c#问题 求解答求解惑啊啊啊啊啊！！！！！！！！！！！！！！！！！！！！

Question

namespace EventSample
{

class M
{ }

class A <T>: M
    where T:M
{
    public static T Instance
    {
        get;
        private set;
    }

    public void cgr()
    {
        if (Instance == null)
        {
            Instance = this as T;
        }
    }

}

class B : A<B>
{
    public int x;

}
class C : A<C>
{

    public int y;
}
class Program
{

    static void Main(string[] args)
    {


        B b = new B();
        C c = new C();
        b.cgr();
        c.cgr();
        B.Instance.x = 10;
        C.Instance.y = 11;
        Console.ReadKey();
    }
}

}
已上代码A类有个静态属性保存子类对象，在mian入口里我先调用了 b.cgr();此时Instance里存了B类对象，由于静态的原因所有子类共用一个静态，那么在调用c.cgr()时，Instance应该不为空，静态厉害应该是B类对象啊，为什么打断点时，Instanse还为空？

Answer 1

 private T instance;
public static T Instance
    {
        get { if (instance == null) instance = new T(); return instance; }
        private set { instance = value; }
    }

 where T:M, new()