How can I output errors when using .less programmatically?

Question

I\'ve written an ASP.NET MVC action method that receives a .less file name, processes it via Less.Parse() and outputs the processed css file.

Answer 1

I am using a wrapper class around dotless, as follows:

public class LessParser : IStylizer
{
    public string ErrorFileName { get; private set; }
    public int ErrorLineNumber { get; private set; }
    public int ErrorPosition { get; private set; }
    public string ErrorMessage { get; private set; }

    string IStylizer.Stylize(Zone zone)
    {
        ErrorFileName = zone.FileName;
        ErrorLineNumber = zone.LineNumber;
        ErrorPosition = zone.Position;
        ErrorMessage = zone.Message;

        return String.Empty;
    }

    public string Compile(string lessContent, string lessPath)
    {
        var lessEngine = new EngineFactory(new DotlessConfiguration
        {
            CacheEnabled = false,
            DisableParameters = true,
            LogLevel = LogLevel.Error,
            MinifyOutput = true
        }).GetEngine();

        lessEngine.CurrentDirectory = lessPath;

        /* uncomment if DisableParameters is false
        if (lessEngine is ParameterDecorator)
            lessEngine = ((ParameterDecorator)lessEngine).Underlying;
        */

        /* uncomment if CacheEnabled is true
        if (lessEngine is CacheDecorator)
            lessEngine = ((CacheDecorator)lessEngine).Underlying;
        */

        ((LessEngine)lessEngine).Parser.Stylizer = this;

        return lessEngine.TransformToCss(lessContent, null);
    }

    public FileInfo SyncCss(FileInfo lessFile)
    {
        var cssFile = new FileInfo(
            lessFile.FullName.Substring(0, lessFile.FullName.Length - lessFile.Extension.Length) + ".css");

        if (!cssFile.Exists || cssFile.LastWriteTimeUtc < lessFile.LastWriteTimeUtc)
        {
            string cssContent = Compile(ReadFileContent(lessFile), lessFile.DirectoryName);

            if (String.IsNullOrEmpty(cssContent))
                return null;

            using (var stream = cssFile.Open(FileMode.Create))
            using (var writer = new StreamWriter(stream, Encoding.UTF8))
            {
                writer.Write(cssContent);
            }
        }

        return cssFile;
    }

    public string ReadFileContent(FileInfo file)
    {
        using (var reader = file.OpenText())
        {
            return reader.ReadToEnd();
        }
    }
}

The trick is to use own implementation of IStylizer interface that is called upon encountering a parse error to format the resulting error message. This allows us to capture discrete pieces of the error, unlike implementation of ILogger interface where the error is already a formatted text.

var parser = new LessParser();
var lessFile = new FileInfo("C:\\temp\\sample.less"));
var cssFile = parser.SyncCss(lessFile);

if (cssFile != null)
    Console.WriteLine(parser.ReadFileContent(cssFile));
else
    Console.WriteLine("Error '{3}' in {0}, line {1}, position {2}",
        parser.ErrorFileName, parser.ErrorLineNumber, parser.ErrorPosition, parser.ErrorMessage);