CODEWITHSUNDEEP
c#arraysheap-memoryallocationstack-memory
morowinder
morowinder

Reputation: 513

When is array allocated on stack in C#?

I've been trying to figure out when things get allocated on stack and I can't figure out how would you make array (or rather values in it) get allocated on stack;

in this example:

public void foo()
{
    int bar[] = new int [10];
}

10 int structs would be allocated on the the heap, only pointer to them would be on stack, correct?

How would one make fixed size array to go on stack? What if I'm using stucts I defined?

What if I want array size passed as parameter to function?

As far as I understand there should be no problem to get array of arbitrary size on stack when function is called, if size is known when function is called.

Should I even be bothered by this? As far as I understand getting this fixed size array on stack would improve performance, because no heap allocation is done.

Upvotes: 38

Views: 24423

Answers (3)

Theodor Zoulias
Theodor Zoulias

Reputation: 43475

Starting from C# 12, it is now possible to allocate a Span<T> on the stack by using a collection expression. The two lines below are equivalent:

Span<int> bar = [1, 2, 3, 4, 5]; // Collection expression C# 12

Span<int> bar = stackalloc int[] { 1, 2, 3, 4, 5 }; // Old style

Upvotes: 1

James Wilkins
James Wilkins

Reputation: 7367

It was mentioned in comments, but no one bother to update any answers, so I'm posting one. C# now has a special struct type that works well for this purpose:

public void foo()
{
    Span<int> bar = stackalloc int [10];
}

Taken from the docs:

Span is a ref struct that is allocated on the stack rather than on the managed heap. Ref struct types have a number of restrictions to ensure that they cannot be promoted to the managed heap, including that they can't be boxed, they can't be assigned to variables of type Object, dynamic or to any interface type, they can't be fields in a reference type, and they can't be used across await and yield boundaries. In addition, calls to two methods, Equals(Object) and GetHashCode, throw a NotSupportedException.

Span<T> Struct

Upvotes: 16

Ondrej Tucny
Ondrej Tucny

Reputation: 27962

10 int structs would be allocated on the the heap, only pointer to them would be on stack, correct?

Yes, correct.

How would one make fixed size array to go on stack? What if I'm using stucts I defined?

The stackalloc keyword serves this purpose. However, this works in unsafe context only, which is a rather unnecessarily limiting factor in most scenarios, not worth the performance tradeoff.

Example:

public void unsafe foo()
{
    int* bar = stackalloc int [10];
}

You will have to use pointer arithmetic to access members of the array.

What if I want array size passed as parameter to function? As far as I understand there should be no problem to get array of arbitrary size on stack when function is called, if size is known when function is called.

Works as expected:

public void unsafe foo(int length)
{
    int* bar = stackalloc int [length];
}

Should I even be bothered by this? As far as I understand getting this fixed size array on stack would improve performance, because no heap allocation is done.

No, in general not. Unless you deal with some very specific performance-critical scenarios, like heavy math computations, encryption, compression etc., it brings no real benefit.

Also, see this question for performance-related discussion.

Upvotes: 35

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