Convert pair of values to byte[]

Question

I need a quick way of converting a pair of integers to a byte array. If it were a single value, I could use the following:

private static unsafe byte[] ToBytes(int value)
{
    var bytes = new byte[4];
    fixed (byte* pointer = bytes)
    {
        *(int*)pointer = value;
    }
    return bytes;
}

However I can't quite figure out how you'd do the same for a pair of integers. The following which I tried doesn't work but should help to show what I'm aiming for

private static unsafe byte[] ToBytes(int value1, int value2)
{
    var bytes = new byte[8];
    fixed (byte* pointer = bytes)
    {
        *(int*)pointer = value1;
        *(int*)pointer + 4 = value2;
    }
    return bytes;
}

(I'm aware I can do the same using BitConverter, but I want to compare the performance of the two)

Can't you put the int's in a long and then do the unsafe stuff with that long? — rene, Nov 23 '16 at 12:18
Try `*((int*)pointer + 4) = value2;` or, since the pointer is a pointer to an int, try +1 instead since you want to access "the next int". — Lasse V. Karlsen, Nov 23 '16 at 12:19
@rene - Thanks. I suspect it'd undo the savings of the micro-optimizations I'm already doing! I'm hoping I can do it without. — Barguast, Nov 23 '16 at 12:20
@LasseV.Karlsen - + 1 seems to do trick; I incorrectly assumed it was in bytes — Barguast, Nov 23 '16 at 12:23
Put two bytes into an UInt16 results = (UInt16)((b[0] <<8) + b[1]); This is faster than BitConverter and gives same results. — jdweng, Nov 23 '16 at 12:36

score 3 · Accepted Answer · edited May 23 '17 at 10:30

Whether this will perform better than alternatives I don't know but the correct syntax is like this:

*((int)pointer + 1) = value2;

This:

*(int)pointer + 4 = value2;

gives an error because this: *(int)pointer is a variable reference and thus your code really looks like this:

i + 4 = value2;

which is not legal syntax in C#.

Instead encapsulate the addition to the address before dereferencing it into a variable reference (an lvalue if you want to google for these things):

*((int)pointer + 1)

+4 is also incorrect since the pointer is a "pointer to int", not a "pointer to byte", which means that +X is really +X * sizeof(int) in terms of byte-level addressing.

Your final method should look like this:

private static unsafe byte[] ToBytes(int value1, int value2)
{
    var bytes = new byte[8];
    fixed (byte* pointer = bytes)
    {
        *(int*)pointer = value1;
        *((int*)pointer + 1) = value2;
    }
    return bytes;
}

Thanks for the thorough answer. It seems to do the trick. (will mark as answer when I can) — Barguast, Nov 23 '16 at 12:24

Convert pair of values to byte[]

1 Answers1