如果使用正確，DataSets能戲劇性地提高你Web站點的性能，然而，如果使用不正確，DataSets會浪費寶貴的服務器資源並降低Web Site的性能。爲了更好地使用DataSets，DataSets可以被cache在存儲器中，響應來自同樣的或是不同的頁面多個請求
Caching DataSets
你可以通過cache對象來緩存DataSet,DataTable或者DataView，爲了緩存DataSet,你應該使用這樣的語句，比如： Cache( "CachedDataSet" ) = dstDataSet ，這個語句增加了一個叫做 CachedDataSet 的項，然後將名爲dstDataSet 的 DataSet 賦值給它
應該意識到，當系統資源變得很少或是應用程序重新啓動，緩存中的項是會被移除的，換句話說，你沒有考慮到當這個項不存在的情況。爲了在任何時候都能從項獲得cache對象，你應該這樣寫：
dstDataSet = Cache( "CachedDataSet" )
If myDataSet Is Nothing Then
' Recreate the DataSet
Cache( "CachedDataSet" ) = dstDataSet
End If
當要從cache中取數據時，總是檢查這個項是否存在，如果不是，重新創建並把這個項重新添加到cache
注意
你必須使用“Is”而不是“=”來比較 cache 中的內容是否存在，你必須使用這個操作符是因爲你要比較的是對象，而不是值。
Displaying Cached Data
假設，你正在構建網上商店，想要把 Products 表的一列商品顯示在 DataGrid 上，或者是，你想要是你的顧客把商品分類顯示在不同的列上。這種情況下，當顧客每次分類商品時，數據沒有變化，只是數據的順序改變了，就沒有必要重新從數據庫列出商品。如下面的例子。
Listing 12.21 CacheProducts.aspx
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.SqlClient" %>
<Script Runat="Server">
Sub Page_Load
If Not IsPostBack Then
' Bind to datagrid
dgrdProducts.DataSource = GetProducts()
dgrdProducts.DataBind()
End If
End Sub
Function GetProducts() As DataView
Dim dstProducts As DataSet
Dim conNorthwind As SqlConnection
Dim dadProducts As SqlDataAdapter
Dim dvwProducts As DataView
dvwProducts = Cache( "Products" )
If dvwProducts Is Nothing Then
dstProducts = New DataSet()
conNorthwind = New SqlConnection(
"Server=localhost;UID=sa;PWD=secret;Database=Northwind" )
dadProducts = New SqlDataAdapter( "Select * from Products", conNorthwind )
dadProducts.Fill( dstProducts, "Products" )
dvwProducts = dstProducts.Tables( "Products" ).DefaultView()
Cache( "Products" ) = dvwProducts
End If
Return dvwProducts
End Function
Sub dgrdProducts_SortCommand( s As Object, e As DataGridSortCommandEventArgs )
Dim dvwProducts As DataView
' Sort DataView
dvwProducts = GetProducts()
dvwProducts.Sort = e.SortExpression
' Rebind to DataGrid
dgrdProducts.DataSource = dvwProducts
dgrdProducts.DataBind()
End Sub
</Script>
<html>
<head><title>CacheProducts.aspx</title></head>
<body>
<form Runat="Server">
<asp:DataGrid
ID="dgrdProducts"
AllowSorting="True"
OnSortCommand="dgrdProducts_SortCommand"
Runat="Server" />
</form>
</body>
</html>
Figure 12.10. Sorting cached products.
在cache中緩存商品戲劇性地改變ASP.NET頁面的性能，通過存儲在內存中的商品，數據庫不必要每次頁面請求就重新查詢一遍，還能保證每次商品都是存在的。如果你要改變商品列表，你可以通過以下代碼移除 Products 項
Cache.Remove( "Products" )
Filtering Cached Data
你可以允許用戶去選擇一個分類來顯示相對應的商品類表，Categories 表和 Products 可以被同時存放在 cache 。 當某個商品類型被選中了，你可以從 cache 中篩選出這個類型的商品。代碼如下：
Listing 12.22 CacheStore.aspx
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.SqlClient" %>
<Script Runat="Server">
Dim dstStore As DataSet
Sub Page_Load
Dim conNorthwind As SqlConnection
Dim dadNorthwind As SqlDataAdapter
dstStore = Cache( "Store" )
If dstStore Is Nothing Then
dstStore = New DataSet()
conNorthwind = New SqlConnection(
"Server=localhost;UID=sa;PWD=secret;Database=Northwind" )
dadNorthwind = New SqlDataAdapter( "Select CategoryID, CategoryName From Categories",
conNorthwind )
conNorthwind.Open()
dadNorthwind.Fill( dstStore, "Categories" )
dadNorthwind.SelectCommand = New SqlCommand( "Select * From Products", conNorthwind )
dadNorthwind.Fill( dstStore, "Products" )
conNorthwind.Close()
Cache( "Store" ) = dstStore
End If
If Not IsPostBack Then
dgrdCategories.DataSource = dstStore
dgrdCategories.DataMember = "Categories"
dgrdCategories.DataBind()
End If
End Sub
Sub dgrdCategories_ItemCommand( s As Object, e As DataListCommandEventArgs )
Dim dvwProducts As DataView
Dim intCategoryID
' Select the Chosen Category
dgrdCategories.SelectedIndex = e.Item.ItemIndex
' Get CategoryID from DataKeys Collection
intCategoryID = dgrdCategories.DataKeys( e.Item.ItemIndex )
' Filter the Products
dvwProducts = dstStore.Tables( "Products" ).DefaultView()
dvwProducts.RowFilter = "CategoryID=" & intCategoryID
dgrdProducts.DataSource = dvwProducts
dgrdProducts.DataBind()
End Sub
</Script>
<html>
<head><title>CacheStore.aspx</title></head>
<body>
<form Runat="Server">
<table width="100%">
<tr><td valign="top" width="200">
<asp:DataList
ID="dgrdCategories"
OnItemCommand="dgrdCategories_ItemCommand"
SelectedItemStyle-BackColor="Yellow"
DataKeyField="CategoryID"
Runat="Server">
<ItemTemplate>
<asp:LinkButton
Text='<%# Container.DataItem( "CategoryName" ) %>'
Runat="Server" />
</ItemTemplate>
</asp:DataList>
</td><td valign="top">
<asp:DataGrid
ID="dgrdProducts"
Runat="Server" />
</td></tr>
</table>
</form>
</body>
</html>
Figure 12.11. Filtering a cached DataSet.
Finding a Row in a Cached DataSet
Imagine that your company wants to build a private intranet Web site that contains a directory of employees. Suppose, furthermore, that your company wants to create a form that enables a user to search through the list of employees to display information on a particular employee (the employee name, phone number, and so on).
你可以緩存所有的僱員信息在 DataView，爲了顯示某個特殊僱員的詳細信息，你可以使用 cache 中 DataView 的 Find 函數。如：
Listing 12.23 CacheEmployees.aspx
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.SqlClient" %>
<Script Runat="Server">
Sub Button_Click( s As Object, e As EventArgs )
Dim dstEmployees As DataSet
Dim conNorthwind As SqlConnection
Dim dadEmployees As SqlDataAdapter
Dim dvwEmployees As DataView
Dim arrValues( 1 ) As Object
Dim intEmployeeIndex As Integer
' Get cached DataView
dvwEmployees = Cache( "Employees" )
If dvwEmployees Is Nothing Then
dstEmployees = New DataSet()
conNorthwind = New SqlConnection(
"Server=localhost;UID=sa;PWD=secret;Database=Northwind" )
dadEmployees = New SqlDataAdapter( "Select * From Employees", conNorthwind )
dadEmployees.Fill( dstEmployees, "Employees" )
dvwEmployees = dstEmployees.Tables( "Employees" ).DefaultView()
dvwEmployees.Sort = "LastName, FirstName"
Cache( "Employees" ) = dvwEmployees
End If
' Find the employee
arrValues( 0 ) = txtLastName.Text
arrValues( 1 ) = txtFirstName.Text
intEmployeeIndex = dvwEmployees.Find( arrValues )
If intEmployeeIndex <> -1 Then
lblName.Text = txtLastName.Text & ", " & txtFirstName.Text
lblPhone.Text = dvwEmployees( intEmployeeIndex ).Row( "HomePhone" )
lblNotes.Text = dvwEmployees( intEmployeeIndex ).Row( "Notes" )
Else
lblError.Text = "Employee Not Found!"
End If
End Sub
</Script>
<html>
<head><title>CacheEmployees.aspx</title></head>
<body>
<h2>Employee Directory</h2>
<form Runat="Server">
<b>First Name:</b>
<asp:TextBox
ID="txtFirstName"
Runat="Server" />
<p>
<b>Last Name:</b>
<asp:TextBox
ID="txtLastName"
Runat="Server" />
<asp:Button
Text="Find!"
OnClick="Button_Click"
Runat="Server" />
<hr>
<asp:Label
ID="lblError"
ForeColor="Red"
EnableViewState="False"
Runat="Server" />
<asp:Label
ID="lblName"
EnableViewState="False"
Runat="Server" />
<p>
<asp:Label
ID="lblPhone"
EnableViewState="False"
Runat="Server" />
<p>
<asp:Label
ID="lblNotes"
EnableViewState="False"
Runat="Server" />
</form>
</body>
</html>