Implementing View State properties

Maintaining State Using View State Properties

If you have data stored in the Page class that is not part of a control, this data gets lost after a postback. For example, a simple field or a property does not maintain its value as the Page class that contains this data is recreated each time the page loads, both on the initial request as well as on subsequent postbacks. To see how this can be problematic, consider this example:

VB

Partial Class WithoutViewState
  Inherits System.Web.UI.Page
  
  Dim counter As Integer
  
  Protected Sub PostbackButton_Click(sender As Object, e As System.EventArgs) _
              Handles PostbackButton.Click
    counter = counter + 1
    Label1.Text = counter.ToString()
  End Sub
End Class

C#

using System;

public partial class WithoutViewState : System.Web.UI.Page
{
  private int counter;
  
  protected void PostbackButton_Click(object sender, EventArgs e)
  {
    counter++;
    Label1.Text = counter.ToString();
  }
}

When you run this code, every time you click the button, the label will display 1. The reason for this is that after each postback the Page is recreated and counter is initialized to zero again.

Fixing this is pretty easy. All you need to do is convert your field or property to a View State property. The Get method (get in C#) gets its data from the ViewState property of the containing Page class, or returns a default value when no value is present in View State (which is the case for the first request). The Set method (set in C#) stores the value in View State. Here's the same page, but now rewritten so Counter (renamed to match the naming conventions for properties) is now able to maintain its value across postbacks.

VB

Partial Class WithViewState
  Inherits System.Web.UI.Page

  Protected Sub PostbackButton_Click(sender As Object, e As System.EventArgs) _
              Handles PostbackButton.Click
    Counter = Counter + 1
    Label1.Text = Counter.ToString()
  End Sub

  Public Property Counter As Integer
    Get
      Dim o As Object = ViewState("Counter")
      If Not o Is Nothing Then
        Return CType(o, Integer)
      Else
        Return 0
      End If
    End Get
    Set(ByVal Value As Integer)
      ViewState("Counter") = Value
    End Set
  End Property
End Class

C#

using System;

public partial class WithViewState : System.Web.UI.Page
{
  protected void PostbackButton_Click(object sender, EventArgs e)
  {
    Counter++;
    Label1.Text = Counter.ToString();
  }
  
  public int Counter
  {
    get
    {
      object counter = ViewState["Counter"];
      if (counter != null)
      {
        return (int)counter;
      }
      else
        return 0;
    }
    set
    {
      ViewState["Counter"] = value;
    }
  }
}

This code uses the Page's ViewState property to store its data. Data you assign to this property is serialized and stored in a hidden field called __VIEWSTATE in the page. For the sample above, that field looks like this:

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" 
     value="/wEPDwUKMTI2NTY4ODI3MQ8WAh4HQ291bnRlcgIIFgICAw9kFgICAQ8PFgIeBFRleHQFAThkZ
	 GThk/CgB5NdiYE7ihUhzVOVuNmLk+zuF14uID9rnQyGIQ==" />

Note that the original code has remained exactly the same (except for renaming the Counter variable which is optional.

ASP.NET sends this hidden field to the browser on each request and the browser sends it back on each postback. At the server, ASP.NET is then able to extract the values stored in this field. This means that the Get method in the Counter property is able to successfully retrieve the number that it stored on a previous request using the Set method. When there's no value, the Get method of the property returns 0 which is the standard default value for an Integer (an int in C#). You could modify this default value to whatever default you want to initialize the Counter property on the first request.

Warning

Values you store in View State are serialized and stored in the hidden __VIEWSTATE field. This adds to the size of the page which in turn affects download time and performance. Don't use View State to store large objects such as DataSets. However, storing a few simple values, like the Counter property in this example is a good use of View State.

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