Destroying a struct object in C#?

Question

I am a bit confused about the fact that in C# only the reference types get garbage collected. That means GC picks only the reference types for memory de-allocation. So what happens with the value types as they also occupy memory on stack ?

Answer 1

For a start, whether they're on the stack or part of the heap depends on what context they're part of - if they're within a reference type, they'll be on the heap anyway. (You should consider how much you really care about the stack/heap divide anyway - as Eric Lippert has written, it's largely an implementation detail.)

However, basically value type memory is reclaimed when the context is reclaimed - so when the stack is popped by you returning from a method, that "reclaims" the whole stack frame. Likewise if the value type value is actually part of an object, then the memory is reclaimed when that object is garbage collected.

The short answer is that you don't have to worry about it :) (This assumes you don't have anything other than the memory to worry about, of course - if you've got structs with references to native handles that need releasing, that's a somewhat different scenario.)

Answer 2

I am a bit confused about the fact that in C# only the reference types get garbage collected.

This is not a fact. Or, rather, the truth or falsity of this statement depends on what you mean by "get garbage collected". The garbage collector certainly looks at value types when collecting; those value types might be alive and holding on to a reference type:

struct S { public string str; }
...
S s = default(S); // local variable of value type
s.str = M(); 

when the garbage collector runs it certainly looks at s, because it needs to determine that s.str is still alive.

My suggestion: clarify precisely what you mean by the verb "gets garbage collected".

GC picks only the reference types for memory de-allocation.

Again, this is not a fact. Suppose you have an instance of

class C { int x; }

the memory for the integer will be on the garbage-collected heap, and therefore reclaimed by the garbage collector when the instance of C becomes unrooted.

Why do you believe the falsehood that only the memory of reference types is deallocated by the garbage collector? The correct statement is that memory that was allocated by the garbage collector is deallocated by the garbage collector, which I think makes perfect sense. The GC allocated it so it is responsible for cleaning it up.

So what happens with the value types as they also occupy memory on stack ?

Nothing at all happens to them. Nothing needs to happen to them. The stack is a million bytes. The size of the stack is determined when the thread starts up; it starts at a million bytes and it stays a million bytes throughout the entire execution of the thread. Memory on the stack is neither created nor destroyed; only its contents are changed.

Answer 3

There are too many verbs used in this question, like destroyed, reclaimed, deallocated, removed. That doesn't correspond well with what actually happens. A local variable simply ceases to be, Norwegian parrot style.

A method has a single point of entry, first thing that happens is that the CPU stack pointer is adjusted. Creating a "stack frame", storage space for the local variables. The CLR guarantees that this space is initialized to 0, not otherwise a feature you use strongly in C# because of the definite assignment rule.

A method has a single point of exit, even if you method code is peppered with multiple return statements. At that point, the stack pointer is simply restored to its original value. In effect it "forgets" that the local variables where ever there. Their values are not 'scrubbed' in any way, the bytes are still there. But they won't last long, the next call in your program are going to overwrite them again. The CLR zero-initialization rule ensures that you can never observe those old values, that would be insecure.

Very, very fast, takes no more than a single processor cycle. A visible side-effect of this behavior in the C# language is that value types cannot have a finalizer. Ensuring no extra work has to be done.

Answer 4

A value type on the stack is removed from the stack when it goes out of scope.