Design Patterns Simplified - Part 2 (Singleton)【设计模式简述--第二部分（单例模式）】
I am here to continue the explanation of Design Patterns. Today we will explain the easiest yet an important design pattern called Singleton.
这里我继续来解释设计模式。今天我将会来解释，最简单但非常重要的设计模式，也就是单例模式。
In case you have not had a look at our first article, go through the following link:
这里假设，你还没有看我的第一篇文章，请先回去阅读吧，下面是链接：
Before talking about its implementation let’s begin with some fundamental questions as in the following.
来讨论单例模式是如何实现之前，我们先看看下面一些基础的问题吧。
Use of the Singleton Pattern【使用单例模式】
As the name suggests, the Singleton Pattern allows only one instance of a class to be created.
就像这个名字一样，单例模式只允许，创建一个类的实例。
When do we need to have only one instance of a class?
为什么我们只需要一个类的实例？
There are many possible requiremetns for a instance of a class but they all tend to have the one objective that we don’t want to change the state of the object or we want to keep the class stateless.
这里有许多可能的requiremetns类的实例，但是它们都想要只有一个对象，所以我们不能去改变对象的状态或者使对象的状态变成无效的。
A simple example could be that you want to load some master data at once and let the consumers of the data make a call to the Singleton class instead of each consumer making various calls by creating a new instance.
举一个简单的例子，你想要立刻加载主表的数据，并且让一个单例类来调用获取客户表的数据，而不是对于每一个客户，都来创建一个类的实例来调用获取数据。
In general, in any complex enterprise application, Repository and Data Access Layer classes can be seen as a Singleton since typically we don’t want them to maintain the state in these layers.
一般来说，在任何复杂点的企业级应用程序中，仓储和数据访问层的类，可以作为单例来看待，因为我们不想要它们在这些层中，保持状态。
Some other example could be cross-cutting concerns like Logging, Configuration, Caching and so forth that can also be implemented as a Singleton since we want a single and global point of access to these classes.
其他的例子就是横切关注点了，例如日志，系统配置，缓存等等，可以同样设计为单例，因为我们想要对这些类，进行全局的，单一的访问。
Apart from the core consideration explained above, I have seen that developers, mostly not so experienced sometimes, create unnecessarily instances that creates not just an overhead to memory but also impacts the overall performance of an application.
除了上面解释的，我看到过很多的开发者，有时候并不是那么有经验，他们创建不必要的实例，这不仅仅增加了内存的开销，同样也影响了系统的性能。
Why not Static classes【为什么不使用静态类】
There can be several reasons why to not use a static class however I can think of a few as follows.
至于为什么不使用静态类，我认为有如下的原因：
- There can be cases where you want to implement interfaces (maybe to implement IOC, I will explain IOC later) in a class that can be done in a Singleton implementation but not in the static one.
可能存在这样的情况：你想要在类中实现某个接口【可能是实现IOC，我后面会降到IOC】，这种情况可以在单例中做到，但是不能在静态类实现。
- If required, you can use a singleton class as a method parameter whereas you cannot do that with a static class.
Special care for Singleton classes【特别要说的就是单例类】
We need to take special care for Singleton classes. The idea of a state of a class comes with some extra care that means we need to handle synchronization issues in multi-threaded environments.
我们需要特别说到单例类，类的状态，有一些需要注意点，也就是我们需要在多线程的环境中处理同步的问题。
Enough theory, now let’s talk about implementation.
好了，理论已经足够了，现在我们来讨论怎么实现单例模式吧。
Let’s have a look at the most basic implementation.
我们先看看最基本的实现。
In the first example below, we have implemented a Singleton with Lazy loading since the instance will not be created until the caller calls the GetInstance method for the first time.
在下面的例子中，我实现了一个懒加载的单例，因为这个实例只有在GetInstance方法第一次被调用的时候，才会创建类的实例。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks; namespace ConsoleApplication1
{
/// <summary>
/// SingletonClass单例模式学习
/// </summary>
public class SingletonClass
{
/// <summary>
/// 创建私有的，静态的，类的变量
/// </summary>
private static SingletonClass instance = null; /// <summary>
/// 创建私有的SingletonClass无参构造函数
/// </summary>
private SingletonClass()
{ } /// <summary>
/// 创建静态的属性GetInstance
/// </summary>
public static SingletonClass GetInstance
{
get
{
if (instance == null)
{
//实例化SingletonClass
instance = new SingletonClass();
}
return instance;
}
}
}
}
Let’s try to fix the sync issue that may arise in multi-threaded environments. For this, we will use a double-lock mechanism.
现在我们来修复，上面例子中在多线程环境中，可能出现的同步问题吧。对于这个，我将会使用一个双锁机制。
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks; namespace ConsoleApplication1
{
/// <summary>
/// SingletonClass单例模式学习
/// </summary>
public class SingletonClass
{
/// <summary>
/// 创建私有的，静态的，类的变量
/// </summary>
private static SingletonClass instance = null; private static object lockMe = new object();
/// <summary>
/// 创建私有的SingletonClass无参构造函数
/// </summary>
private SingletonClass()
{ } /// <summary>
/// 创建静态的属性GetInstance
/// </summary>
public static SingletonClass GetInstance
{
get
{
if (instance == null)
{
lock (lockMe)
{
if (instance == null)
{
//实例化SingletonClass
instance = new SingletonClass();
}
} }
return instance;
}
}
}
}
And in the last, Singleton with static initializations. Please note that the .NET Framework guarantees thread safety for static initialization so we don’t need extra care for sync issues however we may not get the benefit of lazy loading of objects here.
最后，我们看下单例模式静态的初始化。请注意对于静态的初始化，.NET Framework保证了线程安全，我们不必要去关心同步的问题，但是这种情况下，我们不能从懒加载对象中获益。
public class SingletonClass
{
private static SingletonClass instance = new SingletonClass();
private SingletonClass() {}
public static SingletonClass GetInstance
{
get
{
return instance;
}
}
}
I hope you have liked this article. I look forward to your comments/suggestions.
我希望你喜欢，这篇文章，期待你的评论和建议。