In a modern garbage collected runtime such as .NET or Java, how does garbage collection handle circular references between objects?

Introduction / Context:
In languages such as C sharp and Java, memory is managed by a garbage collector rather than by manual deallocation. One common question is how the runtime deals with circular references, where objects reference each other in a loop. Developers sometimes worry that such cycles will cause memory leaks. This question examines how modern tracing garbage collectors handle circular references and why they are usually not a problem when no live references exist from program roots.

Given Data / Assumptions:
- The runtime uses a tracing garbage collector, not pure reference counting.
- Objects can hold references to other objects, forming arbitrary graphs including cycles.
- There are well defined GC roots such as static fields, local variables on stacks, and CPU registers.
- The goal of garbage collection is to reclaim memory used by objects that are no longer reachable from roots.

Concept / Approach:
Tracing garbage collectors work by starting from GC roots and following references to mark all reachable objects. Any object not reached during this traversal is considered garbage and can be reclaimed. Because the decision is based on reachability from roots, not on the raw reference count, entire cycles of objects that are isolated from roots can be safely collected. Circular references only become a problem in systems that rely solely on naive reference counting, which cannot detect cycles without extra logic.

Step-by-Step Solution:
Step 1: The collector identifies GC roots, such as static variables and stack references in running threads. Step 2: It traverses the object graph, following every reference from roots and marking all visited objects as reachable. Step 3: Objects that are not marked reachable at the end of this traversal, including any cycles that are no longer connected to roots, are classified as garbage. Step 4: The collector reclaims memory for these unreachable objects by freeing their space on the managed heap.

Verification / Alternative check:
You can simulate this behavior by writing code that creates two objects that reference each other, then removing all references from local variables and static fields. In a .NET or Java environment, memory profiling tools show that after a collection, those objects are reclaimed even though they formed a cycle. This confirms that reachability analysis, not naive reference counting, governs collection decisions in these runtimes.

Why Other Options Are Wrong:
Option B is wrong because modern .NET and Java collectors do not rely only on reference counting and are specifically designed to reclaim cycles. Option C is incorrect since objects reside on the managed heap in memory, not automatically on disk. Option D is false because circular references are allowed; object graphs often contain cycles such as parent child relationships in trees with back references.

Common Pitfalls:
A common misconception is equating all garbage collection with reference counting, leading to unnecessary fear of circular references. The real sources of memory leaks in managed environments usually involve lingering references from static collections, event handlers, or caches that still point to objects, keeping them reachable. Developers should focus on removing unnecessary references from long lived objects rather than avoiding cycles altogether.

Final Answer:
Tracing garbage collectors handle circular references by marking all objects reachable from GC roots and then reclaiming any remaining unreachable objects, so cycles that are no longer reachable from roots are collected and do not cause memory leaks.