Garbage Collection and Memory in .NET

You do not free memory by hand in C#. The garbage collector (GC) does it. Interviewers want to see that you understand roughly how, what it does not clean up, and how programs still leak memory.

Stack and heap

See value and reference types for the detail on where value types actually live.

How the garbage collector works

The GC finds objects that the program can no longer reach, starting from the roots (local variables, static fields, CPU registers), and reclaims their memory. It then compacts the surviving objects together.

It is generational, based on the observation that most objects die young:

An object that survives a collection is promoted to the next generation. Objects of 85,000 bytes or more go to the large object heap, which is collected together with Gen 2 and is not compacted by default, so allocating many big arrays can hurt.

var item = new object();
Console.WriteLine(GC.GetGeneration(item));     // 0   (just allocated)
GC.Collect();                                  // for demonstration only
Console.WriteLine(GC.GetGeneration(item));     // 1   (it survived, so it was promoted)

Do not call GC.Collect() in real code. The collector knows better than you when to run, and forcing it usually makes performance worse.

What the GC does not clean up

The GC reclaims memory. It does not close files, database connections or sockets in a timely way: those need Dispose(), usually through . See exceptions and IDisposable.

Finalizers

A finalizer () is a last-chance method the GC calls before reclaiming an object. They are slow and unpredictable:

• objects with finalizers survive an extra collection,
• you cannot know when (or in what order) they run,
• they run on a separate thread.

So: implement for deterministic cleanup, and add a finalizer only as a safety net for unmanaged resources, calling in .

Memory leaks in a garbage-collected language

"Memory leak" in C# means an object stays reachable when you no longer need it, so the GC cannot free it. The usual causes:

• Event handlers that are never unsubscribed (the publisher holds the subscriber). See delegates, events and lambdas.
• Static fields and singletons that collect data forever (a static or used as a cache with no limit).
• Lambdas that capture large objects and are stored somewhere long-lived.
• Undisposed unmanaged resources, including -style handles created in a loop.

To find a leak, take memory snapshots in Visual Studio or use and , then look at what holds a reference to the objects that keep growing.

Interview answers in one line each

• What are Gen 0, 1 and 2? Age-based groups: new objects in Gen 0, survivors move up. Young generations are collected often and cheaply.
• Does the GC close file streams? Not promptly. Use or .
• Can you force a collection? Yes (), but you almost never should.
• Can a C# program leak memory? Yes, by keeping unneeded objects reachable.

Practise

1. Run the generation example, then allocate an and print its generation. What do you see, and why?
2. Subscribe an object to a static event, drop your reference to it, and show it is still alive with a .
3. Take the Exceptions, Memory and Async quiz.