How do generations (Gen0, Gen1, Gen2, LOH) work in .NET GC?

Question

How do generations (Gen0, Gen1, Gen2, LOH) work in .NET GC and When would you call GC.Collect() manually — or should you never ? How does IDisposable and using pattern interact with GC?

Deepika Sawant · Answer

The .NET Garbage Collector (GC) is generational to optimize performance. It assumes most objects die young, so it categorizes them by age: Generation Description Collection Frequency Typical Use Gen 0 Newly allocated objects Very frequent Temp variables, short-lived objects Gen 1 Survived Gen 0 Less frequent Intermediate lifespan Gen 2 Survived multiple collections Least frequent Long-lived objects (e.g., caches, static data) LOH (Large Object Heap) Objects > 85,000 bytes Only during Gen 2 Large arrays, bitmaps, buffers How It Works GC starts with Gen 0 because it's fast and cheap. Survivors move to Gen 1 , then Gen 2 . LOH is not compacted by default and can fragment memory — a common pain point in high-throughput apps. Should You Call GC.Collect() Manually? Generally, No The GC is self-tuning . Manual calls can: Stall threads Break generational assumptions Hurt performance Rare Exceptions Use GC.Collect() only when: You’ve released a large amount of memory (e.g., unloading a plugin or image-heavy form) You're benchmarking memory usage You're in a constrained environment (e.g., embedded systems) IDisposable , using , and the GC Purpose of IDisposable GC handles memory , but not resources like: File handles Network sockets Database connections Unmanaged memory These need deterministic cleanup , which GC doesn’t guarantee. The using Pattern Ensures Dispose() is called even if exceptions occur: using (var conn = new SqlConnection(...)) { // Use connection } // Dispose() is called here GC + IDisposable GC may call Finalize() (if implemented), but it’s non-deterministic . You should implement the Dispose pattern to clean up resources and suppress finalization: public void Dispose() { Dispose(true); GC.SuppressFinalize(this); // Prevents Finalize from running } Imagine you're writing a high-performance image editor: You load large bitmaps → LOH You wrap file streams → IDisposable You unload plugins → maybe GC.Collect() (but only after profiling)

Ck Nitin · Answer

Thanks Deepika

How do generations (Gen0, Gen1, Gen2, LOH) work in .NET GC?

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