You’ve probably done this too

If you kicked off a new ASP.NET webapp recently, somewhere after File -> New Project you probably got excited about using a CSS preprocessor like LESS or SASS because:

  • nobody writes CSS directly anymore
  • it’s supposed to make your life easier
  • something with unicorns and rainbows

This post isn’t about that. This is about the little quirks I had to jump through to get a usable development environment for writing LESS code.

NOTE: This uses the bundling features in MVC4. If you’re using a version of MVC which is older than that, go upgrade. Bundling is so nice.

Step 1 - The Installer-ation

To NuGet!

PM> Install-Package dotLess

Go run that in a new project. I’ll wait.

Done? Great, so go write a little LESS code (I’ve just called this file sample.less for the purposes of this demo):

@color: #4D926F;

.site-title a {
  color: @color;
h3 {
  color: @color;

And then include it in your Razor layout…

<!DOCTYPE html>
<html lang="en">
        <meta charset="utf-8" />
        <title>@ViewBag.Title - My ASP.NET MVC Application</title>
        <link href="~/favicon.ico" rel="shortcut icon" type="image/x-icon" />
        <meta name="viewport" content="width=device-width" />
    <!--  and more stuff here obvs -->

But you’re application doesn’t know about how it can transform LESS code into CSS. That’s where bundles come in.

Step 2 - Dude, where’s my bundle?

Bundles are an easy way to merge and minify resources in your application (such as JavaScript files and CSS stylesheets). In addition to being a lovely convenience, it’s actually a great way to improve site performance (fewer HTTP requests, reducing size of response payload).

So go back to NuGet and use this:

PM> Install-Package System.Web.Optimization.Less

And add your bundle to the appropriate location within BundleConfig.cs

public class BundleConfig
    public static void RegisterBundles(BundleCollection bundles)
    	// NOTE: existing bundles are here

    	// add this line
      bundles.Add(new LessBundle("~/Content/less").Include("~/Content/*.less"));

Bundles support a limited subset of wildcard syntax, but you can include multiple folders within a bundle.

So this LessBundle gives you the ability to combine and minify files when running the application in <compilation debug="false" /> mode, and it takes care of transforming LESS code into CSS - without needing to update the layout everytime you add a new file to the project.

Editor’s note: I wasn’t aware of this package on the last runthrough - instead I found this LessBundle code in a gist from @benfosterdev which looks very similar to the source for that package, so I’ll give him kudos for it.

Step 3 - But wait, there’s more!

But what about when testing locally? Well, bundles are rather lazy in development mode:

  • no minification
  • no bundling
  • in fact the files just appear as LESS resources to the browser

So you still need to tell your application how it can serve LESS files to the browser, independent all this bundling work.

dotLess will apply transforms to your web.config when you install it through NuGet to provide handlers which can process a specific LESS file:

<?xml version="1.0" encoding="utf-8"?>
    <section name="dotless" type="dotless.Core.configuration.DotlessConfigurationSectionHandler, dotless.Core" />
  <!-- these probably do something useful -->
  <dotless minifyCss="false" cache="true" web="false" />
      <!-- for IIS7+ -->
      <add name="dotless" path="*.less" verb="GET" type="dotless.Core.LessCssHttpHandler,dotless.Core" resourceType="File" preCondition="" />
      <!-- for IIS6 -->
      <add path="*.less" verb="GET" type="dotless.Core.LessCssHttpHandler, dotless.Core" />

Editor’s Note: when I last ran through these steps I didn’t get these config transforms applied on install. I was really looking forward to a rant here because I’d gone through the hard yards to add these handlers by hand - but of course it worked this time.

So these handlers allow you to compile an individual LESS file and see the results in the browser.

Oh, and don’t forget to remove the handlers in your config transforms when you go to production (this is an example of the Web.Release.config transforms file).

<?xml version="1.0"?>
<configuration xmlns:xdt="">
    <compilation xdt:Transform="RemoveAttributes(debug)" />
      <add xdt:Transform="Remove" xdt:Locator="Match(type)" path="*.less" verb="GET" type="dotless.Core.LessCssHttpHandler, dotless.Core" />
      <add xdt:Transform="Remove" xdt:Locator="Match(name)" name="dotless" path="*.less" verb="GET" type="dotless.Core.LessCssHttpHandler,dotless.Core" resourceType="File" preCondition="" />
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