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maintain login and logout sessions in asp.net web forms

I am trying to implement login and logout in asp.net(web forms). In my web form I have two pages namely Default and Main. From Default page when I login with username and password it redirects to the Main page. When I press back button it directly redirects to the default page. For this I copied javascript code to my default page
<script type = "text/javascript" >
function preventBack() { window.history.forward(); }
setTimeout("preventBack()", 0);
window.onunload = function () { null };
source from stackoverflow question
After login when I click on back button in my browser(chrome) first it shows the Default page and then it shows the Main page. i.e page blinks when I click the back button.
It shows main page successfully with the issue.
What should I implement to stop showing the Default page when I click on back button
Update:
<script type = "text/javascript" >
history.pushState(null, null, 'Default.aspx');
window.addEventListener('popstate', function (event) {
history.pushState(null, null, 'Default.aspx');
});
</script>
I placed this code in default page

There is one important thing that you ought to know here.
One cannot disable the browser back button functionality only thing that can be done is prevent it.
You can't, in anyway diasble that button. What you can however do, is to put some logic and prevent that button from doing what it is meant to do.
Now for the script that you have shown, it should serve fine and the other thing to try here is to put a mediator page between your default and main page. So when you login, the control will flow to mediator page and it will then redirect to main page. Now when the user presses back button on the main page, the control will flow to mediator page which will again redirect the user to his main page.
The effect will be the same as your script, but putting a page can help you write Session handling code and some server side checks if you want.
But one thing is sure, the browser back button will be as it is.
Hope this helps.

As Matt said you can't disagree the back behaviour. However you can check if the user is logged in and redirect them to the main page easily.
All you need to do is, on the default page check if the user is logged in, if they are then redirect them to the main page, this way even if the user clicks back, they will be taken back to the main page. And also the can go to the Default page after logging out.

Another way could be using location.replace("...."), which replace the existing document and user can't "go back" using back button as the page doesn't exist in the url history.
Src: https://developer.mozilla.org/en-US/docs/Web/API/Location/replace

asp.net: how do i prevent users from posting the same data multiple times

i have a little asp.net web application.
it is a front end to a sql-server-2008 database.
after they fill out all the data, they will press submit and the data will be sent to the database.
after this point if the user refreshes the page, the data is posted again!!! how do i disable this from happening?

Send a Location header back to the user that redirects the browser to another page: refresh will then reload that other page rather than resubmit the old form.

This is caused by the last request being a POST request to the page, what do you need to do is a redirect so the last request becomes a GET.
After you have handled the post data you can just do a redirect to the same page with:
Response.Redirect("~/ThePage.aspx");
This will prevent you from presenting a message to the user straight from the code behind, if you want to present a success message using this method you will need to add a querystring or something similar:
Response.Redirect("~/ThePage.aspx?result=success");
And then check on the page bind if the querystring to present a success message is set, such a check could look something like this:
if (Request.QueryString["result"] != null && Request.QueryString["result"].ToString().ToLower() == "success")
{
//Show success message
}
Another solution which probably is superior but might require some more work is to wrap the form in a updatepanel, you can read more about it here: http://ajax.net-tutorials.com/controls/updatepanel-control/
An updatepanel will make the form submit with AJAX instead of full postback.

You need to follow the Post/Redirect/Get pattern which is explained on WikiPedia and alluded to by Femi. In your code after you've done your processing do a Response.Redirect to the desired page.

See this article about the PRG pattern: http://en.wikipedia.org/wiki/Post/Redirect/Get
In short, after the user POSTs (submits) data to your server, you issue a Response.Redirect to have the users browser GET a page. This way, if the user presses the reload button, it is the GET request that is repeated.

ASP.net Redirect to the calling page

I have a page that calls another page with some query string parameters. I want to return back to that page after clicking on a button.
I have to mention that I write that code in a user control and I don't know what page called that second page.
Is there something like Back button in browsers?

Simplest way use javascript on client side with
window.back();
For server side you need to save the url referer in page_load:
if(!Page.IsPostback)
{
ViewState["GoBackTo"] = Request.UrlReferrer;
}
and on a button click using Response.Redirect:
Response.Redirect( ViewState["GoBackTo"].ToString() );
edit: please note ppumkin's comment below!

You could look at Cross Page Posting.
Alternatively, if you are generating the link programatically you could include the returnUrl in the url e.g. http://localhost/secondpage.aspx?returnurl=firstpage.aspx
You can then read this querystring parameter in the secondpage and perform as redirect back once your work is done.

You can use the Request.UrlReferrer, but it is not necessarily sent from the client all the time:
Response.Redirect(Request.UrlReferrer.AbsoluteUri);

put this line of code on the page load event
Btn_Back.Attributes.Add("onClick", "javascript:history.back(); return false;");

Javascript alert() after Response.Redirect

I am calling this in my code behind:
(test.aspx)
Response.Redirect("~/Default.aspx");
I want to include a javascript alert after/before being redirected to Default.aspx, is it possible?
I'm doing this because I'm passing a value to another page (test.aspx) and that page checks the db, if reader HasRow(), then redirect to Default.aspx.

The way to do that is display the alert with javascript and then do the redirect with javascript as well:
ScriptManager.RegisterStartupScript(this,this.GetType(),"Redit","alert('asdf'); window.location='" + Request.ApplicationPath + "Default.aspx';",true);

Lets take a look at what happens when you call response.redirect()
Servers sends HTTP response 302 to browser including the url to navigate to
browser doesn't display content of response and instead gets the content from the url specified
if the request to the new url succeeds, then it is displayed.
Now looking at this, we can deduce that it is impossible to tell the browser to do a alert() from the page that issues the redirect because its content (if any) is discarded.
It is possible to accomplish what you want from the page that you are redirecting to. To do this, just check Request.UrlReferrer to check if you were redirected from the correct page, then display the alert when appropriate.
example:
Page1 redirects to page2
Page2 check if Request.UrlReferrer is equal to page1
If equal, display alert
If not, do nothing special
Another approach is do the alert first then do the redirect from javascript. window.location.href = newurl.

if you have display something message on Defaul.aspx page you must declare it. Because when you use redirect your page is rendering from top. You must set before redirect Sesion state on something flag and on Default.aspx page you must insert section who been added when this Sesion state been set.

Expire a particular page on back button of browser

if users press the browser's back button to reach the prior page, the page should display a message like "web page expired".
can i use javascript for this???
for example:
there are 4 pages in web sites. on page 1,2 and 3 the user can use the back-button, wheras on the 4th page the user gets the desired message.
i thought that i can do this by using counter.
i used following javascript on the master page ..
<script type="text/javascript">
function GoBack() {
window.history.go(+1);
}
</script>
and call the function in body like this:
<body onload="GoBack();">
and on the 4th page_load i do the following:
Response.Cache.SetCacheability(HttpCacheability.NoCache);
it is working for all pages .. but i want to do this only for the 4th page

If you only want it on that page level, and when you use postbacks, then I suggest you simply keep it in ViewState instead of Session state. Session's also still available on other pages, where you might want to have other counters.

You need to keep the variable alive across requests. So one way is to put it in some viewstate or sessionstate. Sessionstate is least preferred. But you can possibly put it in a hidden textbox in the page and simply use it.
Looking at the problem after the much awaited update/edit, I shall suggest you to use SessionState. Please give a try on it.