The EnableViewState Property in VS .NET

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The EnableViewState Property
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From the description I gave in the preceding section, it might appear that the Page_Load event fires both in response to the first request for the page and as a result of a postback. Unless you re jumping to another page, however, you typically want to restore the values that the user typed in each control before sending the page back to the browser.
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This simple task is overly difficult in classic ASP, but requires no action on your part under ASP.NET because ASP.NET automatically saves the value of all controls. As a matter of fact, you must explicitly do something only if you do not want these val ues to be saved. More precisely, you can disable the persistence mechanism for a given control by setting its EnableViewState property to False:
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Don t save the value of ListBox1 between postbacks. (You can set this property also in the Property window at design time.) ListBox1.EnableViewState = False
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If you don t need to save the value of any control on the form, you can disable the ViewState mechanism completely by setting the Page object s EnableViewState prop erty to False:
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Don t save the value of any control between postbacks. Me.EnableViewState = False
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The value of controls whose EnableViewState property is False isn t included in the hidden __VIEWSTATE field, which results in fewer bytes sent to the client browser and then back to the server. For this reason, you should disable the ViewState mechanism for all the controls that don t really need to be saved between postbacks. Regardless of the value of the EnableViewState property, ASP.NET always restores the state of a few controls such as TextBox and CheckBox controls because their value is included in the form that is posted back to the server. To clear these controls, you must explicitly assign a value to them before the page is sent back to the client.
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The IsPostBack Property
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Consider the following code, which loads a DropDownList control with the list of all the states for which there is at least one publisher:
Private Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) _ Handles MyBase.Load InitializeStateList() End Sub BiblioConnString is the connection string that points to Biblio.mdb. Dim cn As New OleDbConnection(BiblioConnString) Display unique state strings in the ddlStates DropDownList control. Sub InitializeStateList() cn.Open() Dim cmd As New OleDbCommand( SELECT DISTINCT State FROM Publishers", cn) Dim dr As OleDbDataReader = cmd.ExecuteReader() ddlStates.DataSource = dr The State field is used for filling the list of the DropDownList control. ddlStates.DataTextField = State" Bind the control to the data source.
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ddlStates.DataBind() dr.Close cn.Close End Sub
(I ll explain data binding in the next chapter.) In the preceding code snippet, the InitializeStateList routine runs each time the page is requested. However, running this code for any request after the first one is useless because the contents of the ddlStates control are embedded in the hidden __VIEWSTATE field and are restored automatically when the page is reloaded after a postback. You can choose from two ways to avoid this overhead:
Set the EnableStateView property of the ddlStates control to False so that its contents aren t added to the __VIEWSTATE hidden field. Run the InitializeStateList routine only the first time the browser requests the page and skip it during a postback operation.
Choosing between these two approaches isn t an easy decision. Sometimes recalculat ing the contents of a control (or reloading it from a database query) is faster than bloat ing each page with a large __VIEWSTATE field, especially if the client is connecting to the server through a slow dial-up connection. In the majority of cases, however, the best approach is to load the data once and store it in the __VIEWSTATE field. To load the data only once, you must distinguish the first page request from postbacks, which you do by means of the page s IsPostBack read-only property. Here s an improved version of the Page_Load event handler that takes advantage of this property:
Private Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) _ Handles MyBase.Load If Not Page.IsPostBack Then InitializeStateList() End If End Sub ...(The remainder of the code is unchanged)...
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