css - 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(<filename>) and outputs the processed css file.

This works fine as long as the .less code is valid, but if there is an error, dotLess just returns an empty string. So if there is an error processing the file, my action method returns an empty css file.

How can I output an error message with a closer description of the syntax error instead?

Answer 1

The dotLess parser traps Exceptions and outputs them to a Logger. The snippet from dotLess's source that performs this is LessEngine.TransformToCss:

public string TransformToCss(string source, string fileName)
{
    try
    {
        Ruleset ruleset = this.Parser.Parse(source, fileName);
        Env env = new Env();
        env.Compress = this.Compress;
        Env env2 = env;
        return ruleset.ToCSS(env2);
    }
    catch (ParserException exception)
    {
        this.Logger.Error(exception.Message);
    }
    return "";
}

Less.Parse has an overload that takes a DotlessConfiguration object, which provides several properties that you can use:

public class DotlessConfiguration
{
    // Properties
    public bool CacheEnabled { get; set; }
    public Type LessSource { get; set; }
    public Type Logger { get; set; }
    public LogLevel LogLevel { get; set; }
    public bool MinifyOutput { get; set; }
    public int Optimization { get; set; }
    public bool Web { get; set; }
}

You will notice that the Logger property is of type Type. Whatever type you supply must implement dotless.Core.Loggers.ILogger:

public interface ILogger
{
    // Methods
    void Debug(string message);
    void Error(string message);
    void Info(string message);
    void Log(LogLevel level, string message);
    void Warn(string message);
}

As we saw in the first snippet, the Error method on the logger will get called when an error is encountered during parsing.

Now, the one sticky point of all this is how exactly an instance of the type that implements ILogger gets instantiated. Internally, dotLess uses an IoC container that is baked into the DLL. Following the method calls, it appears that it will eventually call Activator.CreateInstance to instantiate your ILogger.

I hope this is at least somewhat helpful.