C#：实现 NetworkStream.Peek？

Question

目前，C# 中没有 NetworkStream.Peek 方法。实现这种方法的最佳方法是什么，该方法的功能与 NetworkStream.ReadByte 类似，只是返回的字节实际上并未从 Stream 中删除？

Answer 1

我遇到了同样的“查看幻数，然后决定将流发送到哪个流处理器”的要求，不幸的是，我无法通过传递已经消耗的字节来解决这个问题 - 正如对 Aaronaught 答案的评论中所建议的那样在单独的参数中进入流处理方法，因为这些方法是给定的，它们期望 System.IO.Stream 而不是其他任何东西。

我通过创建一个或多或少通用的包装 Stream 的 PeekableStream 类来解决这个问题。它适用于 NetworkStreams，也适用于任何其他 Stream，前提是您使用 Stream.CanRead。

---

编辑

或者，您可以使用全新的 ReadSeekableStream 并执行

var readSeekableStream = new ReadSeekableStream(networkStream, /* >= */ count);
...
readSeekableStream.Read(..., count);
readSeekableStream.Seek(-count, SeekOrigin.Current);

---

无论如何，PeekableStream 来了：

/// <summary>
/// PeekableStream wraps a Stream and can be used to peek ahead in the underlying stream,
/// without consuming the bytes. In other words, doing Peek() will allow you to look ahead in the stream,
/// but it won't affect the result of subsequent Read() calls.
/// 
/// This is sometimes necessary, e.g. for peeking at the magic number of a stream of bytes and decide which
/// stream processor to hand over the stream.
/// </summary>
public class PeekableStream : Stream
{
    private readonly Stream underlyingStream;
    private readonly byte[] lookAheadBuffer;

    private int lookAheadIndex;

    public PeekableStream(Stream underlyingStream, int maxPeekBytes)
    {
        this.underlyingStream = underlyingStream;
        lookAheadBuffer = new byte[maxPeekBytes];
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
            underlyingStream.Dispose();

        base.Dispose(disposing);
    }

    /// <summary>
    /// Peeks at a maximum of count bytes, or less if the stream ends before that number of bytes can be read.
    /// 
    /// Calls to this method do not influence subsequent calls to Read() and Peek().
    /// 
    /// Please note that this method will always peek count bytes unless the end of the stream is reached before that - in contrast to the Read()
    /// method, which might read less than count bytes, even though the end of the stream has not been reached.
    /// </summary>
    /// <param name="buffer">An array of bytes. When this method returns, the buffer contains the specified byte array with the values between offset and
    /// (offset + number-of-peeked-bytes - 1) replaced by the bytes peeked from the current source.</param>
    /// <param name="offset">The zero-based byte offset in buffer at which to begin storing the data peeked from the current stream.</param>
    /// <param name="count">The maximum number of bytes to be peeked from the current stream.</param>
    /// <returns>The total number of bytes peeked into the buffer. If it is less than the number of bytes requested then the end of the stream has been reached.</returns>
    public virtual int Peek(byte[] buffer, int offset, int count)
    {
        if (count > lookAheadBuffer.Length)
            throw new ArgumentOutOfRangeException("count", "must be smaller than peekable size, which is " + lookAheadBuffer.Length);

        while (lookAheadIndex < count)
        {
            int bytesRead = underlyingStream.Read(lookAheadBuffer, lookAheadIndex, count - lookAheadIndex);

            if (bytesRead == 0) // end of stream reached
                break;

            lookAheadIndex += bytesRead;
        }

        int peeked = Math.Min(count, lookAheadIndex);
        Array.Copy(lookAheadBuffer, 0, buffer, offset, peeked);
        return peeked;
    }

    public override bool CanRead { get { return true; } }

    public override long Position
    {
        get
        {
            return underlyingStream.Position - lookAheadIndex;
        }
        set
        {
            underlyingStream.Position = value;
            lookAheadIndex = 0; // this needs to be done AFTER the call to underlyingStream.Position, as that might throw NotSupportedException, 
                                // in which case we don't want to change the lookAhead status
        }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        int bytesTakenFromLookAheadBuffer = 0;
        if (count > 0 && lookAheadIndex > 0)
        {
            bytesTakenFromLookAheadBuffer = Math.Min(count, lookAheadIndex);
            Array.Copy(lookAheadBuffer, 0, buffer, offset, bytesTakenFromLookAheadBuffer);
            count -= bytesTakenFromLookAheadBuffer;
            offset += bytesTakenFromLookAheadBuffer;
            lookAheadIndex -= bytesTakenFromLookAheadBuffer;
            if (lookAheadIndex > 0) // move remaining bytes in lookAheadBuffer to front
                // copying into same array should be fine, according to http://msdn.microsoft.com/en-us/library/z50k9bft(v=VS.90).aspx :
                // "If sourceArray and destinationArray overlap, this method behaves as if the original values of sourceArray were preserved
                // in a temporary location before destinationArray is overwritten."
                Array.Copy(lookAheadBuffer, lookAheadBuffer.Length - bytesTakenFromLookAheadBuffer + 1, lookAheadBuffer, 0, lookAheadIndex);
        }

        return count > 0
            ? bytesTakenFromLookAheadBuffer + underlyingStream.Read(buffer, offset, count)
            : bytesTakenFromLookAheadBuffer;
    }

    public override int ReadByte()
    {
        if (lookAheadIndex > 0)
        {
            lookAheadIndex--;
            byte firstByte = lookAheadBuffer[0];
            if (lookAheadIndex > 0) // move remaining bytes in lookAheadBuffer to front
                Array.Copy(lookAheadBuffer, 1, lookAheadBuffer, 0, lookAheadIndex);
            return firstByte;
        }
        else
        {
            return underlyingStream.ReadByte();
        }
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        long ret = underlyingStream.Seek(offset, origin);
        lookAheadIndex = 0; // this needs to be done AFTER the call to underlyingStream.Seek(), as that might throw NotSupportedException,
                            // in which case we don't want to change the lookAhead status
        return ret;
    }

    // from here on, only simple delegations to underlyingStream

    public override bool CanSeek { get { return underlyingStream.CanSeek; } }
    public override bool CanWrite { get { return underlyingStream.CanWrite; } }
    public override bool CanTimeout { get { return underlyingStream.CanTimeout; } }
    public override int ReadTimeout { get { return underlyingStream.ReadTimeout; } set { underlyingStream.ReadTimeout = value; } }
    public override int WriteTimeout { get { return underlyingStream.WriteTimeout; } set { underlyingStream.WriteTimeout = value; } }
    public override void Flush() { underlyingStream.Flush(); }
    public override long Length { get { return underlyingStream.Length; } }
    public override void SetLength(long value) { underlyingStream.SetLength(value); }
    public override void Write(byte[] buffer, int offset, int count) { underlyingStream.Write(buffer, offset, count); }
    public override void WriteByte(byte value) { underlyingStream.WriteByte(value); }
}

Answer 2

如果您不需要实际检索字节，可以参考 DataAvailable 属性。

否则，您可以使用 StreamReader并调用其 Peek 方法。

请注意，由于延迟问题，这两种方法对于从网络流中读取数据都不是特别可靠。数据可能在您查看之后立即变得可用（存在于读取缓冲区中）。

我不确定您打算用它做什么，但是 NetworkStream 上的 Read 方法是一个阻塞调用，因此您实际上不需要检查状态，即使您收到的是大块。如果您试图在从流读取时保持应用程序响应，则应使用线程或异步调用来接收数据。

编辑：根据这篇文章，StreamReader.Peek是NetworkStream 上存在错误，或者至少具有未记录的行为，因此如果您选择走这条路，请务必小心。

---

已更新 - 对评论的回复

“查看”实际流本身的概念实际上是不可能的；它只是一个流，一旦接收到字节，它就不再在流上。有些流支持查找，因此您可以从技术上重新读取该字节，但 NetworkStream 不是其中之一。

窥视仅适用于将流读入缓冲区时；一旦数据位于缓冲区中，那么查看就很容易，因为您只需检查缓冲区中当前位置的任何内容。这就是为什么 StreamReader 能够做到这一点；通常，Stream 类不会有自己的 Peek 方法。

现在，具体针对这个问题，我质疑这是否真的是正确的答案。我理解动态选择处理流的方法的想法，但是您真的需要在原始流上执行此操作吗？您不能先将流读入字节数组，甚至将其复制到 MemoryStream 中，然后从该点开始处理它吗？

我看到的主要问题是，如果当您从网络流中读取数据时发生问题，数据就会消失。但如果您先将其读入临时位置，则可以对其进行调试。您可以找出数据是什么以及尝试处理数据的对象中途失败的原因。

一般来说，您想要使用 NetworkStream 做的第一件事就是将其读入本地缓冲区。我能想到不这样做的唯一原因是，如果您正在读取大量数据 - 即使这样，如果内存不适合文件系统，我可能会考虑使用文件系统作为中间缓冲区。

我不知道您的确切要求，但从目前为止我所了解到的情况来看，我的建议是：不要尝试直接从 NetworkStream 处理数据，除非有令人信服的理由这样做。考虑首先将数据读入内存或磁盘，然后处理副本。

Answer 3

如果您有权访问 Socket 对象，则可以尝试 接收方法，传递SocketFlags.Peek。这类似于可以传递给 BSD 套接字或 Winsock 中的 recv 调用的 MSG_PEEK 标志。

Answer 4

这是一个非常简单的 PeekStream 实现，它允许您仅在流的开头查看一定数量的字节（而不是随时查看）。所查看的字节本身作为 Stream 返回，以最大限度地减少对现有代码的更改。

使用方法如下：

Stream nonSeekableStream = ...;
PeekStream peekStream = new PeekStream(nonSeekableStream, 30); // Peek max 30 bytes
Stream initialBytesStream = peekStream.GetInitialBytesStream();
ParseHeaders(initialBytesStream);  // Work on initial bytes of nonSeekableStream
peekStream.Read(...) // Read normally, the read will start from the beginning

GetInitialBytesStream() 返回一个可查找流，其中最多包含基础流的 peekSize 个初始字节（如果流短于 peekSize，则该字节数会更少） ）。

由于其简单性，读取 PeekStream 应该只比直接读取底层流稍微慢一点（如果有的话）。

public class PeekStream : Stream
{
    private Stream m_stream;
    private byte[] m_buffer;
    private int m_start;
    private int m_end;

    public PeekStream(Stream stream, int peekSize)
    {
        if (stream == null)
        {
            throw new ArgumentNullException("stream");
        }
        if (!stream.CanRead)
        {
            throw new ArgumentException("Stream is not readable.");
        }
        if (peekSize < 0)
        {
            throw new ArgumentOutOfRangeException("peekSize");
        }
        m_stream = stream;
        m_buffer = new byte[peekSize];
        m_end = stream.Read(m_buffer, 0, peekSize);
    }

    public override bool CanRead
    {
        get
        {
            return true;
        }
    }

    public override bool CanWrite
    {
        get
        {
            return false;
        }
    }

    public override bool CanSeek
    {
        get
        {
            return false;
        }
    }

    public override long Length
    {
        get
        {
            throw new NotSupportedException();
        }
    }

    public override long Position
    {
        get
        {
            throw new NotSupportedException();
        }
        set
        {
            throw new NotSupportedException();
        }
    }

    public MemoryStream GetInitialBytesStream()
    {
        return new MemoryStream(m_buffer, 0, m_end, false);
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        throw new NotSupportedException();
    }

    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        // Validate arguments
        if (buffer == null)
        {
            throw new ArgumentNullException("buffer");
        }
        if (offset < 0)
        {
            throw new ArgumentOutOfRangeException("offset");
        }
        if (offset + count > buffer.Length)
        {
            throw new ArgumentOutOfRangeException("count");
        }

        int totalRead = 0;

        // Read from buffer
        if (m_start < m_end)
        {
            int toRead = Math.Min(m_end - m_start, count);
            Array.Copy(m_buffer, m_start, buffer, offset, toRead);
            m_start += toRead;
            offset += toRead;
            count -= toRead;
            totalRead += toRead;
        }

        // Read from stream
        if (count > 0)
        {
            totalRead += m_stream.Read(buffer, offset, count);
        }

        // Return total bytes read
        return totalRead;
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        throw new NotImplementedException();
    }

    public override int ReadByte()
    {
        if (m_start < m_end)
        {
            return m_buffer[m_start++];
        }
        else
        {
            return m_stream.ReadByte();
        }
    }

    public override void Flush()
    {
        m_stream.Flush();
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            m_stream.Dispose();
        }
        base.Dispose(disposing);
    }
}

免责声明：上面的 PeekStream 取自一个工作程序，但没有经过全面测试，因此可能包含错误。它对我有用，但您可能会发现一些失败的极端情况。

Answer 5

FWIW，这是一个与不可查找流相比的可查看流，仅针对前面一个字节进行了优化：

public class OneBytePeekableStream : Stream
{
    private readonly bool _disposeStreamOnDispose;
    private readonly Stream _stream;
    private int _buffer; // byte or -1
    private int _bufferLength; // 0 or 1

    public OneBytePeekableStream(Stream stream, bool disposeStreamOnDispose)
    {
        if (stream == null)
            throw new ArgumentNullException(nameof(stream));
            
        _stream = stream;
        _disposeStreamOnDispose = disposeStreamOnDispose;
    }

    public override long Length => _stream.Length;
    public override bool CanRead => _stream.CanRead;
    public override bool CanSeek => _stream.CanSeek;
    public override bool CanWrite => _stream.CanWrite;
    public override bool CanTimeout => _stream.CanTimeout;
    public override int ReadTimeout { get => _stream.ReadTimeout; set => _stream.ReadTimeout = value; }
    public override int WriteTimeout { get => _stream.WriteTimeout; set => _stream.WriteTimeout = value; }
    public override long Position { get => _stream.Position - _bufferLength; set { _stream.Position = value; _bufferLength = 0; } }

    public override int Read(byte[] buffer, int offset, int count)
    {
        if (buffer == null)
            throw new ArgumentNullException(nameof(buffer));

        if (offset < 0)
            throw new ArgumentOutOfRangeException(nameof(offset));

        if (count < 0)
            throw new ArgumentOutOfRangeException(nameof(count));

        if (buffer.Length - offset < count)
            throw new ArgumentOutOfRangeException(nameof(count));

        if (count == 0)
            return 0;

        if (_bufferLength == 0)
            return _stream.Read(buffer, offset, count);

        if (_buffer < 0)
            return 0;

        _bufferLength = 0;
        buffer[offset] = (byte)_buffer;
        if (count == 1)
            return count;

        var read = _stream.Read(buffer, offset + 1, count - 1);
        return read + 1;
    }

    // this is the sole reason of this class
    // returns -1 is stream is EOF
    public virtual int PeekByte()
    {
        if (_bufferLength > 0)
            return _buffer;

        _buffer = _stream.ReadByte();
        _bufferLength = 1;
        return _buffer;
    }

    public override int ReadByte()
    {
        if (_bufferLength == 0)
            return _stream.ReadByte();

        if (_buffer < 0)
            return -1;

        _bufferLength = 0;
        return _buffer;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        var ret = _stream.Seek(offset, origin);
        _bufferLength = 0;
        return ret;
    }

    public override void Flush() => _stream.Flush();
    public override void SetLength(long value) => _stream.SetLength(value);
    public override void WriteByte(byte value) => _stream.WriteByte(value);
    public override void Write(byte[] buffer, int offset, int count) => _stream.Write(buffer, offset, count);

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            if (_disposeStreamOnDispose)
            {
                _stream.Dispose();
            }
        }

        base.Dispose(disposing);
    }
}