavel字段 Select Case Query Case "管理员" strsql = "select * from User_Info where Level ='管理员'" Case " 操作员" strsql = "select * from User_Info where Level ='操作员'" Case "一般用户" strsql = "select * from User_Info where Level ='一般用户'" End Select '查询 reader = common.ExecQuery(strsql) Try While reader.Read() UserLeavel = New Modal.Muser UserLeavel.userID = reader.GetString(0).Trim UserLeavel.PWD = reader.GetString(1).Trim UserLeavel.level = reader.GetString(2).Trim UserLeavel.UserName = reader.GetString(3).Trim UserLeavel.Head = reader.GetString(4).Trim Dlist.Add(UserLeavel) End While Catch ex As Exception MsgBox(ex.Message()) '获取异常 End Try Return Dlist End Function End Class
Modal类：
Public Class Muser
Private _userID As String
Public Property userID As String
Get
Return _userID
End Get
Set(ByVal value As String)
_userID = value
End Set
End Property
Private _PWD As String
Public Property PWD As String
Get
Return _PWD
End Get
Set(ByVal value As String)
_PWD = value
End Set
End Property
Private _Level As String
Public Property level As String
Get
Return _Level
End Get
Set(ByVal value As String)
_Level = value
End Set
End Property
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 _Head As String
Public Property Head As String
Get
Return _Head
End Get
Set(ByVal value As String)
_Head = value
End Set
End Property
End Class
BLL层：
BLL层，进行各种逻辑判断，将获取字段通过参数传到DAL层，同时又将结果返回。
Public Class BJudge
Function GetUser(ByVal leavel As Modal.Muser) As List(Of Modal.Muser)
Dim selectuser As New List(Of Modal.Muser) '初始化
Dim LDal As New DataBase.DabaBase '初始化
'传参（获取字段和查询结果）
selectuser = LDal.SeletUser(leavel)
Return LDal.SeletUser(leavel)
End Function
End Class
UI层：
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles MyBase.Load
'下拉框中显示字段
cmbSelect.Items.Add("管理员")
cmbSelect.Items.Add("一般用户")
cmbSelect.Items.Add("操作员")
End Sub
Private Sub cmbSelect_SelectedValueChanged(ByVal sender As System.Object,ByVal e As System.EventArgs) Handles cmbSelect.SelectedValueChanged
Dim sourse As New BLL.BJudge
Dim users As New Modal.Muser
users.level = cmbSelect.SelectedItem() '选择的字段给users
DataGridView1.DataSource = sourse.GetUser(users)
End Sub
End Class
查询结果：
对于MVC我还没有具体的接触，知道的也就这么一点，我想在不久的将来，我会用到MVC，也会更加深刻的理解他。而对于三层，还没有真正的使用过他，就仅仅是做过两个小例子，简单尝试了下三层的分层，但是它最大的好处——解耦，说实在的，还不是很透彻的理解他。