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。
@H_404_96@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：
@H_404_96@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：
@H_404_96@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。
@H_404_96@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：
@H_404_96@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。