C#单例模式的三种写法

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。
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
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();
}