Garbage Collection in .NET (Interview Guide)

Most developers say:
“GC frees memory.”
That’s correct… but way too shallow to pass a real interview.

What is Garbage Collection in .NET?

❌ Bad answer

It automatically frees unused memory.

✅ Senior answer

Garbage Collection (GC) is an automatic memory management system that reclaims memory occupied by objects that are no longer referenced by the application.
It optimizes performance using a generational model and runs periodically to balance memory usage and CPU cost.

How Garbage Collection works

Objects are allocated on the managed heap
GC tracks references to determine if objects are still in use
Unreferenced objects are marked and removed
Memory is compacted to reduce fragmentation

Generational model (very important)

Gen 0: Short-lived objects (collected frequently)
Gen 1: Medium-lived objects
Gen 2: Long-lived objects (collected less often)

👉 Most objects die young → this is why GC is optimized this way.

Example

void CreateObjects()
{
    for (int i = 0; i < 1000; i++)
    {
        var obj = new object(); // allocated in Gen 0
    }
}

Common mistakes

❌ Forcing GC manually

GC.Collect();

This can hurt performance. Let the runtime decide.

❌ Memory leak misconception

GC does not prevent memory leaks if references are still held.

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