C# - How to get int ref to byte[]

Question

As the title says I need to get int reference into byte array

class ClassFoo {
    private byte[] _data;

    public ref byte GetByteRef(int index) {
        // ...Omitted for brevity
        return ref _data[index];
    }
    public ref int GetIntRef(int index) {
        // This won't work as it returns a integer value not reference
        return ref BitConverter.ToInt32(_data, index * 4);
    } 
}

I am positive that it is possible without switching into an unsafe context. And the answer is buried somewhere in documentation below.

But I failed to find it. Any help is much appreciated.

@JonathanWood That's the other way around, and also involves a copy. — Etienne de Martel
– Etienne de Martel, Commented Nov 20, 2023 at 0:21
You could just ref int myInt = ref Unsafe.As<byte, int>(ref myBytes[index]). Do ensure though that after myBytes[index] there is enough space in your array to load a full size integer from (don't do out of bounds reads). It's basically the managed equivalent of int* pMyInt = (int*)(pMyBytes + index). — Frederik Hoeft
– Frederik Hoeft, Commented Nov 20, 2023 at 0:36
@FrederikHoeft You shouldn't use Unsafe, and anyway you don't need it. See my answer. — Etienne de Martel
– Etienne de Martel, Commented Nov 20, 2023 at 0:38
@EtiennedeMartel "You shouldn't use Unsafe" - well, that depends on the use-case and the performance you want to achieve ;) MemoryMarshal.Cast is just a wrapper around Unsafe.As(). Given that the OP just wanted to read a single element it feels weird to Cast the whole array to an int span first (though the JIT will probably optimize that away), but you'd still have the overhead of all the checks. — Frederik Hoeft
– Frederik Hoeft, Commented Nov 20, 2023 at 0:45

Etienne de Martel · Accepted Answer · 2023-11-20 00:32:42Z

4

First, you can use MemoryMarshal.Cast to convert a byte span to an int span. Since arrays are implicitly convertible to spans, you can just plug the array in there:

var intSpan = MemoryMarshal.Cast<byte, int>(_data);

And then you get a ref to the span's index of your choice:

return ref intSpan[index];

Or, in one go:

return ref MemoryMarshal.Cast<byte, int>(_data)[index];

No pointer required.

edited Nov 20, 2023 at 0:32

answered Nov 20, 2023 at 0:26

Etienne de Martel

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2 Comments

Jan Tajovsky Over a year ago

It works but the bytes are in reverse order. So, the .NET stores the Int32 in 'Little Endian', I guess... Nevertheless I can work with that.

Frederik Hoeft Over a year ago

@JanTajovsky you may want to take a look at the BinaryPrimitives class in System.Buffers.Binary if you need network byte order ;)

Selaka Nanayakkara · Accepted Answer · 2023-11-20 00:35:52Z

0

To my knowledge, you can't directly use a reference to an int for a byte array in old .Net Framework versions. Since arrays are in C# are reference types and the reference itself is already used to reference to the array(like an object reference). The only way is you can use the unsafe keyword and pointers to achieve a similar.

edited Nov 20, 2023 at 0:35

answered Nov 20, 2023 at 0:25

Selaka Nanayakkara

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3 Comments

Etienne de Martel Over a year ago

That was true in the ancient times of .NET Framework, but we have spans now.

Selaka Nanayakkara Over a year ago

Aren't spans underneath use unsafe ?

Etienne de Martel Over a year ago

What they use for their implementation is irrelevant, you can use spans without the unsafe keyword.

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