用C#实现一个类，要求该类只能被实例化一次，怎么写？谢谢
如题，万分感谢！

------解决方案--------------------
Singleton模式：
using System;
public class Singleton
{
private static Singleton instance;
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}

------解决方案--------------------
也可以选择在调用的时候才实例化的

public class sealed Singleton
{
private static volatile Singleton _instance;
private static object syncRoot = new object();

private Singleton() { }

public Singleton Instance
{
get
{
if(_instance == null)
{
Lock(syncRoot)
{
if(_instance == null)
{
_instance = new Singleton();
}
}
}
return _instance;
}
}
}