DecoratorLogger_4

 public class NullLogger : ILogger
    {
        public void Exception(string message, Exception ex) { }

        public void Info(string message) { }

        public void Error(string message) { }

        public void Warning(string message) { }

        public void Progress(ProgressDataEventArgs progressData) { }
    }

    public class MemoryLogger : ILogger
    {
        (...)//nothing changed

        public MemoryLogger(ILogger internalLogger = null) //optional change
        {
            Log = new List<LogLine>();
            _internalLogger = internalLogger ?? new NullLogger(); //optional change
        }

        (...)//nothing changed
   }