Question

What happens when there is a circular reference between two structs? How does memory allocation work for the example below or would it yield and error?

    public struct MultipleChoiceQuestion
    {
        public IEnumerable<Option> Options;
    }

    public struct Option
    {
        public MultipleChoiceQuestion Question;
    }

Answer 1

The Options field in MultipleChoiceQuestion is just a reference here, which will be null by default. The Question field in Option is a value of type MultipleChoiceQuestion which will have its default value by default. There’s no problem here (other than a questionable design in more ways than one).

A genuine circular reference leading to a logical memory problem will fail to compile:

struct Foo
{
    Bar bar;
}

struct Bar
{
    Foo foo;
}

Test.cs(6,9): error CS0523: Struct member ‘Foo.bar’ of type ‘Bar’ causes a cycle in the struct layout
Test.cs(11,9): error CS0523: Struct member ‘Bar.foo’ of type ‘Foo’ causes a cycle in the struct layout