byte array from specific index as struct in c# without making a copy

Question

Currently I code client-server junk and deal a lot with C++ structs passed over network. I know about ways provided here Reading a C/C++ data structure in C# from a byte array, but they all about making a copy.

I want to have something like that:

struct/*or class*/ SomeStruct
{
    public uint F1;
    public uint F2;
    public uint F3;
}

Later in my code I want to have something like that:

byte[] Data; //16 bytes that I got from network
SomeStruct PartOfDataAsSomeStruct { get { return /*make SomeStruct instance based on this.Data starting from index 4, without copying it. So when I do PartOfDataAsSomeStruct.F1 = 132465; it also changes bytes 4, 5, 6 and 7 in this.Data.*/; } }

If this is possible, please, tell how?

Marc Gravell · Accepted Answer · 2020-07-11 17:21:43Z

5

Like so?

byte[] data = new byte[16];
// 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
Console.WriteLine(BitConverter.ToString(data));
ref SomeStruct typed = ref Unsafe.As<byte, SomeStruct>(ref data[4]);
typed.F1 = 42;
typed.F2 = 3;
typed.F3 = 9;
// 00-00-00-00-2A-00-00-00-03-00-00-00-09-00-00-00
Console.WriteLine(BitConverter.ToString(data));

This coerces the data from the middle of the byte-array using a ref-local that is an "interior managed pointer" to the data. Zero copies.

If you need multiple items (like how a vector would work), you can do the same thing with spans and MemoryMarshal.Cast

Note that it uses CPU-endian rules for the elements - little endian in my case.

For spans:

byte[] data = new byte[256];
// create a span of some of it
var span = new Span<byte>(data, 4, 128);
// now coerce the span
var typed = MemoryMarshal.Cast<byte, SomeStruct>(span);
Console.WriteLine(typed.Length); // 10 of them fit
typed[3].F1 = 3; // etc

answered Jul 11, 2020 at 17:21

Marc Gravell

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3 Comments

Marc Gravell Over a year ago

@Kosmos you can also use unsafe and unmanaged pointers if you're using down-level compiler etc, but: the above is a lot easier!

Kosmo零 Over a year ago

Whooohooo, it's working! I wanted to avoid unsafe thing... at least until I run into problems that I can't solve without it. This time problem is solved.

Marc Gravell Over a year ago

@Kosmos just note that even though you didn't use unsafe, it is still "unsafe", because you might have got the size wrong (not enough bytes in the array), meaning you're overwriting someone else's memory. So just: don't get the size wrong :)

Raphael · Accepted Answer · 2020-07-11 18:04:51Z

Thank you for the correction, Marc Gravell. And thank you for the example.

Here is a way using Class and Bitwise Operators, without pointers, to do the samething:

    class SomeClass
    {
        public byte[] Data;

        public SomeClass()
        {
            Data = new byte[16];
        }

            public uint F1
        {
            get
            {
                uint ret = (uint)(Data[4] << 24 | Data[5] << 16 | Data[6] << 8 | Data[7]);
                return ret;
            }
            set
            {
                Data[4] = (byte)(value >> 24);
                Data[5] = (byte)(value >> 16);
                Data[6] = (byte)(value >> 8);
                Data[7] = (byte)value;
            }
        }        
    }

Testing:

            SomeClass sc = new SomeClass();
            sc.F1 = 0b_00000001_00000010_00000011_00000100;
            Console.WriteLine(sc.Data[3].ToString() + " " + sc.Data[4].ToString() + " " + sc.Data[5].ToString() + " " + sc.Data[6].ToString());
            Console.WriteLine(sc.F1.ToString());
//Output:
//1 2 3 4
//16909060

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