image cache-control example

You want to simplify and improve your HTTP cache headers in your ASP.NET site. Caching with HTTP headers is important to every web site, but there are many complicated rules and conflicts among the headers. Here are lots of notes about HTTP headers and their methods in ASP.NET and the C# language.

Use the examples here to improve your HTTP headers.
ASP.NET provides many options for HTTP header caching.

Set Expires headers

First, we know that ASP.NET websites may use ASPX files or ASHX handlers to serve static content like images. Keeping with Yahoo’s guidelines, use this ASP.NET C# code to set Expires in the far future.

Page that uses Response.Cache [C#]

using System;
using System.Web.UI;

public partial class _Default : Page
    protected void Page_Load(object sender, EventArgs e)
	// Set this ASPX response to expire in one year.
 // ... This is essentially 'never'.

Description of the code. IIS7 will set the Cache-Control header automatically when you specify SetExpires. No other caching logic is necessary for static resource caching on client browsers. However, there are other options available. Please see the section on Cache-Control headers.

Yahoo: static pages

Almost all big websites have certain images that never change, such as logos, shopping cart images, rounded borders and gradients, and navigation bars. Yahoo says to use “the Expires header in the HTTP response to tell the client how long a component can be cached.” Please see “Best Practices for Speeding Up Your Web Site” at

Example Expires HTTP Header:
Expires: Thu, 15 Apr 2010 20:00:00 GMT

Description. Yahoo describes the example: “This is a far future Expires header, telling the browser that this response won’t be stale until April 15, 2010. Note that the time format is very specific and you will need to use a special DateTime format in the C# language to create or test it. Please also see the section on DateTime.

Yahoo: dynamic pages

Yahoo recommends the Cache-Control header for dynamic pages. There are several variants of this you can use. Yahoo’s best practices: “For dynamic components: use an appropriate Cache-Control header to help the browser with conditional requests.” Using Cache-Control gives you overriding power on the cache setting, allowing you to specify options for proxies and the server.

Use Cache-Control

Here we look at Cache-Control headers in ASP.NET. The implementation of caching on Response.Cache is very complex and confusing in ASP.NET, and some options will trigger other options. Setting a page for one hour of caching is done like the following example. As Yahoo states, Cache-Control gives you more control and “helps browsers” with conditional requests.

Page that uses Response.Cache and Cache-Control [C#]

using System;
using System.Web;
using System.Web.UI;

public partial class _Default : Page
    protected void Page_Load(object sender, EventArgs e)
	// Set cache for 1 hour on all computers and servers.
 // ... Proxies, browsers, and your server will cache it.
	Response.Cache.SetMaxAge(new TimeSpan(1, 0, 0));

Use Fiddler

Here we see how you can use Microsoft’s Fiddler tool with HTTP headers. When developing ASP.NET applications with caching, you should use Fiddler, or another equivalent tool, to look at the HTTP headers. I have written material on Fiddler.

Fiddler Tool for HTTP DebuggingFiddler and HTTP headers

Use Cache methods

In this section, we look at many of the Cache methods you can use in ASP.NET code-behind and other code files. These methods and properties are used to control the HTTP cache settings on your ASP.NET response. They must be called on the HttpResponse object, using the Response.Cache… style syntax.

Expiration Time

You will need this when using callbacks, which I have no experience with.

You can use this to add a custom header to the Cache-Control header, which could be used for future changes in HTTP 1.1 or proprietary options.

This overrides certain settings made by SetCacheability, such as NoCache and ServerAndNoCache.

This is important and sets the Cache-Control header, which is the preferred mechanism for caching dynamic pages. See the list of HttpCacheability enums below.

This allows you to specify a string that is considered the ‘tag’ of a resource. This is not recommended by Yahoo and not normally needed.

Tells ASP.NET to assign random etags to your resources that are keyed to the file contents. Simplifies ETag usage. Not normally needed.

Very important and useful for static resources such as logo images or web site layout images. Recommended by Yahoo for static resources.

This can be used to date your file and return a 304 when a user requests the same one again. This doesn’t save an HTTP request. Yahoo recommends modified dates over ETags.

Same as above but tells ASP.NET to read in the file metadata automatically.

Very important. This gives you a relative time window you can specify a resource can be cached for. This is an alternative to the Expires header, and it overrides the Expires header.

This seems to remove the HttpCacheability.Server setting. It seems like a really poor design in ASP.NET.

Applies the “Cache-Control: no-store” header. This is useful for advertisements and dynamic responses.

Some proxy caches can change the format of your files when they store them. This setting should tell them not to.

Changes header when using vary parameters. Not often useful.

Not likely to be useful. It suggests that proxy caches can expire or keep resources for a specific time. I doubt they would honor this exactly.

Indicates when validation should occur. See Cache-Control header section.

Changes the logic of when the server expires its cache. Has many quirks and you must test it carefully.

Don’t listen to browsers when they say a resource is expired or stale. Otherwise, they can invalidate caches.

Allows you to set the custom vary header, which is useful when you have the Vary header. See section on Vary.

These are public getters only, meaning you cannot set these properties. They are useful for debugging and diagnostics of your Vary header.

HttpCacheability constants

You need to call SetCacheability on the Response.Cache to set the main Cache-Control header. This header controls the location and general behavior of the cache. You need to combine this setting with other Cache class method calls to achieve many behaviors. However, these enums define the general setting.

SetCacheability Issue

Tells the browser, the server, and proxies to never cache this resource. This can be useful for advertisements and resources that are always changing.

Only cache on the browser. This will provide bandwidth savings for your users, but your server won’t store a cached copy of the output. This is adequate for many sites.

The ultimate cache setting: tells the server to save the page, proxy caches to save the page, and the browser to save the page.

Only cache the page on the server (output caching without browser caching). However, when your visitors click on your static pages, they will be reloaded.

The same as NoCache except it allows the server to store the page. Has slightly different meaning for remote clients and proxies. Not often useful.

Tells proxy caches to never cache this page, but to allow the browser and the server to cache it.Question

Private caches

Private is a term that means the web browser on your users’ computers. By using private, your users won’t download or re-request the pages they have viewed. This reduces the work your server does and the bandwidth you use. It refuses proxy servers and your own server caches, meaning you still have much CPU use and bandwidth use.

Expires and Max-Age

The two HTTP headers both target the client-side caching, but have a semantic difference. Max-Age, part of the Cache-Control, is relative to the current time, making it easier to use in many cases. It “specifies the maximum amount of time that an representation will be considered fresh.” Further, “similar to Expires, this directive is relative to the time of the request, rather than absolute.”

Max-Age overrides Expires

When looking at the specification for HTTP, you will see that Max-Age and Expires conflict. From the HTTP spec: “If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header.”

Public proxy caches (Squid)

There are even more complexities when considering proxy caching. I am not certain how common proxy caches are, but when available, they can reduce your bandwidth and improve access times. Basically, they can serve up your pages for you, for free. The Squid software “reduces bandwidth and improves response times by caching and reusing frequently-requested web pages. [It] optimises the data flow between client and server.”

Vary header

If you look at the HTTP headers of MySpace, for example, you see the header “Vary: Accept-Encoding”. This means that the caches will be separate for each encoding. This improves localization. You can use SetVaryByCustom to change this. Note that this is a suggestion to proxy servers, and does not improve the rate of cache hits.

Handler.ashx cache

First, you can use HTTP handlers in ASP.NET for a faster way to server dynamic content than Web Form pages. Handler.ashx is the default name for an ASP.NET generic handler. You need to use the HttpContext parameter and access the Response that way.

ASHX Handler Tutorial

HTTP Handler that uses caching [C#]

<%@ WebHandler Language="C#" %>

using System;
using System.Web;

public class Handler : IHttpHandler {

    public void ProcessRequest (HttpContext context) {
	// Cache this handler response for 1 hour.
	HttpCachePolicy c = context.Response.Cache;
	c.SetMaxAge(new TimeSpan(1, 0, 0));

    public bool IsReusable {
	get {
	    return false;

Remove Cache-Control

Here we see how you can remove the Cache-Control header in ASP.NET by configuring the application. When you use the Expires header on static resources such as JavaScript, CSS, and images, you do not need the Cache-Control header. Cache-Control and Expires are alternatives and are not both needed. You can disable the HTTP Cache-Control header entirely in Web.config. Note however that this setting is defeated if you change cache headers during execution.

Example Web.config [C#]

<?xml version="1.0"?>
	<httpRuntime sendCacheControlHeader="false"/>
    <!-- etc. -->

Convert DateTime

As noted above, HTTP dates have a very specific format you must follow. Fortunately, the .NET framework provides a format pattern string for this. You can use a special format string, after converting the date into the current time zone. HTTP has a rigid definition of date formats. For more information on HTTP date formats, such as the RFC1123 pattern, see the Microsoft site.

Page that converts DateTime [C#]

using System;
using System.Web.UI;

public partial class _Default : Page
    protected void Page_Load(object sender, EventArgs e)
	DateTime d = DateTime.UtcNow;
	string s = d.ToUniversalTime().ToString("r");

Other cache methods

There is a Cache object you can use to store data programmatically. This is separate from the methods shown here, and has many different options. This document focuses on HTTP headers. On the server-side, the Cache[] object is very useful, but it does not address the client-side caching in browsers, such as Firefox, MSIE, and Google Chrome.

Use Response.Cache

There are several different ways to access the Cache. The examples above show how you can access it in the Page class and also from generic handlers. However, Cache is an intrinsic object, and my research shows what happens when you access it.

HttpContext Request Property

Performance of using properties. My research indicates that accessing the Response.Cache property in each method call may not be ideal, and storing the HttpCachePolicy as a variable is better.

Response.Write Test


In this article, we saw how you can use HTTP headers in ASP.NET to greatly enhance the performance of your pages and make them more usable. We did not cover server-side database caching and other complex topics. Instead, we focused on the client-side HTTP headers and client-side caching.

One thought on “ image cache-control example

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