我该如何准确地读取n从字节流？ [英] How do I read exactly n bytes from a stream?

问题描述

这是一个有点棘手，比我想象中的第一。我试图从流中读取n个字节。

This is a little more tricky than I first imagined. I'm trying to read n bytes from a stream.

借助声称的读不必返回n个字节，它只是必须返回至少1和最多n个字节，0字节是到达流的端部的特殊情况。

The MSDN claims that Read does not have to return n bytes, it just must return at least 1 and up to n bytes, with 0 bytes being the special case of reaching the end of the stream.

通常情况下，我使用的是类似

Typically, I'm using something like

var buf = new byte[size];
var count = stream.Read (buf, 0, size);

if (count != size) {
    buf = buf.Take (count).ToArray ();
}

yield return buf;

我希望的正是尺寸字节，但通过规范的FileStream将被允许返回了大量的1字节的块为好。这必须加以避免。

I'm hoping for exactly size bytes but by spec FileStream would be allowed to return a large number of 1-byte chunks as well. This must be avoided.

要解决，这将是有2缓冲器，一个用于读出和一个用于收集块，直到我们得到请求的字节数的一种方式。这是一个有点麻烦，但。

One way to solve this would be to have 2 buffers, one for reading and one for collecting the chunks until we got the requested number of bytes. That's a little cumbersome though.

我也看了一下 BinaryReader在，但其规格也做没有明确指出n个字节将肯定返回。

I also had a look at BinaryReader but its spec also does not clearly state that n bytes will be returned for sure.

要澄清：当然，在数据流的末尾字节，返回的数字可能小于尺寸 - 这不是一个问题。我只谈论没有接获任何即使他们是在流中可用的字节。

To clarify: Of course, upon the end of the stream the returned number of bytes may be less than size - that's not a problem. I'm only talking about not receiving n bytes even though they are available in the stream.

推荐答案

一个稍微更可读的版本：

A slightly more readable version:

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

或书面作为流类的扩展方法：

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;
    }
}

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