常用的Singleton写法

public class Core
    ...{
        private static Core _instance;

        public static Core Instance
        ...{
            get
            ...{
                if (null == _instance)
                ...{
                    _instance = new Core();
                }
                return _instance;
            }
        }

        private Core()
        ...{
            // Add some code here...
        }

    }