How to use ASP.Net MVC 4 to Bundle LESS files in Release mode?

Question

I\'m trying to have LESS files in my web project, and have the MVC 4 bundling functionality call into the dotLess library to turn the LESS into CSS, then minify the result a

Answer 1

The accepted answer does not work with recent changes to ASP.NET, so is no longer correct.

I've fixed the source in the accepted answer:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Web.Hosting;
using System.Web.Optimization;
using dotless.Core;
using dotless.Core.Abstractions;
using dotless.Core.Importers;
using dotless.Core.Input;
using dotless.Core.Loggers;
using dotless.Core.Parser;

namespace Web.App_Start.Bundles
{
    public class LessTransform : IBundleTransform
    {
        public void Process(BundleContext context, BundleResponse bundle)
        {
            if (context == null)
            {
                throw new ArgumentNullException("context");
            }

            if (bundle == null)
            {
                throw new ArgumentNullException("bundle");
            }

            context.HttpContext.Response.Cache.SetLastModifiedFromFileDependencies();

            var lessParser = new Parser();
            ILessEngine lessEngine = CreateLessEngine(lessParser);

            var content = new StringBuilder(bundle.Content.Length);

            var bundleFiles = new List();

            foreach (var bundleFile in bundle.Files)
            {
                bundleFiles.Add(bundleFile);

                var name = context.HttpContext.Server.MapPath(bundleFile.VirtualFile.VirtualPath);
                SetCurrentFilePath(lessParser, name);
                using (var stream = bundleFile.VirtualFile.Open())
                using (var reader = new StreamReader(stream))
                {
                    string source = reader.ReadToEnd();
                    content.Append(lessEngine.TransformToCss(source, name));
                    content.AppendLine();
                }

                bundleFiles.AddRange(GetFileDependencies(lessParser));
            }

            if (BundleTable.EnableOptimizations)
            {
                // include imports in bundle files to register cache dependencies
                bundle.Files = bundleFiles.Distinct();
            }

            bundle.ContentType = "text/css";
            bundle.Content = content.ToString();
        }

        /// 

        /// Creates an instance of LESS engine.
        /// 
        /// The LESS parser.
        private ILessEngine CreateLessEngine(Parser lessParser)
        {
            var logger = new AspNetTraceLogger(LogLevel.Debug, new Http());
            return new LessEngine(lessParser, logger, true, false);
        }

        /// 

        /// Gets the file dependencies (@imports) of the LESS file being parsed.
        /// 
        /// The LESS parser.
        /// An array of file references to the dependent file references.
        private IEnumerable GetFileDependencies(Parser lessParser)
        {
            IPathResolver pathResolver = GetPathResolver(lessParser);

            foreach (var importPath in lessParser.Importer.Imports)
            {
                yield return
                    new BundleFile(pathResolver.GetFullPath(importPath),
                        HostingEnvironment.VirtualPathProvider.GetFile(importPath));
            }

            lessParser.Importer.Imports.Clear();
        }

        /// 

        /// Returns an  instance used by the specified LESS lessParser.
        /// 
        /// The LESS parser.
        private IPathResolver GetPathResolver(Parser lessParser)
        {
            var importer = lessParser.Importer as Importer;
            var fileReader = importer.FileReader as FileReader;

            return fileReader.PathResolver;
        }

        /// 

        /// Informs the LESS parser about the path to the currently processed file. 
        /// This is done by using a custom  implementation.
        /// 
        /// The LESS parser.
        /// The path to the currently processed file.
        private void SetCurrentFilePath(Parser lessParser, string currentFilePath)
        {
            var importer = lessParser.Importer as Importer;

            if (importer == null)
                throw new InvalidOperationException("Unexpected dotless importer type.");

            var fileReader = importer.FileReader as FileReader;

            if (fileReader == null || !(fileReader.PathResolver is ImportedFilePathResolver))
            {
                fileReader = new FileReader(new ImportedFilePathResolver(currentFilePath));
                importer.FileReader = fileReader;
            }
        }
    }
}

Please note one known issue with this code as is is that LESS @imports must use their full paths, i.e. you must use @import "~/Areas/Admin/Css/global.less"; instead of @import "global.less";.