三种单例模式写法：
public class Singleton
{
private static Singleton instance;
private static readonly object syncRoot = new object();
private Singleton() { }
public static Singleton GetInstance()
{
if (instance == null)//Double-Check Locking 双重锁定，比直接lock性能更好，最完美的模式（懒汉式单例）
{
lock (syncRoot)
{
if (instance == null)
{
instance = new Singleton();
}
}
}
return instance;
}
}
public class Singleton1
{
private static Singleton1 instance;
private static readonly object syncRoot = new object();
private Singleton1() { }
public static Singleton1 GetInstance()
{
lock (syncRoot)//每次调用GetInstance都需要lock，影响性能（懒汉式单例）
{
if (instance == null)
{
instance = new Singleton1();
}
}
return instance;
}
}
public class Singleton2
{
private static Singleton2 instance=new Singleton2();//静态初始化方式（饿汉式单例），类一加载就实例化对象，一般这样用也没问题，缺点：提前占用系统资源
private Singleton2() { }
public static Singleton2 GetInstance()
{
return instance;
}
}
双重锁定性能优化分析：