Structs, Interfaces and Boxing

Question

Take this code:

interface ISomeInterface
{
    public int SomeProperty { get; }
}

struct SomeStruct : ISomeInterface
{
    int someValue;

    public int SomeProperty { get { return someValue; } }

    public SomeStruct(int value)
    {
        someValue = value;
    }
}

and then I do this somewhere:

ISomeInterface someVariable = new SomeStruct(2);

is the SomeStruct boxed in this case?

Answer 1

Yes, it is. Basically whenever you need a reference and you've only got a value type value, the value is boxed.

Here, ISomeInterface is an interface, which is a reference type. Therefore the value of someVariable is always a reference, so the newly created struct value has to be boxed.

Answer 2

The MSDN documentation tells us that structs are value, not reference types. They are boxed when converting to/from a variable of type object. But the central question here is: what about a variable of an interface type? Since the interface can also be implemented by a class, then this must be tantamount to converting from a value to a reference type, as Jon Skeet already said, therefore yes boxing would occur. More discussion on an msdn blog.

Answer 3

Jon's point is true, but as a side note there is one slight exception to the rule; generics. If you have where T : ISomeInterface, then this is constrained, and uses a special opcode. This means the interface can be used without boxing. For example:

public static void Foo<T>(T obj) where T : ISomeInterface {
    obj.Bar(); // Bar defined on ISomeInterface
}

This does not involve boxing, even for value-type T. However, if (in the same Foo) you do:

ISomeInterface asInterface = obj;
asInterface.Bar();

then that boxes as before. The constrained only applies directly to T.

Answer 4

I'm adding this to hopefully shed a little more light on the answers offered by Jon and Marc.

Consider this non-generic method:

public static void SetToNull(ref ISomeInterface obj) {
    obj = null;
}

Hmm... setting a ref parameter to null. That's only possibly for a reference type, correct? (Well, or for a Nullable<T>; but let's ignore that case to keep things simple.) So the fact that this method compiles tells us that a variable declared to be of some interface type must be treated as a reference type.

The key phrase here is "declared as": consider this attempt to call the above method:

var x = new SomeStruct();

// This line does not compile:
// "Cannot convert from ref SomeStruct to ref ISomeInterface" --
// since x is declared to be of type SomeStruct, it cannot be passed
// to a method that wants a parameter of type ref ISomeInterface.
SetToNull(ref x);

Granted, the reason you can't pass x in the above code to SetToNull is that x would need to be declared as an ISomeInterface for you to be able to pass ref x -- and not because the compiler magically knows that SetToNull includes the line obj = null. But in a way that just reinforces my point: the obj = null line is legal precisely because it would be illegal to pass a variable not declared as an ISomeInterface to the method.

In other words, if a variable is declared as an ISomeInterface, it can be set to null, pure and simple. And that's because interfaces are reference types -- hence, declaring an object as an interface and assigning it to a value type object boxes that value.

Now, on the other hand, consider this hypothetical generic method:

// This method does not compile:
// "Cannot convert null to type parameter 'T' because it could be 
// a non-nullable value type. Consider using 'default(T)' instead." --
// since this method could take a variable declared as, e.g., a SomeStruct,
// the compiler cannot assume a null assignment is legal.
public static void SetToNull<T>(ref T obj) where T : ISomeInterface {
    obj = null;
}