Detecting Page Refresh

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Introduction

A common problem that Web Application developers encounter is how to stop the user from refreshing the page. The problem arises, if the previous request to the server was a PostBack, which, for example, inserts the WebForm�s data into a database. This will result in the addition of duplicate rows in the database. But we have a constraint that we can�t stop the user by refreshing the page. So, what to do? Although we can�t stop the user from refreshing the page, but we can determine if this event has already occurred and then take appropriate action.

This article is the result of inspiration from the article �Build Your ASP.NET Pages on a Richer Bedrock� by Dino Esposito in which the author has outlined a mechanism to detect page refreshes. But the problem with this method is that it is cumbersome and complicated, although the fundamental idea is sound and forms the basis of this solution. Dino�s mechanism uses a counter stored on the page and a session variable to store the previous request�s counter on the server, if the two match then we have a page refresh.

Strategy

My strategy will make use of the ViewState feature. As we are using ViewState, it would seem logical to perform the operation in the LoadViewState and SaveViewState methods. Using these two methods, instead of the OnLoad method, has more benefits in that it eliminates the potential problems of sub-classes implementing Page_Load. Let�s have a look at these methods first:

LoadViewState

The LoadViewState method loads the previously saved ViewState. The savedState object contains the saved view state values for the item.

SaveViewState

Saves any server control view-state changes that has occurred since the time the page was posted back to the server.

Returns the server control's current view state. If there is no view state associated with the control, this method returns a null reference (Nothing in Visual Basic).

How the process works

The LoadViewState method, which is part of the page�s initialization phase, is only invoked during PostBack and therefore the SaveViewState is the only method, of the two ViewState related methods, that is called when the page is first requested.

Note: _refreshState (which on initial page request is defaulted to False and on subsequent PostBack requests, is the value of ViewState) is assigned to the Session["__ISREFRESH"] item and the negated _refreshState is saved to the new ViewState.

Protected Overrides Function SaveViewState() As Object
 Session("__ISREFRESH") = _refreshState
 Dim AllStates() As Object = New Object(2) {}
 AllStates(0) = MyBase.SaveViewState
 AllStates(1) = Not (_refreshState)
Return AllStates
End Function

Once a PostBack event takes place the LoadViewState method is called:

Protected Overrides Sub LoadViewState(ByVal savedState As Object)
 Dim AllStates As Object() = savedState
 MyBase.LoadViewState(AllStates(0))
 _refreshState = Boolean.Parse(AllStates(1))
 _isRefresh = _refreshState = Session("__ISREFRESH")
End Sub

Note: The _refreshState is retrieved from the ViewState and compared with the value in the Session["__ISREFRESH"] item. The result is stored in _isRefresh, which is used by the IsRefresh property.

The listing below shows the entire class definition:

Public Class test1
   Inherits System.Web.UI.Page

#Region " Web Form Designer Generated Code "
   'This call is required by the Web Form Designer.

   <System.Diagnostics.DebuggerStepThrough()> 
   Private Sub InitializeComponent()
   End Sub

   Protected WithEvents Button1 As System.Web.UI.WebControls.Button
   Protected WithEvents label1 As System.Web.UI.WebControls.Label

   'NOTE: The following placeholder declaration is 

   'required by the Web Form Designer.


   'Do not delete or move it.


   Private designerPlaceholderDeclaration As System.Object

   Private Sub Page_Init(ByVal sender As System.Object, 
             ByVal e As System.EventArgs) Handles MyBase.Init
   'CODEGEN: This method call is required by the Web Form Designer

   'Do not modify it using the code editor.


   InitializeComponent()

   End Sub

#End Region

   Private _refreshState As Boolean
   Private _isRefresh As Boolean

   Private Sub Page_Load(ByVal sender As System.Object, 
            ByVal e As System.EventArgs) Handles MyBase.Load
     'Put user code to initialize the page here

     Me.label1.Text = _isRefresh.ToString
   End Sub

   Protected Overrides Sub LoadViewState(ByVal savedState As Object)
     Dim AllStates As Object() = savedState
     MyBase.LoadViewState(AllStates(0))
     _refreshState = Boolean.Parse(AllStates(1))
     _isRefresh = _refreshState = Session("__ISREFRESH")
   End Sub

   Protected Overrides Function SaveViewState() As Object
     Session("__ISREFRESH") = _refreshState
     Dim AllStates() As Object = New Object(2) {}
     AllStates(0) = MyBase.SaveViewState
     AllStates(1) = Not (_refreshState)
     Return AllStates
   End Function
End Class

When you click the control which fires PostBack event, LoadViewState event is executed. This function loads the previously saved ViewState and here we retrieve the refresh state to find out whether the page is requested to be refreshed. The crucial part is checking whether the value of _refreshstate and the one stored in session is the same, if yes it means the user has hit the Refresh Button.

The SaveViewState function saves the current _refreshState value in session so that we can check this later on, to find out whether the Refresh button is hit.

Conclusion

In this article I have tried to demonstrate a simplified method of detecting a page refresh event.