理解MemoryMarshal.Cast的返回值

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英文:

Understanding the return value of MemoryMarshal.Cast

问题

我正在尝试理解新的C#类型 Span<T>

在阅读Microsoft的文章 All About Span: Exploring a New .NET Mainstay 时,我看到在 What Is Span<T>? 部分的底部有以下内容:

"Spans 提供了许多超出已经提到的好处。例如,spans 支持重新解释转换的概念,这意味着您可以将 Span<byte> 强制转换为 Span<int>(其中 Span<int> 的第0个索引映射到 Span<byte> 的前四个字节)。这样,如果您读取了一个字节缓冲区,您可以将其传递给安全高效地操作分组字节作为整数的方法。"

在这篇文章中没有提供示例,但我最终在 Adam Storr的文章 中找到了这种新的方式,通过使用 MemoryMarshal.Cast 方法来实现。

然而,当我尝试这样做时,我得到了一个奇怪的结果:

var byteArray = new byte[] { 1,0,0,0, 0,1,0,0};
Span<byte> byteSpan = byteArray;
Span<int> intSpan = MemoryMarshal.Cast<byte, int>(byteSpan);

Microsoft的文章说 "Span的第0个索引映射到Span的前四个字节"。所以,通过创建一个包含8个字节的数组,我得到了一个包含2个整数的Span。

第一个整数的值是 1,这是我预期的值(0001),但第二个整数的值是 256,这是我不理解的地方。

难道我不应该得到值 2 吗,因为我的字节数组的第二半部分是 0010 吗?

英文:

I'm trying to understand the new C# type Span&lt;T&gt;

When reading the Microsoft's article All About Span: Exploring a New .NET Mainstay I saw at the bottom of the What Is Span&lt;T&gt;? section

> Spans provide a multitude of benefits beyond those already mentioned. For example, spans support the notion of reinterpret casts, meaning you can cast a Span&lt;byte&gt; to be a Span&lt;int&gt; (where the 0th index into the Span&lt;int&gt; maps to the first four bytes of the Span&lt;byte&gt;). That way if you read a buffer of bytes, you can pass it off to methods that operate on grouped bytes as ints safely and efficiently.

They don't provide any example in this article but I eventually found the (new way) of doing this in Adam Storr's article by using the MemoryMarshal.Cast method

However when I try to do this, I get a weird result

var byteArray = new byte[] { 1,0,0,0, 0,1,0,0};
Span&lt;byte&gt; byteSpan = byteArray;
Span&lt;int&gt; intSpan = MemoryMarshal.Cast&lt;byte, int&gt;(byteSpan);

Microsoft's articles says the 0th index into the Span maps to the first four bytes of the Span. So by creating an array of 8 bytes I get a Span of 2 integers.

The value of the first integer is 1 which is what I was expecting (0001) but the for 2nd integer I get the value 256 which is what I don't understand.

Am I not supposed to get the value 2 because the 2nd half of my bytes array is 0010?

答案1

得分: 6

1
(可以用十六进制表示为):

0x00000001
0x0001
0x01

具有大端字节模式:

0x00 0x00 0x00 0x01

和小端字节模式:

0x01 0x00 0x00 0x00


256
(可以用十六进制表示为):

0x00000100
0x0100

具有大端字节模式:

0x00 0x00 0x01 0x00

和小端字节模式:

0x00 0x01 0x00 0x00


由于您的机器是小端字节序,因此MemoryMarshal.Cast使用小端表示数字。因此,1 0 0 0 被解释为 1,而 0 1 0 0 被解释为 256

英文:

The number:

1

(which can be written in hex as):

0x00000001
0x0001
0x01

Has the big-endian byte pattern:

0x00 0x00 0x00 0x01

And the little-endian byte pattern:

0x01 0x00 0x00 0x00

The number:

256

(which can be written in hex as):

0x00000100
0x0100

Has the big-endian byte pattern:

0x00 0x00 0x01 0x00

And the little-endian byte pattern:

0x00 0x01 0x00 0x00

Since your machine is little-endian, the MemoryMarshal.Cast is using the little-endian representations of numbers. Therefore 1 0 0 0 gets interpreted as 1, and 0 1 0 0 gets interpreted as 256.

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  • 本文由 发表于 2020年1月3日 19:02:26
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