每个对象只调用一次结构构造函数

结构中是否可以有一个构造函数，每个对象只调用一次？例如，我希望代码的行为与此类似：

struct mystruct { /*...*/ }
//later on
mystruct x = 5; //constructor called once, okay
x = 6; //constructor called twice for this object, exception thrown
mystruct y = 6; //different object, this is okay
mystruct z;
z = 7; //this is also okay since the definition didn't call the constructor

这在C#中可能吗？如果没有，有没有办法模仿这种行为？

我试着保留一个this的静态字典，并测试是否存在新的this，但没有成功。测试this时，ObjectIDGenerator也不会。