All about Viewstate

All about Viewstate

The ASP.NET ViewState is a powerful feature and maintains the state of pages between postbacks.

What is ViewState and what it is not?

There are some common misconceptions about ViewState.  Let me discuss these points here for the benefit of the readers.  ViewState does not hold the controls, rather it holds the values of the form controls and their corresponding ID's that would otherwise be lost due to a post back because they do not post with the form.  ViewState is not used to hold session data or to transmit data between pages.  ViewState does not recreate the dynamically created controls of a page.  It does not restore the values to the controls after a post back operation.  Taken aback? Yes, it is true.  Even when the ViewState for a control is disabled, still the value would be retained after a post back of the page occurs, for input controls like TextBox or DropDownList.  So then, what is ViewState?  ViewState represents the state of a page when it was last processed on the web server.  It holds the values of a control that has been dynamically changed.

Enabling and Disabling ViewState

By default, ViewState is enabled for all server controls.  ViewState can be enabled and disabled in any of the following ways.

·         Page Level

·         Control Level

·         Application Level

·         Machine Level

To enable or disable ViewState in the Page Level, use the following in the Page directive of the ASP.NET page.

Listing 2

<%@ Page EnableViewState ="False" %> 
or 
<%@ Page EnableViewState ="True" %>

To enable or disable ViewState at the Control Level, use the following:

Listing 3

<asp:TextBox id="txtCode" runat="server” EnableViewState="false" />
or
<asp:TextBox id="txtCode" runat="server" EnableViewState="true" />

To enable or disable ViewState in the Application Level, use the following:

Listing 4

<pages enableViewState="false" />
or
<pages enableViewState="true" />

To enable ViewState in the Machine Level, use the following:

Listing 5

<pages enableViewState="true" enableViewStateMac="true" ... />
or
<pages enableViewState="false" ... />

How does Viewstate works?

All server controls have a property called ViewState.  If this is enabled, the ViewState for the control is also enabled.  Where and how is ViewState stored?  When the page is first created all controls are serialized to the ViewState, which is rendered as a hidden form field named __ViewState.  This hidden field corresponds to the server side object known as the ViewState.  ViewState for a page is stored as key-value pairs using the System.Web.UI.StateBag object.  When a post back occurs, the page de-serializes the ViewState and recreates all controls.  The ViewState for the controls in a page is stored as base 64 encoded strings in name - value pairs.  When a page is reloaded two methods pertaining to ViewState are called, namely the LoadViewState method and SaveViewState method.  The following is the content of the __ViewState hidden field as generated for a page in my system.

Listing 1

<input type="hidden" name="__VIEWSTATE"
 value="dNrATo45Tm5QzQ7Oz8AblWpxPjE9MMl0Aq765QnCmP2TQ==" />

Saving and Restoring Values to and from the ViewState

ViewState works with the following types.

·         Primitive types

·         Arrays of primitive types

·         ArrayList and Hashtable

·         Any other serializable object

To add an ArrayList object to the ViewState use the following statements.

Listing 6

ArrayList obj = new ArrayList();
//Some code
ViewState["ViewStateObject"] = obj;

To retrieve the object later use:

Listing 7

obj = ViewState["ViewStateObject"];

ViewState Errors

There is a common ViewState error that is often encountered when transferring the control from one aspx page to another.  Let there be two aspx pages, first.aspx and second.aspx.  Let there be a text box and a submit button in the first.aspx page.  If we now use the Server.Transfer in the handler for the submit button click event in the first.aspx page to transfer the control from the page first.aspx to the page second.aspx, a ViewState error would occur.  This is because the EnableViewStateMac property of the second.aspx page is set to true by default, just as it is in all other aspx pages.  This problem can be overcome by setting the property to false in the second.aspx page.