1. 概论：
本文将介绍如何创建一个三层应用程序，并且将介绍如何创建一个Web Service服务。
ADO.NET创建Windows三层结构应用程序的体系架构如下图所示：
该结构分三个层次：表示层、业务层、数据层。
数据层：代表物理数据库。
业务层：负责数据层与表示层之间的数据传输。
表示层：应用程序的客户端，它通过业务层来访问数据库。
表示层所操作的是驻留在内存中的本地数据，当需要更新数据库数据时，要通过业务层提供的更新方法实现。这样可以大大提高应用程序的性能，而且，什么时候更新数据完全由你决定，提高了编程的灵活性。
2.实例:
这里我们具体做一个实例来看看如何用VB.NET创建三层结构的应用程序。
数据库：我们选择SQL SERVER 的NorthWind数据库。
业务层：我们创建一个WebService作为中间层。（需要安装IIS服务）
表示层：我们写一个Windows Form
第一步：创建WebService。
具体步骤如下：
1. 新建一个项目，选择ASP.NET Web服务，命名为：”WebService For 业务层”。
2. 添加两个Sql DataAdapter，一个为Customer_da,它指向NorthWind数据库的Customers表，另一个为Order_da,指向Northwind数据库的Orders表。
3. 然后生成一个Typed DataSet（选择“数据”菜单的“生成数据集”），命名为：Super_ds.
4. 数据库连接已经完成，下一步我们将考虑它与表示层之间的通信，这里我们定义两个方法。一个为：Get_DataSet,它返回一个Super_ds类型的数据集，另一个为：Update_DataSet，它负责更新数据库数据， 方法代码如下：
<WebMethod()> Public Function Get_Dataset() As super_ds
customer_da.Fill(Super_ds1.Customers)
order_da.Fill(Super_ds1.Orders)
Return Super_ds1
End Function
<WebMethod()> Public Sub Update_Dataset()
Super_ds1.AcceptChanges()
End Sub
你可以运行测试一下你建立的这个WebService。它将提供两个方法。返回的DataSet是以XML表示的。
业务层的完整代码如下：
Imports System.Web.Services
Public Class Service1
Inherits System.Web.Services.WebService
‘Web Services Designer Generated Code…….
<WebMethod()> Public Function Get_Dataset() As super_ds
customer_da.Fill(Super_ds1.Customers)
order_da.Fill(Super_ds1.Orders)
Return Super_ds1
End Function
<WebMethod()> Public Sub Update_Dataset()
Super_ds1.AcceptChanges()
End Sub
' WEB SERVICE EXAMPLE
' The HelloWorld() example service returns the string Hello World.
' To build, uncomment the following lines then save and build the project.
' To test this web service, ensure that the .asmx file is the start page
' and press F5.
'
'<WebMethod()> Public Function HelloWorld() As String
' HelloWorld = "Hello World"
' End Function
End Class
第二步：创建表示层
具体步骤如下：
1. 新建一个Windows应用程序，命名为：“Windows Form For 表示层”。
2. 在窗体上添加一个DataGrid,一个Button,Button1的text为“Load”,作用是：从业务层读取数据。
3. 在解决方案窗体中添加Web 引用，将我们自己建立的Web Service for 业务层引入到当前项目中。
4. 向Button1的Click事件添加如下代码：
Dim Customer_Ds As New localhost.super_ds()
Dim ser1 As New localhost.Service1()
Customer_Ds.Merge(ser1.Get_Dataset)
DataGrid1.DataSource = Customer_Ds
这里我们调用了Web Service的Get_DataSet函数，Update_DataSet方法的调用与此完全相同。
表示层的完整代码如下：
Imports Data_Access_表示层
Public Class Form1
Inherits System.Windows.Forms.Form
#Region " Windows Form Designer generated code "
Public Sub New()
MyBase.New()
'This call is required by the Windows Form Designer.
InitializeComponent()
'Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
If disposing Then
If Not (components Is Nothing) Then
components.Dispose()
End If
End If
MyBase.Dispose(disposing)
End Sub
Friend WithEvents Button1 As System.Windows.Forms.Button
Friend WithEvents Button2 As System.Windows.Forms.Button
Friend WithEvents Button3 As System.Windows.Forms.Button
Friend WithEvents Client_DataSet As Data_Access_表示层.localhost.super_ds
Friend WithEvents DataGrid1 As System.Windows.Forms.DataGrid
'Required by the Windows Form Designer
Private components As System.ComponentModel.Container
'NOTE: The following procedure is required by the Windows Form Designer
'It can be modified using the Windows Form Designer.
'Do not modify it using the code editor.
<System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
Me.Button1 = New System.Windows.Forms.Button()
Me.Button2 = New System.Windows.Forms.Button()
Me.Button3 = New System.Windows.Forms.Button()
Me.Client_DataSet = New Data_Access_表示层.localhost.super_ds()
Me.DataGrid1 = New System.Windows.Forms.DataGrid()
CType(Me.Client_DataSet, System.ComponentModel.ISupportInitialize).BeginInit()
CType(Me.DataGrid1, System.ComponentModel.ISupportInitialize).BeginInit()
Me.SuspendLayout()
'
'Button1
'
Me.Button1.Location = New System.Drawing.Point(88, 360)
Me.Button1.Name = "Button1"
Me.Button1.TabIndex = 0
Me.Button1.Text = "load"
'
'Button2
'
Me.Button2.Location = New System.Drawing.Point(232, 360)
Me.Button2.Name = "Button2"
Me.Button2.TabIndex = 1
Me.Button2.Text = "update"
'
'Button3
'
Me.Button3.Location = New System.Drawing.Point(376, 360)
Me.Button3.Name = "Button3"
Me.Button3.TabIndex = 2
Me.Button3.Text = "clear"
'
'Client_DataSet
'
Me.Client_DataSet.DataSetName = "Client_DataSet"
Me.Client_DataSet.Locale = New System.Globalization.CultureInfo("zh-CN")
Me.Client_DataSet.Namespace = "http://www.tempuri.org/CustomerDs.xsd"
'
'DataGrid1
'
Me.DataGrid1.DataMember = ""
Me.DataGrid1.Location = New System.Drawing.Point(40, 56)
Me.DataGrid1.Name = "DataGrid1"
Me.DataGrid1.Size = New System.Drawing.Size(480, 264)
Me.DataGrid1.TabIndex = 3
'
'Form1
'
Me.AutoScaleBaseSize = New System.Drawing.Size(6, 14)
Me.ClientSize = New System.Drawing.Size(568, 429)
Me.Controls.AddRange(New System.Windows.Forms.Control() {Me.DataGrid1, Me.Button3, Me.Button2, Me.Button1})
Me.Name = "Form1"
Me.Text = "Form1"
CType(Me.Client_DataSet, System.ComponentModel.ISupportInitialize).EndInit()
CType(Me.DataGrid1, System.ComponentModel.ISupportInitialize).EndInit()
Me.ResumeLayout(False)
End Sub
#End Region
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim Customer_Ds As New localhost.super_ds()
Dim ser1 As New localhost.Service1()
Customer_Ds.Merge(ser1.Get_Dataset)
DataGrid1.DataSource = Customer_Ds
End Sub
End Class
总结：可见，表示层窗体上完全没有数据库连接控件，它与数据库的连接任务是通过业务层来完成的，这样，程序的结构更加清晰，当然业务层的实现也可以用其他方法，比如：写一个自己的类来完成与数据库的数据传输。
……