Converting 2 bytes to Short in C#

Question

I'm trying to convert two bytes into an unsigned short so I can retrieve the actual server port value. I'm basing it off from this protocol specification under Reply Format. I tried using BitConverter.ToUint16() for this, but the problem is, it doesn't seem to throw the expected value. See below for a sample implementation:

int bytesRead = 0;

while (bytesRead < ms.Length)
{
    int first = ms.ReadByte() & 0xFF;
    int second = ms.ReadByte() & 0xFF;
    int third = ms.ReadByte() & 0xFF;
    int fourth = ms.ReadByte() & 0xFF;
    int port1 = ms.ReadByte();
    int port2 = ms.ReadByte();
    int actualPort = BitConverter.ToUInt16(new byte[2] {(byte)port1 , (byte)port2 }, 0);
    string ip = String.Format("{0}.{1}.{2}.{3}:{4}-{5} = {6}", first, second, third, fourth, port1, port2, actualPort);
    Debug.WriteLine(ip);
    bytesRead += 6;
}

Given one sample data, let's say for the two byte values, I have 105 & 135, the expected port value after conversion should be 27015, but instead I get a value of 34665 using BitConverter.

Am I doing it the wrong way?

Mark Wilkins · Accepted Answer · 2011-04-24 13:18:33Z

37

If you reverse the values in the BitConverter call, you should get the expected result:

int actualPort = BitConverter.ToUInt16(new byte[2] {(byte)port2 , (byte)port1 }, 0);

On a little-endian architecture, the low order byte needs to be second in the array. And as lasseespeholt points out in the comments, you would need to reverse the order on a big-endian architecture. That could be checked with the BitConverter.IsLittleEndian property. Or it might be a better solution overall to use IPAddress.HostToNetworkOrder (convert the value first and then call that method to put the bytes in the correct order regardless of the endianness).

edited Apr 24, 2011 at 13:18

answered Apr 21, 2011 at 21:14

Mark Wilkins

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3 Comments

Lasse Espeholt Over a year ago

Are you sure this will work if the application is run on a different architecture?

Mark Wilkins Over a year ago

@lasseespeholt: That's a good point. It probably does need a check using IsLittleEndian. For a big-endian architecture, the order would need to be as given in the OP.

shelbypereira Over a year ago

solution from @BrokenGlass ushort value2 = (ushort)(port1 + (port2 << 8)); should be preferred in speed critical situations as bitconverter can and allocating the extra array can be slow.

BrokenGlass · Accepted Answer · 2011-04-21 21:16:34Z

14

BitConverter is doing the right thing, you just have low-byte and high-byte mixed up - you can verify using a bitshift manually:

byte port1 = 105;
byte port2 = 135;

ushort value = BitConverter.ToUInt16(new byte[2] { (byte)port1, (byte)port2 }, 0);
ushort value2 = (ushort)(port1 + (port2 << 8)); //same output

answered Apr 21, 2011 at 21:16

BrokenGlass

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3 Comments

Lasse Espeholt Over a year ago

+1 for the "manual" approach. In my opinion BitConverter does way to many checks and an endian check which can break the code.

shelbypereira Over a year ago

from your solution: ushort value2 = (ushort)(port1 + (port2 << 8)); //same output this is the way to go, in most cases we need this type of conversion for a large volume of bytes and speed is likely to be an issue. I am currently using this in imaging applications. Overhead of creating the array and using bitconverter can be huge.

Jroonk Over a year ago

Lest anyone else goes down the same path I did. Arithmetic operations in C# on anything less than 16 bits get converted to 16 bits to avoid overflow during the operation. Hence the value2 operation converts port1 and the (port2 << 8) to ints, adds them together, and then he casts it back to a ushort. So @shelypereira may be correct that BitConverter has overhead, so does the basic math, its just invisible. See, this stack overflow link: stackoverflow.com/questions/10065287/…

Peter Mortensen · Accepted Answer · 2016-12-12 17:24:30Z

11

To work on both little and big endian architectures, you must do something like:

if (BitConverter.IsLittleEndian)
    actualPort = BitConverter.ToUInt16(new byte[2] {(byte)port2 , (byte)port1 }, 0);
else
    actualPort = BitConverter.ToUInt16(new byte[2] {(byte)port1 , (byte)port2 }, 0);

edited Dec 12, 2016 at 17:24

Peter Mortensen

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answered Apr 21, 2011 at 21:21

Andrei Pana

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