Array of structs inheriting interface appear to be reference types

Question

I have a struct called Foo and Bar - both if these implement an interface called IFoo.

public interface IFoo
{
    string StrTest { get; set; }
    int NumTest { get; set; }
}

public struct Foo : IFoo
{
    public Foo(string backTest, int numTest)
    {
        StrTest = backTest;
        NumTest = numTest;
    }

    public string StrTest { get; set; }
    public int NumTest { get; set; }
}

public struct Bar : IFoo
{
    public Bar(string backTest, int numTest)
    {
        StrTest = backTest;
        NumTest = numTest;
    }

    public string StrTest { get; set; }
    public int NumTest { get; set; }
}

Let's say I have an array of IFoo, at index "0" I have a Foo struct. The problem comes when I move that struct over to a new index - such as "1" (which should copy it). Now, you would expect both of them to be separate, meaning you change one and the other shouldn't change as well.

However, I find that if I change one of the properties for the newly moved one - both change... isn't this class (reference type) functionality?

Take a look at the following example:

// Create a list of "BackInterface"s - with foo and bar.
List<IFoo> arr = new List<IFoo>
{
    new Foo("A", 2),
    new Bar("B", 4)
};

// Now, place the SAME struct at index "0" (the foo) at the end - it should be copied?
arr.Add(arr[0]);

// Modify the item at the last newly-created index
arr[2].StrTest = "C";

Both arr[0] and arr[2]'s StrTest are now "C".

If I have an array of just Foo or Bar I don't have this problem - only when I have an array of IFoo. I have also tried normal arrays (of type "object) and ArrayList (which doesn't have generics) - none of which have worked.

Any idea what is happening in here, and how to still be able to have an array of both Foo and Bar, both having to have an implementation of something?

Read up on boxing. E.g. https://stackoverflow.com/questions/3032750/structs-interfaces-and-boxing — Joe, Dec 21 '18 at 19:07
In order to dispatch an interface method, boxing is required. You could for example have also created a class implementing `IFoo`. — Ian Mercer, Dec 21 '18 at 19:07
Note a struct doesn't inherit from an interface, it implements an interface. — Servy, Dec 21 '18 at 19:12
@Servy Just asking: What is particularly bad about mutable structs? — ABPerson, Dec 21 '18 at 19:30
@ABPerson There's lots of readily accessible information on the topic. Just do some searching. — Servy, Dec 21 '18 at 19:31

score 2 · Accepted Answer · answered Dec 21 '18 at 19:36

You can't have an array of both Foo and Bar, both structs, without the boxing you're seeing.

You get boxing if you cast a struct to an interface, which happens when you create List<IFoo> and then put your struct in it. You can see your structs have become reference types by doing

bool test = arr[0].GetType().IsValueType;

after you create arr: it will be false for List<IFoo> and true for List<Foo>.

There are some workarounds, such as having two arrays one for Foo and one for Bar and then another array to index into both of them. But that's horrible. See this earlier Stack Overflow answer for more.

Because of all this in general it's not a great idea to put interfaces on structs: structs are intended to be lightweight things that live on the stack. If you need this behavior you're better off using classes.

I'll mark this answer as the answer since it does go into a bit more detail - as well as *possible* workarounds - but, I guess I'll just stick with a class! Even if I will have to clone it *a lot*. — ABPerson, Dec 21 '18 at 19:43

score 1 · Answer 2 · answered Dec 21 '18 at 19:08

By storing the value types in a variable that's of an interface type it gets boxed. Interfaces are always reference types, so implicitly converting a value type implementing it to the interface's type needs to box it. A boxed value type is a reference type, so you're observing reference type behavior because arr[0] is in fact a reference type.

Array of structs inheriting interface appear to be reference types

2 Answers2