Why do structs need to be boxed?

c# struct boxing
Red Hyena picture Red Hyena · Dec 30, 2009 · Viewed 8.4k times · Source

In C#, any user-defined struct is automatically a subclass of System.Struct System.ValueType and System.Struct System.ValueType is a subclass of System.Object.

But when we assign some struct to object-type reference it gets boxed. For example:

struct A
{
    public int i;
}

A a;
object obj = a;  // boxing takes place here

So my question is: if A is an descendant of System.Object, can't the compiler up-cast it to object type instead of boxing?

Answer

mmx picture mmx · Dec 30, 2009

A struct is a value type. System.Object is a reference type. Value types and reference types are stored and treated differently by the runtime. For a value type to be treated as a reference type, it's necessary for it to be boxed. From a low level perspective, this includes copying the value from the stack where it originally lives to the newly allocated memory on the heap, which also contains an object header. Additional headers are necessary for reference types to resolve their vtables to enable virtual method dispatches and other reference type related features (remember that a struct on stack is just a value and it has zero type information; it doesn't contain anything like vtables and can't be directly used to resolve dynamically dispatched methods). Besides, to treat something as a reference type, you have to have a reference (pointer) to it, not the raw value of it.

So my question is - if A is an descendant of System.Object, can't compiler upcast it to object type instead of boxing?

At a lower level, a value does not inherit anything. Actually, as I said before, it's not really an object. The fact that A derives from System.ValueType which in turn derives from System.Object is something defined at the abstraction level of your programming language (C#) and C# is indeed hiding the boxing operation from you pretty well. You don't mention anything explicitly to box the value so you can simply think the compiler has "upcasted" the structure for you. It's making the illusion of inheritance and polymorphism for values while none of the tools required for polymorphic behavior is directly provided by them.

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