Daniel Hoelbling-Inzko talks about programming

.Less now supports files from the VirtualPathProvider!

Posted by Daniel Hölbling on January 6, 2010

Ok, I’ve just spent almost the whole day refactoring the hell out of our .less codebase just to add one tiny change:

Allow users to not only load files present on the current file system, but also directly from in-memory strings and VirtualPaths (as requested on the list). The problem here being that not everyone wants to serve his .less files right from the server’s file system but sometimes people have pretty sophisticated virtualized storage systems in place that require them to use the VirtualPathProvider abstraction that was added to .NET 2.0.
You can read up on that in ScottGu’s blog or David Ebbo’s blog, but in a nutshell it’s just another way to open files besides using System.IO.File.Open(), letting you forget about all the nasty stuff of where the file is really located. 

To do so I had to allow uses to plug in different “Sources” for .less code, and so I also had to make a breaking change to the main ILessEngine interface. The Interface was taking a filename as parameter, but in light of our recent support emails on the development list I decided that has to go away in favor of a more open approach (mainly to allow users to simply throw in-memory strings at the engine).

Because I know this will break some code, all implementors of ILessEngine still offer the old string parameter as an overload that then defaults to the default FileSource.

Now, for changing the source provider:

If you want to use anything besides the default filesystem based FileSource provider, you now have the ability to plug in a type implementing ILessSource through the DotLessConfiguration (thus through web.config).
.Less comes with three sources built in: VirtualPathSource, FileSource (default).

FileSource by default just opens a file through System.IO.File, while VirtualPathSource will use the HostingEnvironment.VirtualPathProvider.GetFile() method to open a Stream and read the .less code from there.

To enable the VirtualPathSource in your web application you simply need to modify your web.config a bit:

<dotless minifyCss="false" cacheEnabled="true" source="dotless.Core.VirtualPathSource">

The important part is the source attribute as it has to reference a type name implementing ILessSource. So if you want to create your own less source you could simply create another implementation of ILessSource and reference it’s name in the .less config.

If you want to use .less directly from your code to transform something you can just new up a LessSourceObject (the very output we get from ILessSource) and throw your code in there like this:

ILessEngine lessEngine = new EngineFactory().GetEngine(new DotlessConfiguration());
var output = lessEngine.TransformToCss(new LessSourceObject() {Content = "my .less code here"});

As always you can get the latest code from GitHub, or the latest binary release through our TeamCity build server. You can read more about .less (pronounced dot-less) on the project’s website at http://www.dotlesscss.com.

And while at it, we’d appreciate it if you spread the word about .less :).

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