source:
http://stevenbey.co.uk/detectingPageRefresh.aspx
begin:
Detecting Page Refresh
Steven Bey
12.11.2004
Introduction
A common problem that Web Application Developers encounter is how to stop the user from refreshing the page. This is a problem if the previous request to the server was a PostBack, which, for example, inserted the WebForm’s data into a database. The result: duplicate rows in the database. The answer to this problem is that you can’t stop the user from refreshing the page, however there is a way to determine if this event has occurred.
In his article “Build Your ASP.NET Pages on a Richer Bedrock” Dino Esposito outlined a mechanism to detect a page refresh. This method is cumbersome and more complicated than necessary, 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.
A Simpler Method
Keeping the whole process within a base Page Class ensures that the mechanism is completely encapsulated (and simple to implement) and if we use ViewState we eliminate the need to use an additional hidden field. Also, as we simply want to test if the two storage devices contain the same value, we can use two boolean variables, which further simplifies the process.
The last decision to make is where, in the Page’s lifecycle, should the process take place. As we are using ViewState it would seem logical to perform the operation in the LoadViewState and SaveViewState methods. Using these two methods, and not the OnLoad method, has further benefits in that it eliminates potential problems with sub-classes implementing Page_Load.
How The Process Works
The LoadViewState method, which is part of the Page’s initialisation phase, is only invoked during PostBack and therefore SaveViewState is the only method, of the two ViewState related methods, to be called when the page is first requested.
protected override object SaveViewState()
{
Session["__ISREFRESH"] = _refreshState;
object[] allStates = new object[2];
allStates[0] = base.SaveViewState();
allStates[1] = !_refreshState;
return allStates;
}
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.
First of all the Session["__ISREFRESH"] item is initialised/updated with the value of _refreshState (and therefore matching it with latest page request). Secondly, an object array is created. The base class’s SaveViewState is called and the returning value is assigned to the first item in the array. The negated value of _refreshState is assigned to the second item of the arrya. Finally, the objet array is returned to the calling method.
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Once a PostBack event takes place the LoadViewState method is called.
protected override void LoadViewState(object savedState)
{
object[] allStates = (object[]) savedState;
base.LoadViewState(allStates[0]);
_refreshState = (bool) allStates[1];
_isRefresh = _refreshState == (bool) Session["__ISREFRESH"];
}
Note: The _refreshState is retrieved from ViewState and compared with the value in the Session["__ISREFRESH"] item. The result is stored in _isRefresh, which is used by the IsRefresh Property.
First of all savedState is converted into an object array and the first item is passed to the base class’s LoadViewState method. The value of the second item is then stored in _refreshState. Finally, _refreshState is compared to the value stored in the Session["__ISREFRESH"] item and the result stored in _isRefresh, which is then used by the IsRefresh property.
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The listing below shows the entire class definition:
namespace StevenBey.Web.UI
{
public class Page : System.Web.UI.Page
{
private bool _refreshState;
private bool _isRefresh;
public bool IsRefresh
{
get
{
return _isRefresh;
}
}
protected override void LoadViewState(object savedState)
{
object[] allStates = (object[]) savedState;
base.LoadViewState(allStates[0]);
_refrehState = (bool) allStates[1];
_isRefresh = _refreshState == (bool) Session["__ISREFRESH"];
}
protected override object SaveViewState()
{
Session["__ISREFRESH"] = _refreshState;
object[] allStates = new object[2];
allStates[0] = base.SaveViewState();
allStates[1] = !_refreshState;
return allStates;
}
}
}
Testing The Process<%@ Page Inherits="StevenBey.Web.UI.Page" %>
<html>
<head>
<title>Detecting Page Refresh</title>
</head>
<body>
<form runat="server">
<asp:button Text="Test Refresh" runat="server" />
</form>
IsRefresh = <%= IsRefresh %>
</body>
</html>
Clicking the “Test Refresh” button invokes a PostBack, however the value of IsRefresh doesn’t change until you click on the browser’s Refresh button or press F5 on the keyboard (and then click “Retry”). Clicking the “Test Refresh” button once again resets the value of IsRefresh to false.
Conclusion
In this article I have demonstrated a simplified method of detecting a page refresh event.
Download the files used in this this article.
