.Net设计模式之单例模式(Singleton)

public sealed class Singleton
{
    private static Singleton uniqueInstance;// 定义一个静态变量来保存类的实例

    private Singleton() // 定义私有构造函数，使外界不能创建该类实例
    {
    }

    /// 

    /// 定义公有方法提供一个全局访问点,同时你也可以定义公有属性来提供全局访问点
    /// 
    /// 
    public static Singleton GetInstance()
    {
        if (uniqueInstance == null)// 如果类的实例不存在则创建，否则直接返回
        {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

public sealed class Singleton
{
    private static volatile Singleton uniqueInstance;// volatile关键字知识此成员变量能被多个线程访问。
    
    private static readonly object locker = new object();// 定义一个标识确保线程同步

    private Singleton()// 定义私有构造函数，使外界不能创建该类实例
    {
    }

    public static Singleton GetInstance()
    {
        if (uniqueInstance == null)//先判断不存在再枷锁
        {
            lock (locker)
            {
                if (uniqueInstance == null)// 如果类的实例不存在则创建，否则直接返回
                {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

public sealed class Singleton
{
    private static readonly Singleton instance = new Singleton();

    public static Singleton GetInstance()
    {
        return Singleton.instance;
    }

    private Singleton()//私有构造函数，防止外界调用
    {
       //...
     }
}

public sealed class Singleton
{
    public static readonly Singleton instance;

    //静态构造函数，初始化静态变量。CLR只执行一次
    static Singleton()
    {
        instance = new Singleton();
    }

    public static Singleton GetInstance()
    {
        return Singleton.instance;
    }

    private Singleton()//私有构造函数，防止外界调用
    {
       //...
    }
}

public sealed class Singleton
{
    private static readonly Lazy lazy =
        new Lazy(() => new Singleton());

    public static Singleton Instance => lazy.Value;

    private Singleton()
    {
    }
}