Reading 2 bytes together

[Harsh Gupta]

Hi,

I am reading a file into byte array and consists of header information.

The format specs say that the first 2 bytes gives you a static value of 474. Right now I am converting the bytes (necessarily numeric values) into binary format (representing them as strings), append the first 2 bytes together and then convert it into integer again.

That's working, but I just wanted to make sure if there is any other way to read more than 1 byte together and display the result.

Or any other way, instead of reading into byte array, something long that line?

Thank you

[jmcilhinney]

byte[] data = File.ReadAllBytes("file path here");
short header = BitConverter.ToInt16(data, 0);

if (header == 474)
{
    // Valid file.
}

[Harsh Gupta]

Thanks JMC, didn't knew about the BitConverter class.

But, please correct me if I am wrong. How does BitConverter read the data off byte array? When I tried your code, it didn't give me the correct result. But when I swapped the bytes, it gave me correct result.

byte[] data = System.IO.File.ReadAllBytes("rgbtest.jpg");
 
            byte[] nb = new byte[2];
            nb[0] = data[1];
            nb[1] = data[0];
 
            ushort magic = BitConverter.ToUInt16(nb, 0);
 
// swapping works
            MessageBox.Show(string.Format("Magic Number: {0}", magic));

I tried to read a JPG file too, whose first byte is FF, second byte is D8, so 2 bytes must be FFD8 and Decimal equivalent is 65496, and I get this only when I swap the values.

What am I doing wrong?

[jmcilhinney]

I don't think you're doing anything wrong. I haven't really used the BitConverter all that much and I didn't take its endianness into account. It looks like it's little-endian, which makes sense, so you will, in fact, have to reverse the order of the bytes you want to test. Assuming .NET 3.5, LINQ can help you do that succinctly:

byte[] data = File.ReadAllBytes("file path here");
short header = BitConverter.ToInt16(data.Take(2).Reverse().ToArray(), 0);
 
if (header == 474)
{
    // Valid file.
}

[Arrow_Raider]

This is how I would do it:

string filename = "filename here";
FileStream fs = File.OpenRead(filename);
BinaryReader reader = new BinaryReader(fs);
byte[] magicBytes = reader.ReadBytes(2);
 
//474 in big endian is 0x01DA
if (magicBytes[0] == 0x01 && magicBytes[1] == 0xDA)
{
    //yay
}

Make sure you put this in the using directives:

using System.IO;

Of course, you also need to close the stream or the reader when you are done with it:

reader.Close()