C# 使用 Memory 或 ArraySegment 访问非托管数组？

Question

随着 C# 7.2 中 Memory、Span 和 ArraySegment 的引入，我想知道是否可以将非托管数组表示为可枚举数组对象，存在于堆中。

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[]，也许有一些方法可以欺骗 C# 传递 byte* 而不是 byte[]？

Answer 1

是 Memory<T> , 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) { }
}

现在您可以使用:

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 - 然后使用 .Slice(...)在 .Memory 上代表整个地区。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，此实现假设您要控制其他地方的内存生命周期 - Dispose()这里不试图通过Marshal释放内存等