如果去除一段 C# 代码中的 unsafe fixed

Question

下面是 EnyimMemcachedCore 中的一段代码，由于进行了指针操作，所以用到了 unsafe fixed ，请问有没有办法可以去掉 unsafe fixed ？

public unsafe IList<ArraySegment<byte>> CreateBuffer(IList<ArraySegment<byte>> appendTo)
{
    // key size 
    byte[] keyData = BinaryConverter.EncodeKey(this.Key);
    int keyLength = keyData == null ? 0 : keyData.Length;

    if (keyLength > 0xffff) throw new InvalidOperationException("KeyTooLong");

    // extra size
    ArraySegment<byte> extras = this.Extra;
    int extraLength = extras.Array == null ? 0 : extras.Count;
    if (extraLength > 0xff) throw new InvalidOperationException("ExtraTooLong");

    // body size
    ArraySegment<byte> body = this.Data;
    int bodyLength = body.Array == null ? 0 : body.Count;

    // total payload size
    int totalLength = extraLength + keyLength + bodyLength;

    //build the header
    byte[] header = new byte[24];

    fixed (byte* buffer = header)
    {
        buffer[0x00] = 0x80; // magic
        buffer[0x01] = this.Operation;

        // key length
        buffer[0x02] = (byte)(keyLength >> 8);
        buffer[0x03] = (byte)(keyLength & 255);

        // extra length
        buffer[0x04] = (byte)(extraLength);

        // 5 -- data type, 0 (RAW)
        // 6,7 -- reserved, always 0

        buffer[0x06] = (byte)(this.Reserved >> 8);
        buffer[0x07] = (byte)(this.Reserved & 255);

        // body length
        buffer[0x08] = (byte)(totalLength >> 24);
        buffer[0x09] = (byte)(totalLength >> 16);
        buffer[0x0a] = (byte)(totalLength >> 8);
        buffer[0x0b] = (byte)(totalLength & 255);

        buffer[0x0c] = (byte)(this.CorrelationId >> 24);
        buffer[0x0d] = (byte)(this.CorrelationId >> 16);
        buffer[0x0e] = (byte)(this.CorrelationId >> 8);
        buffer[0x0f] = (byte)(this.CorrelationId & 255);

        ulong cas = this.Cas;
        // CAS
        if (cas > 0)
        {
            // skip this if no cas is specfied
            buffer[0x10] = (byte)(cas >> 56);
            buffer[0x11] = (byte)(cas >> 48);
            buffer[0x12] = (byte)(cas >> 40);
            buffer[0x13] = (byte)(cas >> 32);
            buffer[0x14] = (byte)(cas >> 24);
            buffer[0x15] = (byte)(cas >> 16);
            buffer[0x16] = (byte)(cas >> 8);
            buffer[0x17] = (byte)(cas & 255);
        }
    }

    var retval = appendTo ?? new List<ArraySegment<byte>>(4);

    retval.Add(new ArraySegment<byte>(header));

    if (extraLength > 0) retval.Add(extras);

    // NOTE key must be already encoded and should not contain any invalid characters which are not allowed by the protocol
    if (keyLength > 0) retval.Add(new ArraySegment<byte>(keyData));
    if (bodyLength > 0) retval.Add(body);

    return retval;
}

Answer 1

后来想到，这不正是 Span<T> 与 stackalloc 的用武之地吗，于是进行了这样的修改： 1）去掉 unsafe 2）将 fixed (byte* buffer = header) 改为 Span<byte> header = stackalloc byte[24]; 3）将 buffer 变量名都改为 header 4）将 retval.Add(new ArraySegment<byte>(header)); 改为 `retval.Add(new ArraySegment<byte>(header.ToArray())); 这样修改后，持续集成通过

Answer 2

也没其他操作，直接去掉就行了。

Answer 3

@dudu: 去掉 buffer，直接给header赋值操作就行了。无非慢点点而已。

Answer 4

@花飘水流兮: 这地方性能要求很高

Answer 5

@花飘水流兮: 这个提问本来就是为了解决如何兼得鱼和熊掌

Answer 6

为什么要去掉呢。必要的时候 指针操作的高性能是普通的操作难以企及的。

Answer 7

去掉的只是是 unsafe fixed ，没有去掉指针