Tuesday, May 16, 2006

UrlReferrer - Handle with Care

What page was the user on before this one? Hey! Reponse.UrlReferrer seems to have that information.

Ok, I'll admit I'm a little scared of this feature. It seems to undermine the atomicity of web requests. If I could trust it though, UrlReferrer would be extremely handy for a web app with sophisticated navigation. Imagine multiple ways to get to a screen (as any good app should allow), and the user hits the 'cancel' button, and magically is returned to the previous screen. After all, the user's natural expectation is to be returned to the screen they were just on isn't it?. Here's some code that does just that.

private void btnCancel_Click(object sender, System.EventArgs e)
if(Request.UrlReferrer != null)

This code ensures that the UrlReferrer HTTP Header is set, and if it is redirects the browser to the referrer page. Otherwise the user is sent to a default page. Note here, NUnitAsp doesn't set the UrlReferrer header hence my null check. But...


If the page posts back, the Url Referrer is set to the current page, and your navigation is now broken. You either have to store the referrer in the page load event for later use, or not put any postbacks in the page (danger! danger! you will probably forget about this and break your redirection). I don't know about you, but my pages post back a lot especially in their immature state.


You can't use Reponse.Redirect to navigat to pages that reference the UrlReferrer. Reponse.Redirect causes the browser to send a GET request for the new page. The subsequent requests, such a POST to go to another page, now have the Url Referrer of the current page because of the GET request made by Response.Redirect (or something like that, trace the packets and you'll see what I mean).

The more I write about it, the more convinced I am to avoid Url.Referrer. The design limitations to 'make it work' are extremely constraining and easy to forget. It doesn't work with NUNitAsp (important for me, anyway). I am also unsure of which browsers even support this header.


You can potentially change your navigation strategy and use the bread crumb trail approach. Just save a navigation tree in the user's Session. I have also had some success passing navigation information as URL Query parameters (I.e. page.aspx?PreviousPage=Main.aspx). UrlReferrer is a fragile construct, try to find another way.

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