C# 使用 Memory<T> 或 ArraySegment<T> 访问非托管数组？

Question

随着在 C# 7.2 中引入 Memory、Span 和 ArraySegment，我想知道是否可以将非托管数组表示为可枚举的对象，它存在于堆上。

后一个要求排除了Span，它基本上实现了我想要的：例如

unsafe { bytes = new Span<byte>((byte*)ptr + (index * Width), Width); 

是否可以对ArraySegment 或Memory 做同样的事情？他们的构造函数只接受byte[]，也许有办法欺骗C# 传递byte* 而不是byte[]？

Answer 1

Memory<T> 可以，但您需要创建自己的 MemoryManager<T>。别担心 - 这并不像听起来那么可怕 - here's one I wrote earlier...:

/// <summary>
/// A MemoryManager over a raw pointer
/// </summary>
/// <remarks>The pointer is assumed to be fully unmanaged, or externally pinned - no attempt will be made to pin this data</remarks>
public sealed unsafe class UnmanagedMemoryManager<T> : MemoryManager<T>
    where T : unmanaged
{
    private readonly T* _pointer;
    private readonly int _length;

    /// <summary>
    /// Create a new UnmanagedMemoryManager instance at the given pointer and size
    /// </summary>
    /// <remarks>It is assumed that the span provided is already unmanaged or externally pinned</remarks>
    public UnmanagedMemoryManager(Span<T> span)
    {
        fixed (T* ptr = &MemoryMarshal.GetReference(span))
        {
            _pointer = ptr;
            _length = span.Length;
        }
    }
    /// <summary>
    /// Create a new UnmanagedMemoryManager instance at the given pointer and size
    /// </summary>
    public UnmanagedMemoryManager(T* pointer, int length)
    {
        if (length < 0) throw new ArgumentOutOfRangeException(nameof(length));
        _pointer = pointer;
        _length = length;
    }
    /// <summary>
    /// Obtains a span that represents the region
    /// </summary>
    public override Span<T> GetSpan() => new Span<T>(_pointer, _length);

    /// <summary>
    /// Provides access to a pointer that represents the data (note: no actual pin occurs)
    /// </summary>
    public override MemoryHandle Pin(int elementIndex = 0)
    {
        if (elementIndex < 0 || elementIndex >= _length)
            throw new ArgumentOutOfRangeException(nameof(elementIndex));
        return new MemoryHandle(_pointer + elementIndex);
    }
    /// <summary>
    /// Has no effect
    /// </summary>
    public override void Unpin() { }

    /// <summary>
    /// Releases all resources associated with this object
    /// </summary>
    protected override void Dispose(bool disposing) { }
}

现在你可以使用了：

var mgr = new UnmanagedMemoryManager((byte*)ptr + (index * Width), Width);
Memory<byte> memory = mgr.Memory;

和memory 可以存储在堆上。

但是，为了尽量减少分配，您可能希望创建一个单个 UnmanagedMemoryManager<byte> 覆盖整个 区域 - 仅一次 - 然后在 @ 上使用 .Slice(...) 987654329@ 代表整个区域。这样你就有了一个对象和很多切片（切片是结构，而不是对象）。

请注意，此实现假定您将在其他地方控制内存的生命周期 - 这里的 Dispose() 不会尝试通过 Marshal 等释放内存。