Reference type in struct in C#

Question

I was going through type values in C# and learned that they don't get allocated on the heap as normal reference types do. How does a struct with reference type get allocated?

e.g.

struct simple {
    public Employee e;
    public bool topEmployee;
    public void printSomething()
    {
        Console.WriteLine("Progress " + e.GetProgressReport());
        Console.WriteLine("TopEmployee " + topEmployee);
    }
};

The Employee is a class. Will e get allocated to a heap when initialized? Does it defeat the point of having a struct?

Answer 1

The "kind" of type (value/reference) has little to do with how instances are allocated. It's all about life time, and there are more ways to allocate than "heap" and "stack". Read The Truth About Value Types.

But insofar your question makes sense: A struct's member types do not affect how struct instances are allocated, because they do not affect the lifetime of the object. Same goes for classes, by the way.

The member e will be a part of the value type object and allocated where it may be. This member is a reference, and hence any actual Employee object e refers to will be allocated somewhere else¹. Though it sounds like one, this is not a special rule; locals and class members and array items behave the same way. It does not defeat the point of value types, rather maintains the benefits of both value and reference types. The value type instances are still separate values instead of being aliased, and they still have simpler and shorter life time allowing better allocation choices with less effort. The reference type instances are still shared and (potentially) long-lived.

¹ At least conceptually and in the current implementations; in very simple cases optimizations (escape analysis+allocation sinking) could merge these allocations, but no CLR I'm aware of does that.

Answer 2

Struct memory is allocated 'in-line'. Class memory is allocated on the heap, with a reference (pointer) allocated 'in-line'.

If you see a class variable named C in a program, the storage for that variable will be equivalent to a pointer (say 4 bytes), and the actual storage for the class will be on the heap.

But if you see a struct variable named S in a program, the storage for that variable is simply the size of the variable at the point of declaration. There is no heap allocated storage for it.

If C contains an S then S will be located in the heap storage for C.

If S contains a C then the reference to C will be located in the storage for S, and the storage for C is on the heap. This is the answer to your question about simple and e.

So struct storage can actually be in static memory, on the stack or on the heap (inside a class). Class memory is always on the heap.

Answer 3

The reference variable 'e' will be allocated wherever the struct is allocated (e.g. a local variable of unboxed struct type is likely to go on stack). The instance of Employee the 'e' is pointing to will be allocated on heap. This may vary between .NET implementations, but is most likely true for all current implementations.