Question

I’m building a simple game in an ASP.NET/VB.NET web app. The game has a UI made up of several ImageButtons.

The web page’s code behind file holds an instance to the game object which will manage each turn taken by players.

Everything worked when the Game object’s methods were Shared.

The problem occurred after refactoring to make the game object work as an instance instead of a shared class. Now, when the action returns to the code behind, the instance of the game object is nothing.

I suspect that this has something to do with view state, but uh… Google hasn’t helped.

The code bits:

Public Class _Default
        Inherits System.Web.UI.Page

        Private _gamePanel As Panel
        Private _updatePanel as UpdatePanel
        Private _game as Game

        Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

            If Not Page.IsPostBack Then

                'create a new instance of the game object on first loading page
                _game = New Game(width, height, cellsToWin)

            End If

            ' DisplayGameBoard() does the following:
            '  * Add images to the GameBoard panel inside of the GameBoardUpdatePanel
            '  * Attach click event handler to each image (addressOf located in this 
            '      code behind file
            '  * DisplayGameBoard() works fine the first time but fails on
            '      subsequent post backs because there is no game object instance
            Me.DisplayGameBoard()

        End Sub

(From the page directive)

Language="vb" 
AutoEventWireup="false" 
CodeBehind="Default.aspx.vb" 
Inherits="Game._Default" 
ValidateRequest="false" 
EnableEventValidation="false"
EnableViewState="true"

(update panel on the web page)

    <asp:UpdatePanel ID="GameBoardUpdatePanel" 
                     runat="server" 
                     UpdateMode="Conditional" 
                     RenderMode="Block" 
                     EnableViewState="true"
                     ViewStateMode="Enabled" 
                     ChildrenAsTriggers="true" >

        <ContentTemplate>

            <asp:Label ID="PlayerName"
                        runat="server"></asp:Label>

            <asp:Panel ID="GameBoard"
                        runat="server"
                        cssclass="gameBoard"></asp:Panel>

        </ContentTemplate>
    </asp:UpdatePanel>

Answer 1

It’s not ViewState, it’s simply the life time of the _Default instance.

You create an instance of the Game class and store that as a member of the page, and expect that instance to survive. The problem is that the instance of the page doesn’t survive.

Each request for the page will result in a new instance of the _Default class to be created, and when the response is created the instance is thrown away. The reference to the instance of the Game class that you have stored in the page is also thrown away, and you lose any way to access it.

If you want to keep the instance of the Game class, you can store it in the Session collection, which is user specific:

If Not Page.IsPostBack Then

  'create a new instance of the game object on first loading page
  _game = New Game(width, height, cellsToWin)
  ' store the reference in the user session
  Session("game") = _game

Else

  ' get the reference back from the user session
  _game = DirectCast(Session("game"), Game)

End If

However, you should be carful about how much you store in the user session. Normally objects that are created in a page are short lived (i.e. milliseconds), so they have a small impact on the server resources. Anything that you store in the user session will be extremely long lived in comparison. Consider how large the Game object is, and if you really need to keep the entire object, or if you can keep just the information needed to recreate it for each request.