Garbage collector and circular reference
Consider these two classes:
public class A
{
B b;
public A(B b) { this.b = b; }
}
public class B
{
A a;
public B() { this.a = new A(this); }
}
If I have classes designed like above, would the objects of such classes be collected by Garbage Collector (GC)?
Suppose I do this:
void f()
{
B b = new B();
}
In this method, I create an instance of B called b, and when the method returns, b goes out of scope, and the GC should be able to collect it, but if it were to collect it, it would have to collect a first which is the member of B, and to collect a, it needs to collect b first which is the member of A. It becomes circular. So my question is : is such circular reference going to prevent GC from collecting the objects?
If yes, then how can we avoid this problem? How can we make sure that we don't have circular reference in our class design? Is there any tool (or compiler option) that helps us detecting circular reference?
If no, where and why do we use WeakReference class? What is its purpose?
A cannot depend on B, because A must get made first, as a member of B, and so A can only hold a weak reference to B. Thus it is safe to destroy B first and then A. Arguing recursively, every computer program must be thus. There can never be a true, fully symmetric circular dependence.
why do we use WeakReference class? What is its purpose? I use it -for example- for caching data(in a dictionary). If it is alive ,good, if not, I load it again at first request.
The .NET garbage collector can absolutely handle circular references. The very high level view of how the garbage collector works is ...
Start with locals, statics and GC pinned objects. None of these can be collected
Mark every object which can be reached by traversing the children of these objects
Collect every object which is not marked.
This allows for circular references to be collected just fine. So long as none of them are reachable from an object known to be uncollectable then the circular reference is essentially irrelevant.
Note: I realize I've left out many fun details in order to keep this answer simple and direct
No, This won't be a problem because GC can handle Circular References
MSDN Says
If a group of objects contain references to each other, but none of these object are referenced directly or indirectly from stack or shared variables, then garbage collection will automatically reclaim the memory.
Several answers already explained that circular references are not a problem.
As to the weak references - the reason to use them is caching.
When GC walks object dependency trees he ignores weak references. In other words if the only reference to an object is a weak one(s), it will be garbage collected, but if there was no garbage collection between reference creation and your attempt to use, you can still access the object.
No that circular reference will not affect the garbage collector, and it will be perfectly able to collect the instance of B.
The garbage collector knows that no-one can reference the instance of B after going out of scope, and consequently, no-one can use the instance of B to indirectly reference A.
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