//第一種最簡單，但沒有考慮線程安全，在多線程時可能會出問題，不過俺從沒看過出錯的現象
public class Singleton
{
    private static Singleton _instance = null;
    private Singleton(){}
    public static Singleton CreateInstance()
    {
        if(_instance == null)

        {
            _instance = new Singleton();
        }
        return _instance;
    }
}

 
//第二種考慮了線程安全，不過有點煩，但絕對是正規寫法，經典的一叉 

public class Singleton
{
    private volatile static Singleton _instance = null;
    private static readonly object lockHelper = new object();
    private Singleton(){}
    public static Singleton CreateInstance()
    {
        if(_instance == null)
        {
            
 
lock(lockHelper)
            {
                if(_instance == null)
                     _instance = new Singleton();
            }
        }
        return _instance;
    }
}

//第三種可能是C#這樣的高級語言特有的，實在懶得出奇

public class Singleton
{

    private Singleton(){}
    public static readonly Singleton instance = new
 
 Singleton();
}  

單例模式 的三種寫法 c#