意图: 保证一个类仅有一个实例，并提供一个访问它的全局访问点。
使用性:
代码:
namespace Singleton_DesignPattern
{
using System;
class Singleton
{
private static Singleton _instance;
public static Singleton Instance()
{
if (_instance == null)
_instance = new Singleton();
return _instance;
}
protected Singleton(){}
// Just to prove only a single instance exists
private int x = 0;
public void SetX(int newVal) {x = newVal;}
public int GetX(){return x;}
}
public class Client
{
public static int Main(string[] args)
{
int val;
// can't call new, because constructor is protected
Singleton FirstSingleton = Singleton.Instance();
Singleton SecondSingleton = Singleton.Instance();
// Now we have two variables, but both should refer to the same object
// Let's prove this, by setting a value using one variable, and
// (hopefully!) retrieving the same value using the second variable
FirstSingleton.SetX(4);
Console.WriteLine("Using first variable for singleton, set x to 4");
val = SecondSingleton.GetX();
Console.WriteLine("Using second variable for singleton, value retrieved = {0}", val);
return 0;
}
}
}