Understanding Garbage Collection in .NET

Garbage collection (GC) is an automatic memory management feature in the .NET runtime. Its primary purpose is to reclaim memory that is no longer being used by an application, preventing memory leaks and simplifying development.

How .NET GC Works

The .NET GC operates on a generational approach. It divides the managed heap into generations (Gen 0, Gen 1, and Gen 2) to optimize the collection process. New objects are allocated in Gen 0. When Gen 0 becomes full, a collection occurs. Objects that survive this collection are promoted to Gen 1. If objects in Gen 1 survive a collection, they are promoted to Gen 2. Gen 2 is the oldest generation, and when it becomes full, a full GC collection occurs.

  • Generational Collection: This approach is based on the hypothesis that most objects have a short lifespan. By focusing collections on the youngest generation (Gen 0), the GC can reclaim memory efficiently without having to scan the entire heap.
  • Mark and Sweep: The GC identifies objects that are no longer reachable by the application (i.e., not referenced by any active part of the code). These objects are then marked for deletion.
  • Compaction: After marking unreachable objects, the GC compacts the memory by moving the remaining reachable objects closer together. This reduces fragmentation and creates larger contiguous blocks of free memory.

GC Modes

The .NET GC can operate in different modes, primarily:

  • Workstation GC: Optimized for responsiveness, typically used in client applications. It prioritizes shorter GC pauses.
  • Server GC: Optimized for throughput, typically used in server applications. It can use multiple threads to perform collections concurrently, leading to higher overall performance but potentially longer individual pauses.

The default mode depends on the application type and the number of CPU cores available.

Configuring GC Behavior

You can influence GC behavior through configuration settings:

  • Using the <gcServer> element in your application's configuration file (app.config or web.config) to enable or disable Server GC.
  • Using environment variables like COMPlus_gcServer.
  • Programmatically using the System.Runtime.GCSettings class, though this is less common for typical applications.

Important Note: While GC automates memory management, understanding its behavior can be crucial for performance-sensitive applications. Excessive object allocation and long-lived objects can still impact performance.

Key Concepts

  • Managed Heap: The region of memory where objects created by .NET code are allocated.
  • Root Objects: Objects that are directly accessible by the application (e.g., static fields, local variables on the stack).
  • Object Lifetime: The period during which an object is considered "alive" and therefore should not be garbage collected.

Further Reading