• In page code, set the value of the variable in the ViewState property.

  The following code example shows how you can get an ArrayList object named arrayListInViewState from view state and then bind a GridView control to the object as a data source.

  [Visual Basic]

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  Dim arrayList As ArrayList arrayList = CType(ViewState("arrayListInViewState"), ArrayList) Me.GridView1.DataSource = arrayList Me.GridView1.DataBind()

  C#В	Copy Code
  arrayList = new ArrayList(); arrayList = (ArrayList)ViewState["arrayListInViewState"]; this.GridView1.DataSource = arrayList; this.GridView1.DataBind();

Values in view state are typed as String. In Visual Basic, if you set Option Strict On, you must cast view state values to the appropriate type before using them, as shown in the example. In C#, you should always cast to the appropriate type when reading view state values.

No exception is thrown if you attempt to get a value out of view state that does not exist. To be sure that the value you want is in view state, check first for the existence of the object with a test such as the following:

Visual BasicВ	Copy Code
If ViewState("color") Is Nothing Then ' No such value in view state, take appropriate action. End If

C#В	Copy Code
if (ViewState["color"] == null) // No such value in view state, take appropriate action.

If you attempt to use a nonexistent view state entry in some other way (for example, to examine its type), a NullReferenceException exception is thrown.

View state information is stored using base64 encoding and is included in the page during rendering, increasing the size of the page. When the page is posted back, the contents of view state are sent as part of the page postback information. Because this can significantly increase network traffic and slow down connections, it is recommended that you do not store large quantities of information in view state.

Another important consideration is that if the amount of data in a hidden field becomes large, some proxies and firewalls will prevent access to the page that contains them. Because the maximum amount can vary with different firewall and proxy implementations, large hidden fields can be sporadically problematic. For more information see ASP.NET State Management Recommendations.

Some mobile devices do not allow hidden fields at all. Therefore, view state will not work for those devices. For more information, see ASP.NET Mobile Application Development Overview.