C# 用Singleton类构建多线程单例模式

public sealed class Singleton   {       private static volatile Singleton uniqueInstance;        private static readonly object locker = new object();        private Singleton()       {           var cache = HttpContext.Current.Cache;       }       public static Singleton GetInstance()       {           if (uniqueInstance == null)           {               lock (locker)               {                    if (uniqueInstance == null)                   {                       uniqueInstance = new Singleton();                   }               }           }           return uniqueInstance;       }       public int Number()       {           return 1;       }       public int Number(int i = 0)       {           return i;       }   }