Understanding the return value of MemoryMarshal.Cast

Question

I'm trying to understand the new C# type Span<T>

When reading the Microsoft's article All About Span: Exploring a New .NET Mainstay I saw at the bottom of the What Is Span<T>? 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<byte> to be a Span<int> (where the 0th index into the Span<int> maps to the first four bytes of the Span<byte>). 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<byte> byteSpan = byteArray;
Span<int> intSpan = MemoryMarshal.Cast<byte, int>(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?

Answer 1

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.