Csharp 创建临时Collection 最有效的方法是什么堆栈分配和集合表达式之间的区别

我真的不太习惯看IL.

你不需要(在这种情况下)，看看C# decompilation.

以下是实施细节，可能会更改.

反编译过程如下所示:

<>y__InlineArray2<int> buffer = default(<>y__InlineArray2<int>);
<PrivateImplementationDetails>.InlineArrayElementRef<<>y__InlineArray2<int>, int>(ref buffer, 0) = 1;
<PrivateImplementationDetails>.InlineArrayElementRef<<>y__InlineArray2<int>, int>(ref buffer, 1) = 2;
Span<int> span = <PrivateImplementationDetails>.InlineArrayAsSpan<<>y__InlineArray2<int>, int>(ref buffer, 2);

[CompilerGenerated]
internal sealed class <PrivateImplementationDetails>
{
    internal static Span<TElement> InlineArrayAsSpan<TBuffer, TElement>(ref TBuffer buffer, int length)
    {
        return MemoryMarshal.CreateSpan(ref Unsafe.As<TBuffer, TElement>(ref buffer), length);
    }

    internal static ref TElement InlineArrayElementRef<TBuffer, TElement>(ref TBuffer buffer, int index)
    {
        return ref Unsafe.Add(ref Unsafe.As<TBuffer, TElement>(ref buffer), index);
    }
}

[StructLayout(LayoutKind.Auto)]
[InlineArray(2)]
internal struct <>y__InlineArray2<T>
{
    [CompilerGenerated]
    private T _element0;
}

这里主要感兴趣的是用InlineArrayAttribute标记的生成的<>y__InlineArray2<T>.这是C#12-inline arrays引入的新功能.

Inline arrays are used by the runtime team and other library authors to improve performance in your apps. Inline arrays enable a developer to create an array of fixed size in a struct type. A struct with an inline buffer should provide performance characteristics similar to an unsafe fixed size buffer.
You likely won't declare your own inline arrays, but you use them transparently when they're exposed as System.Span<T> or `System.ReadOnlySpan objects from runtime APIs.

基本上，它是在堆栈上分配的特殊类型，不应该导致额外的堆分配.

此外，当谈论性能时，最重要的是实际测量它.例如，我已经做了下一个小的合成基准测试(稍后将try 判断它有多大意义=):

[MemoryDiagnoser]
public class CollectionBench
{
    [Benchmark]
    public int NewArray()
    {
        Span<int> array = new int[]{ 1, 2 };

        return  Math.Max(array[0], array[1]);
    }
    
    [Benchmark]
    public unsafe int Stackalloc()
    {
        Span<int> stack = stackalloc int[2];
        stack[0] = 1;
        stack[1] = 2;

        return  Math.Max(stack[0], stack[1]);
    }
    
    [Benchmark]
    public int CollectionExpr()
    {
        Span<int> span = [1, 2];

        return Math.Max(span[0], span[1]);
    }
}

这给出了《On My Machine》(via BenchmarkDotNet):

正如您所看到的，没有为集合表达式进行额外的分配.但一定要根据您的实际代码/数据/硬件进行测量.