c# - 好奇心 : Converting a C# struct into an object still copies it

Question

这个问题更多是出于好奇而不是真正的问题。考虑以下代码(C# 4.0，如果重要的话):

class Program {
  static Point myPoint = new Point(3, 5);

  static void Main(string[] args) {
    Console.WriteLine("Point Struct Before: " + myPoint);
    object point = GetPoint();
    Console.WriteLine("Point Object Before: " + point);
    myPoint.X = 10;
    Console.WriteLine("Point Struct After: " + myPoint);
    Console.WriteLine("Point Object After: " + point);
  }

  static object GetPoint() {
    return myPoint;
  }
}

输出如下:

Point Struct Before: 3;5
Point Object Before: 3;5
Point Struct After: 10;5
Point Object After: 3;5

现在，它可以正常工作，这意味着 GetPoint() 返回的点被复制而不是被引用。 (否则最后一行也会显示为“10;5”。)

我现在的问题是:为什么这样做有效？编译器如何知道是复制还是引用一个对象？这是否意味着该决定是在运行时而不是编译期间完成的？

此外，这现在允许我将 point 设置为 null 而 struct 不能设置为 null。结构是否自动转换为可空类型？

Answer 1

Also, this now allows me to set point to null whereas structs can't be set to null. Is the struct automatically converted into a nullable type?

你误解了整个过程:

int i = 5;
object o = i; //Creates a new Int32 with value 5 and assigns it to the object'o' reference storing the value on the garbage collected heap. This is what is called boxing.
object o = null //Simply assigns 'null' to the object reference, it does nothing to the boxed value type 'o' was referencing before.

在上面的示例中，装箱的 int 最终将由 GC 从垃圾收集堆中收集，因为不再有事件引用指向它。