如何从流中读取n个字节

Question

这比我最初想象的要棘手一些。我正在尝试从流中读取n个字节。

MSDN声明Read不必返回n个字节，它必须返回至少1到最多n个字节，其中0个字节是到达流结束的特殊情况。

一般来说，我用的是

var buf = new byte[size];
var count = stream.Read (buf, 0, size);
if (count != size) {
    buf = buf.Take (count).ToArray ();
}
yield return buf;

我希望确切的size字节，但通过规范FileStream将被允许返回大量的1字节块为好。这必须避免。

解决这个问题的一种方法是有两个缓冲区，一个用于读取，一个用于收集块，直到我们得到请求的字节数。不过这有点麻烦。

我也看了一下BinaryReader，但它的规范也没有明确说明n字节将肯定返回。

澄清一下:当然，在流结束时返回的字节数可能小于size -这不是问题。我只是在说没有接收到n个字节，即使它们在流中是可用的。

Answer 1

更易于阅读的版本:

int offset = 0;
while (offset < count)
{
    int read = stream.Read(buffer, offset, count - offset);
    if (read == 0)
        throw new System.IO.EndOfStreamException();
    offset += read;
}

或者写成Stream类的扩展方法:

public static class StreamUtils
{
    public static byte[] ReadExactly(this System.IO.Stream stream, int count)
    {
        byte[] buffer = new byte[count];
        int offset = 0;
        while (offset < count)
        {
            int read = stream.Read(buffer, offset, count - offset);
            if (read == 0)
                throw new System.IO.EndOfStreamException();
            offset += read;
        }
        System.Diagnostics.Debug.Assert(offset == count);
        return buffer;
    }
}

Answer 2

简简单单;你循环;

int read, offset = 0;
while(leftToRead > 0 && (read = stream.Read(buf, offset, leftToRead)) > 0) {
    leftToRead -= read;
    offset += read;
}
if(leftToRead > 0) throw new EndOfStreamException(); // not enough!

在此之后，buf应该已经从流中填充了正确数量的数据，或者将抛出EOF。

Answer 3

从这里的答案中收集了所有内容，我提出了以下解决方案。它依赖于源流的长度。工作在。net core 3.1

/// <summary>
/// Copy stream based on source stream length
/// </summary>
/// <param name="source"></param>
/// <param name="destination"></param>
/// <param name="bufferSize">
/// A value that is the largest multiple of 4096 and is still smaller than the LOH threshold (85K).
/// So the buffer is likely to be collected at Gen0, and it offers a significant improvement in Copy performance.
/// </param>
/// <returns></returns>
private async Task CopyStream(Stream source, Stream destination, int bufferSize = 81920)
{
    var buffer = new byte[bufferSize];
    var offset = 0;
    while (offset < source.Length)
    {
        var leftToRead = source.Length - offset;
        var lengthToRead = leftToRead - buffer.Length < 0 ? (int)(leftToRead) : buffer.Length;
        var read = await source.ReadAsync(buffer, 0, lengthToRead).ConfigureAwait(false);
        if (read == 0)
            break;
        await destination.WriteAsync(buffer, 0, lengthToRead).ConfigureAwait(false);
        offset += read;
    }
    destination.Seek(0, SeekOrigin.Begin);
}