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Detecting Refresh or Postback in ASP.NET

, , 25 Mar 2010
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This article will first explain the fundamentals of how to solve the above problem and later this article will go in depth of how the source code looks like.

The Problem

There are situations where we would like to detect if the postback is from a form interaction (i.e. submit or button clicks) or if it is by hitting the browser F5 refresh button.

Many of you will jump saying how about checking Ispostback value. IsPostback will always have the value which was set previously. So for instance, if the page was posted back before refresh, then the value will be true and if the page is not posted back before refresh, then the value will be false.

This article will first explain the fundamentals of how to solve the above problem and later this article will go in depth of how the source code looks like.

Please feel free to download my free 500 question and answer eBook which covers .NET , ASP.NET, SQL Server, WCF, WPF, WWF, SilverLight, Azure by clicking here.

Reference

The solution which this article proposes first came in an ASP.NET book written by Dino Esposito and it was taken further by Mr. SimoneB to make a small open source code which you can get here.

A brief explanation is also provided at this link.

The article will go step by step to explain the ideas proposed by Mr. Dino Esposito in a simplified manner.

The Fundamental

Step 1: We have created a JavaScript which will generate unique fresh GUID in submit.

Step 2 (User presses submit click): If user presses submit button, it will call the necessary JavaScript function to create the new fresh GUID again.

Step 3: In HttpHandler or HttpModule, the new GUID value is checked with the old GUID value. If the values are not equal, then it means this was not called from a submit click and it’s a refresh event. Accordingly the HttpContext session value is set.

Step 4: In the page load, we can then check if this was a refresh or postback using the session variables.

3 Important Parts of the Code

There are 3 important parts of the code to be understood:

  • JavaScript which generates the unique GUID.
  • HtppHandler which checks if the old value is equal to the new value.
  • ASP.NET page which finally checks if it’s a refresh or postback and handles logic accordingly.

JavaScript Code

So first let's start with the JavaScript code. The genereateRandomSequence function generates a unique GUID by using math functions like random and floor.

The onPostBack function calls generateRandomSequence function to generate GUID and attach the same to a hidden field with name hdnGuid.

This hidden field is generated on the fly in the HttpHandler module which will be explained shortly. Below is the code snippet for the same:

function onPostBack()
{
var y=generateRandomSequence();
var hdnGuid=document.getElementById("hdnGuid"); 
hdnGuid.value=y;
}

function generateRandomSequence() 
{ 
var g = ""; 
for(var i = 0; i < 32; i++) 
g += Math.floor(Math.random() * 0xF).toString(0xF) 
return g; 
}

The above JavaScript function is referred in a JS file and called on the button submit click as shown in the below HTML code snippet:

<title>Untitled Page</title>
<script type="text/javascript" language="javascript" src="Client-Side_Validn.js"></script>
</script>
</head>
<body>
<form id="form1" runat="server" onsubmit="onPostBack()">
<asp:Button ID="Button1" runat="server" Text="Button" />
</form>
</body>
</html>

HttpModule Code

The next important code is the HttpModule code. As a first step, let’s create a simple GUID class which will help us store the GUID values as shown in the below figure:

public class GuidClass
{
private string guid;

public string Guid
{
get
{
return guid;
}

set
{
guid = value;
}
}
}

The next step is to create a simple HttpModule class which overrides the page_Init event and the page_Load event. In the page_Init event, we have created a simple hidden field by name hdnGuid which is attached to the page on the first hit itself.

void _page_Init(object sender, EventArgs e)
{
HiddenField hdnGuid = new HiddenField();
hdnGuid.ID = "hdnGuid";
if (!_page.IsPostBack)
hdnGuid.Value = Guid.NewGuid().ToString();
_page.Form.Controls.Add(hdnGuid);
}

In the page_Load event, we check if the hidden field value is the same as the old value. In case the value is not the same, that means it's a ‘postback’ and if the value is the same, then it's ‘refresh’. As per the situation, we set the httpContent.Items[“Refresh”] value.

void _page_Load(object sender, EventArgs e)
{
HiddenField h1 = (HiddenField)(_page.Form.FindControl("hdnGuid"));
GuidClass currentGuid =new GuidClass();
currentGuid.Guid= h1.Value;
System.Web.HttpContext _httpContext = System.Web.HttpContext.Current;

if (temp.Contains<string>(currentGuid.Guid))
{
_httpContext.Items.Add("IsRefresh",true);
}
else
{
if(!(currentGuid.Guid.Equals(null)||currentGuid.Guid.Equals("")))
temp.Enqueue(currentGuid.Guid);
_httpContext.Items.Add("IsRefresh",false);
}
}

We also need to ensure that the handler is registered in the httpModules tag.

<httpModules>
<add name="Myhandler" type="Myhandler"/>
</httpModules>

ASP.NET Page Code

The final part is to detect in the ASP.NET page whether it’s a refresh or postback. The below code demonstrates how HttpContext session can be referred to check the same and act accordingly.

if ((bool)HttpContext.Current.Items["IsRefresh"])
{
Response.Write("refreshed");
}
else
{
Response.Write("Postback");
}

History

  • 25th March, 2010: Initial post

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Authors

Shivprasad koirala
Architect http://www.questpond.com
India India

I am a Microsoft MVP for ASP/ASP.NET and currently a CEO of a small
E-learning company in India. We are very much active in making training videos ,
writing books and corporate trainings. Do visit my site for 
.NET, C# , design pattern , WCF , Silverlight
, LINQ , ASP.NET , ADO.NET , Sharepoint , UML , SQL Server  training 
and Interview questions and answers


Archak Sainanee
Software Developer (Senior) Lionbridge
India India
I am an engineer at a midsized software firm.
My preferred language of choice is T-SQL.
And I am a big Pink Floyd fan...

Comments and Discussions

 
QuestionQuestion PinmemberSujit Keng23-May-14 3:52 
QuestionDoesn't work with ScriptManager inplace Pinmemberphamdacloc27-Oct-13 18:56 
Questionhow do i do it for a page having more than one button? Pinmembertamal8867219-Aug-13 23:46 
AnswerRe: how do i do it for a page having more than one button? PinmemberArchak Sainanee20-Aug-13 4:35 
QuestionNot working for me. PinmemberAjay Vaddoria7-Mar-13 5:33 
QuestionHow to add For MasterPage Pinmemberkaruguvel24-Nov-12 0:22 
AnswerRe: How to add For MasterPage PinmemberMember 125202523-Dec-12 6:58 
QuestionCheck Page Refresh PinmemberMember 812804011-Oct-12 7:05 
GeneralMy vote of 3 PinmemberHaBiX18-Sep-12 19:52 
QuestionCursor Control after Postback and F5 Refresh Pinmembersarndt16-Mar-12 6:26 
AnswerPossible alternative PinmemberAnkur\m/19-Nov-10 1:26 
GeneralRe: Possible alternative Pinmembercjc13327-Apr-11 13:56 
GeneralRe: Possible alternative PinmemberAnkur\m/27-Apr-11 18:14 
GeneralOverload AppServer PinmemberRodrigo S. Teixeira14-May-10 11:25 
GeneralAn eloquent solution through JS PinmemberShaunnyBoy29-Mar-10 22:01 
QuestionWhat if javascript fail to run ? PinmemberCoolVini27-Mar-10 6:44 
AnswerRe: What if javascript fail to run ? PinmemberArchak Sainanee27-Mar-10 23:43 
GeneralHi Companion PinmemberAnil Srivastava25-Mar-10 19:27 
GeneralThe Pictures(Diagrams) in this article Pinmemberthatraja25-Mar-10 19:14 
GeneralRe: The Pictures(Diagrams) in this article PinmemberSHartmann29-Mar-10 22:08 
GeneralRe: The Pictures(Diagrams) in this article PinmvpShivprasad koirala30-Mar-10 0:57 
GeneralNice Pinmembersejaldesai25-Mar-10 8:30 
GeneralMy vote of 1 Pinmemberdanidanidani25-Mar-10 5:26 
GeneralRe: My vote of 1 PinmvpShivprasad koirala25-Mar-10 7:37 
GeneralRe: My vote of 1 Pinmembersejaldesai25-Mar-10 15:28 
danidanidani wrote:
Lack of originality

 
If you see the original article , this article is different and explains the same thing in a simplified manner. The original article has a full open source but no code explanation which makes it quiet uncomfortable to use in projects.
 

danidanidani wrote:
adds no value

 
I do not know how many times i have felt the need to check if the page is post back or refresh.
 

Some one is atleast trying ....These someone's make the world better.
 
Sorry for the long and stupid lecture.

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