Question

I was able to setup some ASP.NET Image controls to render images from a database by using an HttpHandler.

// Notice the ImageHandler.ashx
<asp:Image ID="imgSrvcGrp" runat="server" ImageUrl='<%# DataBinder.Eval(Container.DataItem, "ServiceGroupID", "~/ImageHandler.ashx?id={0}") %>' />

It works great and was surprisingly easy to setup. My question follows…

I noticed that the HttpHandler (ImageHandler.ashx) gets hit at certain times when I don’t expect it to. For example, I have a break point at the Page_Load event of the page that contains the Image tags consuming the HttpHandler and another break point in the handler itself, and I found that even though there is no postback (ie – the Page_Load break point is not hit) the handler sometimes gets hit (ie – the break point in the handler is hit).

Oddly, I found this occurs when I close a jQuery dialog on the page. I have a jQuery dialog popup on the page, and I found that when I close the jQuery popup (by clicking on the X) the image handler is hit for every image on the page that consumes it and yet no postback occurs.

The only other detail that I can think to add is my webpage is using Telerik’s RadTabStrip, and it is possible that it is somehow having an effect upon this matter.

I realize that my explanation was rather long-winded, so to be more succinct with my question: Why does my image handler get hit so often even though there are no postbacks occurring?

Answer 1

Because loading a page resource and posting a form are different actions.

Don’t think of it from an ASP.NET perspective with postbacks and Page_Load handlers and whatnot. Think of it from the perspective of the actual HTML in the browser. A “postback” is nothing more than a form being posted to a page. If you’re not doing anything to trigger a form post, the page won’t post back.

However, the page is hitting the server when it requests other resources. CSS files, JavaScript files, images, and so on. The resources referenced within the HTML which the page needs to load. So when a page is loaded, it doesn’t just hit the server once. It hits the server to get the HTML document (this is where Page_Load for the page gets triggered), and while it renders that HTML it hits the server again and again for every resource referenced in the HTML document.

If the page is hiding/showing resources with dynamic style tweaks, then it shouldn’t need to re-load those resources each time it shows them. However, if the page is doing something which causes it to have to re-render the loaded content (and that content isn’t cached in the browser), then it will hit the server. This would be what causes that ASHX handler to get hit.

One suggestion for you would be to load up the page with some debugging tools (FireBug in Firefox continues to be my personal choice), and watch the actual traffic coming and going on the page (the “Net” tab in FireBug, possibly something similar in other tools.) This can help you determine exactly what is being requested and possibly what is requesting it. For example, if that RadTabStrip is for whatever reason removing img elements from the page and re-adding them, it’ll request the image from the server each time.