一个简单的单例实例

Singleton模式的实现

Singleton模式的实现基于两个要点：

1）不直接用类的构造函数，而另外提供一个Public的静态方法来构造类的实例。通常这个方法取名为Instance。Public保证了它的全局可见性，静态方法保证了不会创建出多余的实例。

2）将类的构造函数设为Private，即将构造函数"隐藏"起来，任何企图使用构造函数创建实例的方法都将报错。这样就阻止了开发人员绕过上面的Instance方法直接创建类的实例。

通过以上两点就可以完全控制类的创建：无论有多少地方需要用到这个类，它们访问的都是类的唯一生成的那个实例。以下C＃代码展现了两种实现Singleton模式的方式，开发人员可以根据喜好任选其一。

实现方式一：Singleton.cs

using System;
class SingletonDemo
{	private static SingletonDemo theSingleton = null;
private SingletonDemo() {}
public static SingletonDemo Instance()
{	if (null == theSingleton)
{
theSingleton = new SingletonDemo();
}
return theSingleton;
}
static void Main(string[] args)
{	SingletonDemo s1 = SingletonDemo.Instance();
SingletonDemo s2 = SingletonDemo.Instance();
if (s1.Equals(s2))
{	Console.WriteLine("see, only one instance!");
}
}
}

与之等价的另外一种实现方式是：Singleton.cs：

using System;
class SingletonDemo
{	private static SingletonDemo theSingleton = new SingletonDemo();
private SingletonDemo() {}
public static SingletonDemo Instance()
{	return theSingleton;
}
static void Main(string[] args)
{	SingletonDemo s1 = SingletonDemo.Instance();
SingletonDemo s2 = SingletonDemo.Instance();
if (s1.Equals(s2))
{	Console.WriteLine("see, only one instance!");
}
}
}

编译执行：

Csc Singleton.cs

得到运行结果：

see, only one instance!


//Creational Pattern: SINGLETON
//Implemenation in C#
 
/*The constructor should be private. Provide a static method, which returns an instance of the class. use a static variable to check whether already one instance is created or not. if already an instance is there , returns a null */
 
using System;
class SingleInstanceClass
{
            private static SingleInstanceClass sic= null;
            private static bool instanceFlag = false;
            private SingleInstanceClass()
            {
            }
            
            public static SingleInstanceClass Create()
            {
                        if(! instanceFlag)
                        {
                                    sic = new SingleInstanceClass();
                                    instanceFlag = true;
                                    return sic;
                        }
                        else
                        {
                                    return null;
                        }
            }
            
            protected void Finalize()
            {
                        instanceFlag = false;
            }
}


class MyClient
{
            public static void Main()
            {
                        SingleInstanceClass sic1,sic2;
                        sic1 = SingleInstanceClass.Create();
                        if(sic1 != null)
                                    Console.WriteLine("OK");
                        sic2 = SingleInstanceClass.Create();
                        if(sic2 == null)
                                    Console.WriteLine("NO MORE OBJECTS");
            }
}