(1)单例模式
https://www.cnblogs.com/zuoxiaolong/p/pattern2.html(java)
https://mp.weixin.qq.com/s?src=11×tamp=1575942361&ver=2025&signature=1R2lijy-RNQFmTa2YYMQu3cc6AzsUA023Xy3Oq*4epmzh7vGYfNpTmAMEo6o7FtWehe058K8CWRBklqOPig1QWYWrdKg8XXKTAdicgycm*sOnVT-Uo*4AV8f7q4DUlwx&new=1 (C#)