C# parameters by reference and .net garbage collection

Question

I have been trying to figure out the intricacies of the .NET garbage collection system and I have a question related to C# reference parameters. If I understand correctly, variables defined in a method are stored on the stack and are not affected by garbage collection. So, in this example:

public class Test
{
 public Test()
 {
 }

 public int DoIt()
 {
  int t = 7;
  Increment(ref t);
  return t;
 }

 private int Increment(ref int p)
 {
  p++;
 }
}

the return value of DoIt() will be 8. Since the location of t is on the stack, then that memory cannot be garbage collected or compacted and the reference variable in Increment() will always point to the proper contents of t.

However, suppose we have:

public class Test
{
 private int t = 7;

 public Test()
 {
 }

 public int DoIt()
 {
  Increment(ref t);
  return t;
 }

 private int Increment(ref int p)
 {
  p++;
 }
}

Now, t is stored on the heap as it is a value of a specific instance of my class. Isn't this possibly a problem if I pass this value as a reference parameter? If I pass t as a reference parameter, p will point to the current location of t. However, if the garbage collector moves this object during a compact, won't that mess up the reference to t in Increment()? Or does the garbage collector update even references created by passing reference parameters? Do I have to worry about this at all? The only mention of worrying about memory being compacted on MSDN (that I can find) is in relation to passing managed references to unmanaged code. Hopefully that's because I don't have to worry about any managed references in managed code. :)

Answer 1

No, you don't need to worry about it. Basically the calling method (DoIt) has a "live" reference to the instance of Test, which will prevent it from being garbage collected. I'm not sure whether it can be compacted - but I suspect it can, with the GC able to spot which variable references are part of objects being moved.

In other words - don't worry. Whether it can be compacted or not, it shouldn't cause you a problem.

Answer 2

It is exactly how you mention it in the last sentence. The GC will move all needed references when it compacts the heap (except for references to unmanaged memory).

Note that using the stack or heap is related to an instance variable being of a value or reference type. Value types (structs and 'simple' types like int, double, etc) are always on the stack, classes are always in the heap (what is in the stack is the reference - the pointer - to the allocated memory for the instance).

Edit: as correctly noted below in the comment, the second paragraph was written much too quickly. If a value type instance is a member of a class, it will not be stored in the stack, it will be in the heap like the rest of the members.

Answer 3

If I understand correctly, variables defined in a method are stored on the stack and are not affected by garbage collection.

It depends on what you mean by "affected". The variables on the stack are the roots of the garbage collector, so they surely affect garbage collection.

Since the location of t is on the stack, then that memory cannot be garbage collected or compacted and the reference variable in Increment() will always point to the proper contents of t.

"Cannot" is a strange word to use here. The point of using the stack in the first place is because the stack is only used for data which never needs to be compacted and whose lifetime is always known so it never needs to be garbage collected. That why we use the stack in the first place. You seem to be putting the cart before the horse here. Let me repeat that to make sure it is clear: the reason we store this stuff on the stack is because it does not need to be collected or compacted because its lifetime is known. If its lifetime were not known then it would go on the heap. For example, local variables in iterator blocks go on the heap for that reason.

Now, t is stored on the heap as it is a value of a specific instance of my class.

Correct.

Isn't this possibly a problem if I pass this value as a reference parameter?

Nope. That's fine.

If I pass t as a reference parameter, p will point to the current location of t.

Yep. Though the way I prefer to think of it is that p is an alias for the variable t.

However, if the garbage collector moves this object during a compact, won't that mess up the reference to t in Increment()?

Nope. The garbage collector knows about managed references; that's why they're called managed references. If the gc moves the thing around, the managed reference is still valid.

If you had passed an actual pointer to t using unsafe code then you would be required to pin the container of t in place so that the garbage collector would know to not move it. You can do that using the fixed statement in C#, or by creating a GCHandle to the object you want to pin.

does the garbage collector update even references created by passing reference parameters?

Yep. It would be rather fragile if it didn't.

Do I have to worry about this at all?

Nope. You're thinking about this like an unmanaged C++ programmer -- C++ makes you do this work, but C# does not. Remember, the whole point of the managed memory model is to free you from having to think about this stuff.

Of course, if you enjoy worrying about this stuff you can always use the "unsafe" feature to turn these safety systems off, and then you can write heap and stack corrupting bugs to your heart's content.