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HowTo: Reset browser cache of CSS files upon ASP.net MVC app publish

On an ASP.net MVC webapp I’m maintaining, I had the issue that due to caching of older CSS (stylesheet) files in the browser, if the user didn’t press F5/refresh, it wasn’t showing you some message (since I had added the class .center-horiz-vert in the CSS that didn’t exist in the older cached css the browser had).

Instead of changing web.config to stop cachine of CSS files (in which case it would bring the CSS on every page load which is an overkill), I expanded on an idea mentioned by Maxim Kornilov on SO (https://stackoverflow.com/a/12992813/903783), on making the CSS URLs webapp version specific.

I added a fake version parameter to the URLs with the build number as value so that till I publish a new build the browser caches the CSS, but when I upload a new build it brings the new one since it cache with the url as a key (that now includes the build number as a dummy url parameter that the webserver will ignore and just fetch the CSS file when requested)

Maxim’s example was in ASP/ASP.net WebForms syntax instead of MVC’s and Razor Pages’ newer Razor syntax), so I contributed my solution for the case of an ASP.net MVC webapp that wants to serve a fresh copy of CSS files on every new build that you publish (will do this whether the CSS file has changed or not) so that browsers don’t use older cached copies of the file. Obviously this expands to any kind of files you link/load into your webpages via a URL.

1) Added to the webapp’s main class (was called MvcApplication) in Global.asax.cs

#region Versioning

public static string Version => 
typeof(MvcApplication).Assembly.GetName().Version.ToString();
//note: syntax requires C# version >=6 public static DateTime LastUpdated =>
File.GetLastWriteTime(typeof(MvcApplication).Assembly.Location); #endregion

the someProperty => someReadOnlyExpression syntax is just shorthand for someProperty { get { return … ;} } possible since C# 6

2) in its Content/_Layout.cshtml file I used to have the following to show build number and build datetime (based on the webapp’s main assembly) on the page footer:

Version @ViewContext.Controller.GetType().Assembly.GetName().Version 
(@string.Format("{0:yyyy/MM/dd-HH:mm:ss}",
@File.GetLastWriteTime(ViewContext.Controller.GetType().Assembly.Location)))

which I changed to the simpler:

Version @somewebappname.MvcApplication.Version
(@string.Format("{0:yyyy/MM/dd-HH:mm:ss}",
somewebappname.MvcApplication.LastUpdated))

3) it was loading the CSS via hardcoded link in _Layout.cshtml (still refactoring it) which I changed to:

<link href='@Url.Content("~/Content/Site.css?version=" + 
somewebappname.MvcApplication.Version)' rel="stylesheet" type="text/css" />

so if one right-clicks in the webpage and they do view source they see:

<link href='/Content/Site.css?version=2.1.5435.22633' 
rel="stylesheet" type="text/css" />

that is the CSS url is version specific thanks to the dummy parameter version

If a random number was used instead it would fetch the CSS at every page load which is usually undesired, especially if you are already pushing a new webapp build instead of individual page changes to the web server (so that you do have access to a build number that you can inject into URLs).

Note that to achieve auto-incrementing of build number, at Properties/AssemblyInfo.cs I have (see How to have an auto incrementing version number (Visual Studio)?):

// Version information for an assembly consists of the following four values:
//
//      Major Version
//      Minor Version 
//      Build Number
//      Revision
//
// You can specify all the values or you can default the Revision 
// and Build Numbers by using the '*' as shown below: [assembly: AssemblyVersion("1.0.*")] //[assembly: AssemblyFileVersion("1.0.*")]
// don't use boh AssemblyVersion and AssemblyFileVersion with auto-increment
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