Allow sizeof on variables

[thatbakamono]

As described in the title, currently something like this is not possible:

var numbers = new int[] 
{
	1, 2, 3, 4, 5, 6, 7, 8, 9
};

Console.WriteLine(sizeof(numbers));

It's not a big deal with simple arrays because you can solve it like this:

var numbers = new int[] 
{
	1, 2, 3, 4, 5, 6, 7, 8, 9
};

Console.WriteLine(sizeof(int) * numbers.Length);

but it gets a little bit more complicated with stackalloc because you have to store the size somewhere or have magic numbers floating randomly in your code

var numbers = stackalloc int[]
{
	1, 2, 3, 4, 5, 6, 7, 8, 9
};

It would be very helpful, especially when working with unmanaged code which often expects size.

[bernd5]

In the first example, sizeof(members) is not allowed because it is implementation defined. The array does not store only the integers itself, it stores in addition a virtual method table, a length and maybe something more.

But for stackallow, the result is no array. It is a pointer or a Span in new code. You can get the size now via numbers.Length (Link).

(Span is a struct which combines a pointer with a length).

[thatbakamono]

The proposal isn't about the count of elements of an array (or a stack allocated array), it's about the size that all elements take up in the memory (length * sizeof(T)). I don't think things like length or virtual method table should be counted by the proposed sizeof because they are internals of the language and they are implementation specific.

[CyrusNajmabadi]

Can you clarify when/how this is helpful?

[thatbakamono]

It's mainly useful for working with unmanaged code, I'm not aware of any other use cases.

[bernd5]

With Span of T, you don't need a second variable to store the stack allocated array length. So you can write code similar to heap allocated arrays:

using System;

Span<int> numbers = stackalloc int[]
{
	1, 2, 3, 4, 5, 6, 7, 8, 9
};
Console.WriteLine(numbers.Length * sizeof(int));

[thatbakamono]

The entire point of the proposal is to do sizeof(sth) instead of length * sizeof(T)

[bernd5]

Yes, I know. But in my eyes sizeof(sth) could mean different things. It could mean:

• size of the variable itself (for array = sizeof(void*))
• size of user data (like you proposed)
• size of implementation definied memory requirements (including VMT...)
• size after Marshalling

In addition sizeof should always return a constant value - which would be not the case with your proposal.

[thatbakamono]

Yeah, you are 100% right. It's not constant and a little bit confusing because it could mean 4 different things. Got any idea how it could be solved?

[theunrepentantgeek]

... it could mean 4 different things. Got any idea how it could be solved?

If there isn't a clearly obvious and correct meaning for sizeof then it's probably not a good idea to introduce the feature.