Decorator

using System;

namespace Decorator
{
    public abstract class Writer
    {
        public abstract void Write(string message);
    }

    public class BasicWriter : Writer
    {
        public override void Write(string message)
        {
            // nothing
        }
    }

    public abstract class WriterDecorator : Writer
    {
        protected Writer Writer;

        protected WriterDecorator(Writer writer)
        {
            Writer = writer;
        }

        public override void Write(string message)
        {
            Writer.Write(message);
        }
    }

    public class ConsoleLogWriter : WriterDecorator
    {
        public ConsoleLogWriter(Writer writer) : base(writer) { }

        public override void Write(string message)
        {
            base.Write(message);

            Console.WriteLine("ConsoleLogWriter:" + message);
        }
    }

    public class FileLogWriter : WriterDecorator
    {
        public FileLogWriter(Writer writer) : base(writer) { }

        public override void Write(string message)
        {
            base.Write(message);

            Console.WriteLine("FileLogWriter:" + message);
        }
    }

    public class WriterController
    {
        public void Write(Writer writer, string message)
        {
            if (writer != null)
            {
                writer.Write(message);
            }
            else
            {
                Console.Write("No writers");
            }
        }
    }

    class Program
    {
        public static bool UseConsole = true;
        public static bool UseFile = true;

        static void Main(string[] args)
        {
            var writerController = new WriterController();
            var basicWriter = new BasicWriter();

            Writer writer = null;

            if (UseConsole)
                writer = new ConsoleLogWriter(basicWriter);

            if (UseFile)
                writer = new FileLogWriter(writer);

            if (writer != null)
                writerController.Write(writer, "my message");

            Console.ReadKey();
        }
    }
}