第一种最简单,但没有考虑线程安全,在多线程时可能会出问题,不过俺从没看过出错的现象,表鄙视我……
public class Singleton
{
    private static Singleton _instance = null;
    private Singleton(){}
    public static Singleton CreateInstance()
    {
        if(_instance == null)
        {
            _instance = new Singleton();
        }
        return _instance;
    }
}
第二种考虑了线程安全,不过有点烦,但绝对是正规写法,经典的一叉 
public class Singleton
{
    private volatile static Singleton _instance = null;
    private static readonly object lockHelper = new object();
    private Singleton(){}
    public static Singleton CreateInstance()
    {
        if(_instance == null)
        {
            lock(lockHelper)
            {
                if(_instance == null)
                     _instance = new Singleton();
            }
        }
        return _instance;
    }
}
第三种可能是C#这样的高级语言特有的,实在懒得出奇
public class Singleton
{
    private Singleton(){}
    public static readonly Singleton instance = new Singleton();
} 
哦,shit!
 
                
                
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