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Taking a Bite Out of ASP.NET ViewState
Susan Warren
Microsoft Corporation
November 27, 2001
When I meet and talk to new ASP.NET page developers, one of the first things they usually ask me is, "What is that ViewState thing, anyway?" And often you can hear in their voices that same queasy fascination I feel when a waiter in some exotic restaurant parks a plateful of some previously unknown food in front of me. Somebody must think it''''s good; otherwise, they wouldn''''t be serving it. So, I''''ll try it, and maybe even love it, but it sure looks odd!
It''''s that way with ViewState too. Once you get past how it looks, you''''ll find many circumstances where you''''ll be delighted to have ViewState in your ASP.NET application, because it lets you do much more with much less code. But there will also be times when you''''ll want to definitely leave ViewState on the plate. We''''ll look at both scenarios, but first, let''''s answer that question about what is ViewState.
Answer: ViewState Maintains the UI State of a Page
The Web is stateless, and so are ASP.NET Pages. They are instantiated, executed, rendered, and disposed on every round trip to the server. As a Web developer, you can add statefulness using well-known techniques like storing state on the server in Session state or by posting a page back to itself. Take the sign up form in Figure 1 as an example.
Figure 1. Restoring posted form values
You can see I''''ve picked an invalid value for my potluck item. Like most forms on the Web, this one is friendly enough to put a helpful error message and a star next to the field in error. In addition, all of the valid values I entered in the other text boxes and drop-down lists still appear in the form. This is possible, in part, because HTML form elements post their current values from the browser to the server in the HTTP header. You can use ASP.NET tracing to see the form values that are posted back, as in Figure 2.
Figure 2. Values posted in HTTP Form, as shown by ASP.NET trace
Before ASP.NET, restoring the values back into the form fields across multiple postbacks was entirely the responsibility of the page developer, who had to pick them out, one-by-one, from the HTTP form, and push them back into the fields. Happily, ASP.NET does this trick automatically, thus eliminating both a lot of grunt work and a lot of code for forms. But that''''s not ViewState.
ViewState is the mechanism ASP.NET uses to keep track of server control state values that don''''t otherwise post back as part of the HTTP form. For example, the text shown by a Label control is saved in ViewState by default. As a developer, you can bind data or programmatically set the Label just once when the page first loads, and on subsequent postbacks, the label text will be repopulated automatically from ViewState. So in addition to less grunt work and less code, the benefit of ViewState is often fewer trips to the database.
How ViewState Works
There''''s really nothing magical about ViewState. It''''s a hidden form field managed by the ASP.NET page framework. When ASP.NET executes a page, the ViewState values from the page and all of the controls are collected and formatted into a single encoded string, and then assigned to the value attribute of the hidden form field (specifically, <input type=hidden>). Since the hidden form field is part of the page sent to the client, the ViewState value is temporarily stored in the client''''s browser. If the client chooses to post the page back to the server, the ViewState string is posted back too. You can actually see the ViewState form field and its postback value in Figure 2 above.
Upon postback, the ASP.NET page framework parses the ViewState string and populates the ViewState properties for the page and each of the controls. The controls, in turn, use the ViewState data to rehydrate themselves to their former state.
There are three more small, but useful things to know about ViewState.
- You must have a server-side form tag (<form runat=server>) in your ASPX page if you want to use ViewState. A form field is required so the hidden field that contains the ViewState information can post back to the server. And, it must be a server-side form so the ASP.NET page framework can add the hidden field when the page is executed on the server.
- The page itself saves 20 or so bytes of information into ViewState, which it uses to distribute PostBack data and ViewState values to the correct controls upon postback. So, even if ViewState is disabled for the page or application, you may see a few remaining bytes in ViewState.
- In cases where the page does not post back, you can eliminate ViewState from a page by omitting the server side <form> tag.
Getting More from ViewState
ViewState is a marvelous way to track the state of a control across postbacks since it doesn''''t use server resources, doesn''''t time out, and works with any browser. If you are a control author, you''''ll definitely want to check out Maintaining State in a Control.
Page authors can also benefit from ViewState in much the same way. Occasionally your pages will contain UI state values that aren''''t stored by a control. You can track values in ViewState using a programming syntax is similar to that for Session and Cache:
[Visual Basic]'''' save in ViewState
ViewState("SortOrder") = "DESC"
'''' read from ViewState
Dim SortOrder As String = CStr(ViewState("SortOrder"))
[C#]// save in ViewState
ViewState["SortOrder"] = "DESC";
// read from ViewState
string sortOrder = (string)ViewState["SortOrder"];
Consider this example: you want to display a list of items in a Web page, and each user wants to sort the list differently. The list of items is static, so the pages can each bind to the same cached set of data, but the sort order is a small bit of user-specific UI state. ViewState is a great place to store this type of value. Here''''s the code:
[Visual Basic]<%@ Import Namespace="System.Data" %>
<HTML>
<HEAD>
<title>ViewState for Page UI State Values</title>
</HEAD>
<body>
<form runat="server">
<H3>
Storing Non-Control State in ViewState
</H3>
<P>
This example stores the current sort order for a static
list of data in ViewState.<br>
Click the link in the column header to sort the data by that field.<br>
Click the link a second time to reverse the sort direction.
<br><br><br>
<asp:datagrid id="DataGrid1" runat="server"
OnSortCommand="SortGrid" BorderStyle="None" BorderWidth="1px"
BorderColor="#CCCCCC" BackColor="White" CellPadding="5" AllowSorting="True">
<HeaderStyle Font-Bold="True" ForeColor="White"
BackColor="#006699">
</HeaderStyle>
</asp:datagrid>
</P>
</form>
</body>
</HTML>
<script runat="server">
'''' SortField property is tracked in ViewState
Property SortField() As String
Get
Dim o As Object = ViewState("SortField")
If o Is Nothing Then
Return String.Empty
End If
Return CStr(o)
End Get
Set(Value As String)
If Value = SortField Then
'''' same as current sort file, toggle sort direction
SortAscending = Not SortAscending
End If
ViewState("SortField") = Value
End Set
End Property
'''' SortAscending property is tracked in ViewState
Property SortAscending() As Boolean
Get
Dim o As Object = ViewState("SortAscending")
If o Is Nothing Then
Return True
End If
Return CBool(o)
End Get
Set(Value As Boolean)
ViewState("SortAscending") = Value
End Set
End Property
Private Sub Page_Load(sender As Object, e As EventArgs) Handles MyBase.Load
If Not Page.IsPostBack Then
BindGrid()
End If
End Sub
Sub BindGrid()
'''' Get data
Dim ds As New DataSet()
ds.ReadXml(Server.MapPath("TestData.xml"))
Dim dv As New DataView(ds.Tables(0))
'''' Apply sort filter and direction
dv.Sort = SortField
If Not SortAscending Then
dv.Sort += " DESC"
End If
'''' Bind grid
DataGrid1.DataSource = dv
DataGrid1.DataBind()
End Sub
Private Sub SortGrid(sender As Object, e As DataGridSortCommandEventArgs)
DataGrid1.CurrentPageIndex = 0
SortField = e.SortExpression
BindGrid()
End Sub
</script>
[C#]<%@ Page Language="C#" %>
<%@ Import Namespace="System.Data" %>
<HTML>
<HEAD>
<title>ViewState for Page UI State Values</title>
</HEAD>
<body>
<form runat="server">
<H3>
Storing Non-Control State in ViewState
</H3>
<P>
This example stores the curr[1] [2] [3] [4] 下一页
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