C# 12 – Inline arrays

Inline arrays enable you to create an array of fixed size in a struct type. Such a struct, with an inline buffer, should provide performance comparable to an unsafe fixed size buffer.

Inline arrays are mosly to be used by the runtime team and some library authors to improve performance in certain scenarios. You likely won’t declare your own inline arrays, but you will use them transparently when they are exposed as Span<T> or ReadOnlySpan<T> objects by the runtime.

How to declare an inline array

You can declare an inline array by creating a struct and wrapping it with the InlineArray attribute, which takes in the array length as a parameter in the constructor.

[System.Runtime.CompilerServices.InlineArray(10)]
public struct MyInlineArray
{
    private int _element;
}
Code language: C# (cs)

Note: the name of the private member is irrelevant. You can use private int _abracadabra; if you wish. What matters is the type, as that decides the type of your array.

InlineArray usage

You can use an inline array similar to any other array, but with some small differences. Let’s take an example:

var arr = new MyInlineArray();

for (int i = 0; i < 10; i++)
{
    arr[i] = i;
}

foreach (var item in arr)
{
    Console.WriteLine(item);
}
Code language: C# (cs)

First thing to note is that during the initialization, we do not specify the size. Inline arrays are fixed size and their length is defined through the InlineArray attribute that’s applied to the struct. Besides that, everything looks as it would if you were using a normal array, but there’s actually more.

InlineArray doesn’t have a Length property

Some of you might have noticed that in the for loop above we iterated until 10 instead of arr.Length – and that is because inline arrays don’t have a Length property exposed like notmal arrays do.

It gets even weirder…

InlineArray does not implement IEnumerable

And as a result, you cannot call GetEnumerator on an inline array. The main drawback of this is that you cannot use LINQ on inline arrays – at least not at the moment, this is something which might change in the future.

Despite not implementing IEnumerable, you can still use them inside a foreach loop.

foreach (var item in arr) { }
Code language: C# (cs)

In a similar fashion, you can also use the spread operator in combination with inline arrays.

int[] m = [1, 2, 3, ..arr];
Code language: C# (cs)