VS2013 C# function declaration => sign

Question

This code gives me error in vs2013 ... I don't now why ... somebody can help me ?! my problem is i don't know what the => is meant for and how to fix the errors

using System;

namespace SmartStore.Core.Logging
{
    public static class LoggingExtensions
    {
        public static bool IsDebugEnabled(this ILogger l)                                               => l.IsEnabledFor(LogLevel.Debug);


    public static void Fatal(this ILogger l, string msg)                                            => FilteredLog(l, LogLevel.Fatal, null, msg, null);
    public static void Fatal(this ILogger l, Func<string> msgFactory)                               => FilteredLog(l, LogLevel.Fatal, null, msgFactory);

    public static void ErrorsAll(this ILogger logger, Exception exception)
    {
        while (exception != null)
        {
            FilteredLog(logger, LogLevel.Error, exception, exception.Message, null);
            exception = exception.InnerException;
        }
    }

    private static void FilteredLog(ILogger logger, LogLevel level, Exception exception, string message, object[] objects)
    {
        if (logger.IsEnabledFor(level))
        {
            logger.Log(level, exception, message, objects);
        }
    }

    private static void FilteredLog(ILogger logger, LogLevel level, Exception exception, Func<string> messageFactory)
    {
        if (logger.IsEnabledFor(level))
        {
            logger.Log(level, exception, messageFactory(), null);
        }
    }
}

Answer 1

In this case, the => is denoting an expression-bodied member and is equivalent to:

public static void Fatal(this ILogger l, Func<string> msgFactory) 
{ 
    return FilteredLog(l, LogLevel.Fatal, null, msgFactory); 
}

However, this syntax was introduced in C# 6 which requires the Visual Studio 2015 compiler or newer. Switch to the method syntax or upgrade your version of visual studio.