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

As a complement to the accepted answer, I created a LessBundle class, which is the Less eqivalent of the StyleBundle class.

LessBundle.cs code is:

using System.Web.Optimization;

namespace MyProject
{
    public class LessBundle : Bundle
    {
        public LessBundle(string virtualPath) : base(virtualPath, new IBundleTransform[] {new LessTransform(), new CssMinify()})
        {

        }

        public LessBundle(string virtualPath, string cdnPath)
            : base(virtualPath, cdnPath, new IBundleTransform[] { new LessTransform(), new CssMinify() })
        {

        }
    }
}

Usage is similar to the StyleBundle class, specifying a LESS file instead of a CSS file.

Add the following to your BundleConfig.RegisterBundles(BundleCollection) method:

bundles.Add(new LessBundle("~/Content/less").Include(
                 "~/Content/MyStyles.less"));

Update

This method works fine with optimization switched off, but I ran into some minor problems (with CSS resource paths) when optimization was switched on. After an hour researching the issue I discovered that I have reinvented the wheel...

If you do want the LessBundle functionality I describe above, check out System.Web.Optimization.Less.

The NuGet package can be found here.

Edited 12/8/2019 This is no longer an acceptable answer to this issue as there have been breaking changes in ASP.NET over the years. There are other answers further down that have modified this code or supplied other answers to help you fix this issue.

It appears that the dotless engine needs to know the path of the currently processed bundle file to resolve @import paths. If you run the process code that you have above, the result of the dotless.Core.Less.Parse() is an empty string when the .less file being parsed has other less files imported.

Ben Foster's response here will fix that by reading the imported files first:

Import Files and DotLess

Change your LessTransform file as follows:

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<FileInfo>();

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

            SetCurrentFilePath(lessParser, bundleFile.FullName);
            string source = File.ReadAllText(bundleFile.FullName);
            content.Append(lessEngine.TransformToCss(source, bundleFile.FullName));
            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();
    }

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

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

        foreach (var importPath in lessParser.Importer.Imports)
        {
            yield return new FileInfo(pathResolver.GetFullPath(importPath));
        }

        lessParser.Importer.Imports.Clear();
    }

    /// <summary>
    /// Returns an <see cref="IPathResolver"/> instance used by the specified LESS lessParser.
    /// </summary>
    /// <param name="lessParser">The LESS parser.</param>
    private IPathResolver GetPathResolver(Parser lessParser)
    {
        var importer = lessParser.Importer as Importer;
        var fileReader = importer.FileReader as FileReader;

        return fileReader.PathResolver;
    }

    /// <summary>
    /// Informs the LESS parser about the path to the currently processed file. 
    /// This is done by using a custom <see cref="IPathResolver"/> implementation.
    /// </summary>
    /// <param name="lessParser">The LESS parser.</param>
    /// <param name="currentFilePath">The path to the currently processed file.</param>
    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;
        }
    }
}

public class ImportedFilePathResolver : IPathResolver
{
    private string currentFileDirectory;
    private string currentFilePath;

    public ImportedFilePathResolver(string currentFilePath)
    {
        if (string.IsNullOrEmpty(currentFilePath))
        {
            throw new ArgumentNullException("currentFilePath");
        }

        CurrentFilePath = currentFilePath;
    }

    /// <summary>
    /// Gets or sets the path to the currently processed file.
    /// </summary>
    public string CurrentFilePath
    {
        get { return currentFilePath; }
        set
        {
            currentFilePath = value;
            currentFileDirectory = Path.GetDirectoryName(value);
        }
    }

    /// <summary>
    /// Returns the absolute path for the specified improted file path.
    /// </summary>
    /// <param name="filePath">The imported file path.</param>
    public string GetFullPath(string filePath)
    {
        if (filePath.StartsWith("~"))
        {
            filePath = VirtualPathUtility.ToAbsolute(filePath);
        }

        if (filePath.StartsWith("/"))
        {
            filePath = HostingEnvironment.MapPath(filePath);
        }
        else if (!Path.IsPathRooted(filePath))
        {
            filePath = Path.GetFullPath(Path.Combine(currentFileDirectory, filePath));
        }

        return filePath;
    }
}