using System;

namespace mutil {

public interface MessageToolImpl{
    void send(string message);
}
public class MessageTool : MessageToolImpl {
    // strategy pattern
    //private messageToolImpl MessageToolImpl = null;
    public MessageToolImpl MessageToolImpl {private get; set;}

    public void send(string message) {
        if (MessageToolImpl == null) throw new System.Exception("set MessageToolImpl before sending a message.");
        MessageToolImpl.send(message);
    }
    // singleton
    static private MessageTool theInstance = null;

    static public MessageTool getInstance() {
        if (theInstance == null) theInstance = new MessageTool();
        return theInstance;
    }
}

public class ConsoleMessageTool : MessageToolImpl {
    public void send(string message) {
        Console.WriteLine(message);
    }
}

} // namespace