Question

In C#

var buffer = new byte[] {71, 20, 0, 0, 9, 0, 0, 0};

var g = (ulong) ((uint) (buffer[0] | buffer[1] << 8 | buffer[2] << 16 | buffer[3] << 24) |
                    (long) (buffer[4] | buffer[5] << 8 | buffer[6] << 16 | buffer[7] << 24) << 32);

In C++

#define byte unsigned char
#define uint unsigned int
#define ulong unsigned long long

byte buffer[8] = {71, 20, 0, 0, 9, 0, 0, 0};

ulong g = (ulong) ((uint) (buffer[0] | buffer[1] << 8 | buffer[2] << 16 | buffer[3] << 24) |
                    (long) (buffer[4] | buffer[5] << 8 | buffer[6] << 16 | buffer[7] << 24) << 32);

C# outputs 38654710855, C++ outputs 5199.

Why? I have been scratching my head on this for hours…

Edit: C# has the correct output.

Thanks for the help everyone 🙂 Jack Aidley’s answer was the first so I will mark that as the accepted answer. The other answers were also correct, but I can’t accept multiple answers :\

Answer 1

The C++ is not working because you’re casting to long which is typically 32-bits in most current C++ implementation but whose exact length is left to the implementor. You want long long.

Also, please read Bikeshedder’s more complete answer below. He’s quite correct that fixed size typedefs are a more reliable way of doing this.