GarbageCollector

Garbage Collector?

• A system that finds objects that are no longer used and releases them from memory.
• Similar in concept to C++ delete and free(), but C# performs it automatically.
• It exists in heap memory and manages memory there.
• It is mainly invoked when memory becomes insufficient after allocating new objects.

GC Alloc …

• Means a new heap allocation has occurred.
• Equivalent to GC Alloc in the Unity Profiler.
• Think of it like a potential risk warning.
• It does not mean GC has already happened, only that the possibility has been created.

void Update()
{
    // Allocated on the heap every frame.
    var list = new List<int>();
    list.Add(1);
}

• The ideal state is when GC Alloc remains 0, which is often called a GC-free frame.

Unity developers should …

• Proper memory management means minimizing heap allocations so that GC does not occur.
• Minimizing heap allocations means not creating unnecessary objects.
• Because GC runs on the main thread, logic stops while it runs, so frame drops are unavoidable.

Common cases where this can happen

String concatenation

string profile = "Level " + player.Level + " " + player.Nickname;

• When the + operator is used on strings like this, new instances are repeatedly created because strings are immutable, which causes GC.
• Conclusion: use StringBuilder or string interpolation.

LINQ

• Often called a black box of heap allocations.
• If performance is sensitive, iterating with for or foreach is usually preferred over LINQ.

var result = list.Where(x => x.IsActive);

• Because it includes a lambda, a delegate object is created on the heap.
• GC Alloc occurs every time it is called.
• LINQ methods such as ToList, Select, and OrderBy create new collections, which also allocates on the heap.
• It uses IEnumerator<T>, and boxing can happen when value types are involved.

List<int> numbers = new ();
numbers.Where(x => x > 0;) // int -> object

• Conclusion: use it appropriately for one-time execution, compile-time work, or loading-time work.

foreach

foreach (Transform trf in transforms)
{
    //
}

• When using struct-based custom enumerators such as Unity’s Transform component, the foreach statement may cause boxing because it internally casts to the IEnumerator interface.
• More generally, when iterating structs with foreach, boxing can occur during the conversion to the IEnumerator interface.
• Conclusion: it is safe for iterating reference-type classes, small collections, structurally clear loops, or generic collections.

Lambda expressions and anonymous methods

button.onClick.AddListener(() => Debug.Log("Clicked"));

• If a lambda captures variables, an object is created on the heap.
• Even without captures, creating the delegate itself can still cause GC Alloc.
• Conclusion: define the function in advance and pass a reference to it.

Instantiate / Destroy

• As expected, creating and destroying directly also causes GC Alloc.
• Destroy happens after the frame ends, and memory cleanup is delayed as well. If used continuously, GC pressure builds up and can cause severe frame drops.
• Conclusion: use pooling.