class Program
{
    static void Main(string[] args)
    {
      Logger log = Logger.GetInstance();
      log.WriteToFile();
      Console.Read();
    }
}
public class Logger
{
    private static Logger logger = new Logger();
    private Logger(){}
    public static Logger GetInstance()
    {
      return logger;
    }
    public void WriteToFile()
    {
      Console.WriteLine("错误被写入文件了~~");
    }
}

public class Logger
{
    private static Logger logger = null;
    private Logger(){}
    public static Logger GetInstance()
    {
      if (null == logger)
      {
        logger = new Logger();
      }
      return logger;
    }
    public void WriteToFile()
    {
      Console.WriteLine("错误被写入文件了~~");
    }
}

public class Logger
{
    private static Logger logger = null;
    private static object lockObj = new object();
    private Logger(){}
    public static Logger GetInstance()
    {
      lock (lockObj)
      {
        if (logger == null)
        {
          logger = new Logger();
        }
        return logger;
      }
    }
    public void WriteToFile()
    {
      Console.WriteLine("错误被写入文件了~~");
    }
}