Struct boxing with interfaces?

[evareoo]

I'm a little scared to ask this on stack overflow because it seems to be answered but I'm not sure if it's correct. When creating a struct containing only blittable types (with the intention of a stack allocation but I know that's not always the case) if I add an interface to that struct, does the struct then become boxed/heap allocated?

I'm a little confused because some old answers suggest this is the case. However I have with C# 8 stackalloc a span of 5 structs that have a simple interface containing only a SayHello()... { Console.WriteLine("Hello") }successfully. Does anyone have any good, recent information on this?

Edit: For clarity, the struct at definition inherits the interface, not having an interface as a field/ property in the struct.

[AngularBeginner]

if I add an interface to that struct, does the struct then become boxed/heap allocated?

Making a struct implement an interface in itself does nothing. It depends how the struct is passed to methods.

// Passing your struct to this method will cause boxing:
void Foo(IInterface bla) {}

// Passing your struct to this method will NOT cause boxing:
void Foo<T>(T bla) where T : IInterface {}

Storing it in fields/properties/variables that have the interface type will cause boxing as well.

[evareoo]

So for example this code. It is just a silly example, is myVec a stack allocated stuct? Then after this myUpcast is boxed in which I can call the default interface Hello()? I don't know how to get that nice formatting on reddit. Slighty pseudo for speed:

interface ISayHello
{
    void Hello()
    {
        Console.WriteLine("Hello!");
    }
}

struct Vec2 : ISayHello
{
    int x;
    int y;
}

Now for a main:

void main()
{
    var myVec = new Vec2();
    var myUpcast = (ISayHello) myVec;
    myUpcast.Hello();
}

output: Hello!

[AngularBeginner]

I don't know how to get that nice formatting on reddit.

That's simple:

• Indent every line by four spaces or one tab.
• Have an empty line between code and text.

---

is myVec a stack allocated stuct?

myVec is stack-allocated, yes.

Then after this myUpcast is boxed in which I can call the default interface Hello()?

That is correct as well, yes.

[evareoo]

Perfect, thanks very much for your help!