Why is my CSS bundling not working with a bin deployed MVC4 app?

Just to clarify a few things, System.Web.Optimization (aka Bundling/Minification) will work against 4.0. It is not depending on anything in 4.5, so there should be no problems there.

If script bundling is working, and its only an issue with CSS, perhaps the issue is with relative URLs?

I'd first look at the rendered page and see if you are getting references to the CSS bundle, i.e. something like:

<link href="/app/Content/css?v=oI5uNwN5NWmYrn8EXEybCI" rel="stylesheet"/>

If you are, then bundling is working, but something inside your CSS bundle is messed up. Usually this is due to relative URLs inside your CSS bundle being incorrect, i.e. if your images live under ~/Content, but you name your bundle ~/bundles/css, the browser will incorrectly look for images under ~/bundles.

Also, the default behavior is to disable bundling and minification when debug=true. So, if you do want optimizations enabled even when debug=true, you will need to force:

BundleTable.EnableOptimizations = true

Updated: With the new info that v="", that means the bundle was empty, you should verify that you are adding files to the bundle correctly, and that it found them. How are you including files to the bundle?

UPDATE 11/4/2013:

The reason why this happens is because you have .js or .css at the end of your bundle name which causes ASP.NET to not run the request through MVC and the BundleModule.

The recommended way to fix this for better performance is to remove the .js or .css from your bundle name.

So /bundle/myscripts.js becomes /bundle/myscripts

Alternatively you can modify your web.config in the system.webServer section to run these requests through the BundleModule (which was my original answer)

<modules runAllManagedModulesForAllRequests="true">
  <remove name="BundleModule" />
  <add name="BundleModule" type="System.Web.Optimization.BundleModule" />
</modules>

Edit: I also noticed that if the name ends with 'css' (without the dot), that is a problem as well. I had to change my bundle name from 'DataTablesCSS' to 'DataTablesStyles' to fix my issue.

Omitting runAllManagedModulesForAllRequests="true" also worked for me. Add the following configuration in web.config:

<modules>
  <remove name="BundleModule" />
  <add name="BundleModule" type="System.Web.Optimization.BundleModule" />
</modules>

runAllManagedModulesForAllRequests will impose a performance hit on your website if not used appropriately. Check out this article.

The CSS and Script bundling should work regardless if .NET is running 4.0 or 4.5. I am running .NET 4.0 and it works fine for me. However in order to get the minification and bundling behavior to work your web.config must be set to not be running in debug mode.

<compilation debug="false" targetFramework="4.0">

Take this bundle for jQuery UI example in the _Layout.cshtml file.

@Styles.Render("~/Content/themes/base/css")

If I run with debug="true" I get the following HTML.

<link href="/Content/themes/base/jquery.ui.core.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.resizable.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.selectable.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.accordion.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.autocomplete.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.button.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.dialog.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.slider.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.tabs.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.datepicker.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.progressbar.css" rel="stylesheet"/>
<link href="/Content/themes/base/jquery.ui.theme.css" rel="stylesheet"/>

But if I run with debug="false". I'll get this instead.

<link href="/Content/themes/base/css?v=myqT7npwmF2ABsuSaHqt8SCvK8UFWpRv7T4M8r3kiK01" rel="stylesheet"/>

This is a feature so you can easily debug problems with your Script and CSS files. I'm using the MVC4 RTM.

If you think it might be an MVC dependency problem, I'd recommend going into Nuget and removing all of your MVC related packages, and then search for the Microsoft.AspNet.Mvc package and install it. I'm using the most recent version and it's coming up as v.4.0.20710.0. That should grab all the dependencies you need.

Also if you used to be using MVC3 and are now trying to use MVC4 you'll want to go into your web.config(s) and update their references to point to the 4.0 version of MVC. If you're not sure, you can always create a fresh MVC4 app and copy the web.config from there. Don't forget the web.config in your Views/Areas folders if you do.

UPDATE: I've found that what you need to have is the Nuget package Microsoft.AspNet.Web.Optimization installed in your project. It's included by default in an MVC4 RTM app regardless if you specify the target framework as 4.5 or 4.0. This is the namespace that the bundling classes are included in, and doesn't appear to be dependent on the framework. I've deployed to a server that does not have 4.5 installed and it still works as expected for me. Just make sure the DLL gets deployed with the rest of your app.