我们可以使用公共构造函数的单例类吗？ [英] Can we have singleton class with public constructor?

问题描述

可以用public构造函数实现单一吗？如果是这样做有什么好处？

解决方案

嗯，你可以...但是它不是一个单身人士类。



单例类的想法是它只允许类的单个实例存在 - 所以通过允许类外的世界调用构造函数来使公共构造函数公开。创建任意数量的实例：除非构造函数查找并抛出运行时错误。哪个是糟糕的设计，因为它会产生应该作为编译时捕获的运行时问题！

嗯，是的，你可以这样做：

 < span class =code-comment> //  非静态Ctor：静态内部实例 
  public   class  OpenDoorSingleton1 
 {
  private   static  OpenDoorSingleton1 _instance; 
 
  private   string  _aString; 
 
  public   string  AString 
 {
  set  { if （_instance！=  null ）_instance ._aString =  value ; } 
 
  get  
 {
  if （_ instance ！=  null ）
 {
  return  _instance._aString; 
} 
 其他 
 {
 返回 串 .Empty; 
} 
} 
} 
 
  //  隐藏无参数ctor  
 受保护 OpenDoorSingleton1（）
 {
} 
 
  public   static  OpenDoorSingleton1 StaticInstantiate（）
 {
  if （_instance ==  null ）_ instance =  new  OpenDoorSingleton1（）; 
 
  return  _instance; 
} 
 
  public  OpenDoorSingleton1（ string  astring）
 {
 _instance = StaticInstantiate（）; 
 _instance.AString = astring; 
} 
} 

这有一个允许你使用参数的公共构造函数，但是，看看你必须经历的所有麻烦，以避免递归和堆栈溢出！ />


我永远不会使用这样的代码：你隐藏的目的是让它成为一个只有一个实例的类！

<浏览：Jon Skeet：[ ^ ]，以及Singleton的DoFactory样本（免费）：[ ^ ]。

Can implement single with public constructor? and if yes what is the advantage of doing this?

解决方案

Well, you can...but then it isn't a singleton class.

The idea of a singleton class is that it only allows a single instance of the class to exist - so making the constructor public circumvents this by allowing the world outside the class to call the constructor and create as many instances as it wanted: unless the constructor looked for this and threw a run-time error. Which is poor design, because it creates runtime problems that should have been caught as compile time!

Well, yes, you can do something like this:

// non-static Ctor: static internal instance
public class OpenDoorSingleton1
{
    private static OpenDoorSingleton1 _instance;

    private string _aString;

    public string AString
    {
        set { if (_instance != null) _instance._aString = value; }

        get
        {
            if (_instance != null)
            {
                return _instance._aString;
            }
            else
            {
                return string.Empty;
            }
        }
    }

    // hidden parameterless ctor
    protected OpenDoorSingleton1()
    {
    }

    public static OpenDoorSingleton1 StaticInstantiate()
    {
        if (_instance == null) _instance = new OpenDoorSingleton1();

        return _instance;
    }

    public OpenDoorSingleton1(string astring)
    {
        _instance = StaticInstantiate();
        _instance.AString = astring;
    }
}

This has a public constructor that lets you use parameters, but, look at all the trouble you had to go through to avoid recursion and stack overflow here !

I would never use code like this: you are hiding the intention to make this a one-and-only-one instance Class !

See: Jon Skeet: [^], and, the DoFactory sample for Singleton (free): [^].

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