避免在异步方法中使用 Span<T>.ToArray() ？

Question

将部分代码升级为异步，并意识到

Span<T>

不能在

async

方法中使用。

调用

Span<T>.ToArray()

可以解决问题，但代价是每次都进行分配。

您能否建议一种在这种情况下如何避免分配的模式？

using ISO9660.Logical;

namespace ISO9660.Physical;

public static class DiscExtensions
{
    public static async Task ReadFileRawAsync(this Disc disc, IsoFileSystemEntryFile file, Stream stream)
    {
        await ReadFileAsync(disc, file, stream, ReadFileRaw);
    }

    public static async Task ReadFileUserAsync(this Disc disc, IsoFileSystemEntryFile file, Stream stream)
    {
        await ReadFileAsync(disc, file, stream, ReadFileUser);
    }

    private static async Task ReadFileAsync(Disc disc, IsoFileSystemEntryFile file, Stream stream, ReadFileHandler handler)
    {
        int position = (int)file.Position;

        Track track = disc.Tracks.FirstOrDefault(s => position >= s.Position)
                      ?? throw new InvalidOperationException("Failed to determine track for file.");

        int sectors = (int)Math.Ceiling((double)file.Length / track.Sector.GetUserDataLength());

        for (int i = position; i < position + sectors; i++)
        {
            ISector sector = await track.ReadSectorAsync(i);

            byte[] array = handler(file, stream, sector).ToArray(); // sucks

            await stream.WriteAsync(array);
        }
    }

    private static Span<byte> ReadFileRaw(IsoFileSystemEntryFile file, Stream stream, ISector sector)
    {
        Span<byte> data = sector.GetData();

        int size = data.Length;

        Span<byte> span = data[..size];

        return span;
    }

    private static Span<byte> ReadFileUser(IsoFileSystemEntryFile file, Stream stream, ISector sector)
    {
        Span<byte> data = sector.GetUserData();

        int size = (int)Math.Min(Math.Max(file.Length - stream.Length, 0), data.Length);

        Span<byte> span = data[..size];

        return span;
    }

    private delegate Span<byte> ReadFileHandler(IsoFileSystemEntryFile file, Stream stream, ISector sector);
}

ISector接口：

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.Marshalling;

namespace ISO9660.Physical;

public interface ISector
{
    int Length { get; }

    Span<byte> GetData();

    Span<byte> GetUserData();

    int GetUserDataLength();

    public static Span<byte> GetSpan<T>(scoped ref T sector, int start, int length)
        where T : struct, ISector
    {
        var span = MemoryMarshal.CreateSpan(ref sector, 1);

        var bytes = MemoryMarshal.AsBytes(span);

        var slice = bytes.Slice(start, length);

        return slice;
    }
    
    public static ISector Read<T>(Stream stream)
        where T : struct, ISector
    {
        var size = Unsafe.SizeOf<T>();

        Span<byte> span = stackalloc byte[size];

        stream.ReadExactly(span);

        var read = MemoryMarshal.Read<T>(span);

        return read;
    }

    public static async Task<ISector> ReadAsync<T>(Stream stream)
        where T : struct, ISector
    {
        var buffer = new byte[Unsafe.SizeOf<T>()];

        await stream.ReadExactlyAsync(buffer);

        var sector = MemoryMarshal.Read<T>(buffer);

        return sector;
    }
}

ISector 实现示例：

using JetBrains.Annotations;

namespace ISO9660.Physical;

public unsafe struct SectorRawMode1 : ISector
{
    private const int UserDataLength = 2048;

    private const int UserDataPosition = 16;

    [UsedImplicitly]
    public fixed byte Sync[12];

    [UsedImplicitly]
    public fixed byte Header[4];

    [UsedImplicitly]
    public fixed byte UserData[UserDataLength];

    [UsedImplicitly]
    public fixed byte Edc[4];

    [UsedImplicitly]
    public fixed byte Intermediate[8];

    [UsedImplicitly]
    public fixed byte PParity[172];

    [UsedImplicitly]
    public fixed byte QParity[104];

    public readonly int Length => 2352;

    public Span<byte> GetData()
    {
        return ISector.GetSpan(ref this, 0, Length);
    }

    public Span<byte> GetUserData()
    {
        return ISector.GetSpan(ref this, UserDataPosition, UserDataLength);
    }

    public readonly int GetUserDataLength()
    {
        return UserDataLength;
    }
}

Answer 1

将

Span<T>

更改为

Memory<T>

并将更改向上推。

Memory<T>

类似于异步的

Span<T>

（实际上）。这个想法与

Span

不同，

Memory

可以存在于堆上（因此不能引用

stackalloc

）。