Passing build timestamp to code - asp.net

Question

I would like to give my CSS and javascript files far-future headers and add a token onto the URLs referrring to them. That way browsers don't have to re-download CSS and javascript unless I've released a new build of the site.
http://example.com/css/styles.css?build=23424
How can I pass a build number or timestamp to my code-behind so it can add the timestamp?
Obviously C# doesn't have macros, which is what I would use in C/C++.
I realise that this will force browsers to download assets whenever I do a new build - regardless of whether or not the build involved changing the assets. However, I would like to put a simple scheme in place before I implement anything more advanced like looking at individual file modification times.

Answer 1

Here's a bit of code that I use to extract the build id from the current assembly at application start. It reads the version number from the assembly, a version designator (dev/qa/blank) from the web config, then constructs a version number string to stuff into the application. This code goes in Global.asax.cs. You can then refer to it in your markup from the Application instance.
var webAssembly = Assembly.GetAssembly( typeof(...known class...) );
var designator = WebConfigurationManager.AppSettings["VersionDesignator"];
string version = webAssembly.GetName().Version + designator;
this.Application.Add( "Version", version );
Here's an example of how you could use it in an MVC context (sorry I don't have any WebForms examples).
<link type="text/css" rel="stylesheet"
href="/Content/styles/screen.css?build=<%= this.Application["Version"] %>" />

Answer 2

Interesting idea.
Here's one way to do it:
Go into your AssemblyInfo.cs class under the Properties folder in your project.
Change your assembly version to include the star wildcard: [assembly: AssemblyVersion("1.0.*")].
In your code, you can retrieve the current build version like this:
_
var buildNumber = Assembly.GetExecutingAssembly().GetName().Version.Build;
_
That's it, you're done.
Changing the AssemblyVersion to 1.0.* will cause Visual Studio/MSBuild to increment the build number automatically for you each build. Then, you can access this build number at runtime using the above code snippet. Easy as cheese.

Answer 3

Browsers and servers know how to handle caching via HTTP. So, I misunderstand, what are you trying to accomplish with providing this kind of caching. A browser will not re-download your CSS and Javascript if it has seen it recently. And it will re-download it if you do a push of your new build on the server. See cache headers like Cache-control and Expires etc. Here's a tutorial http://www.mnot.net/cache_docs/