How to calculate CheckSum in FIX manually?

Question

I have a FixMessage and I want to calculate the checksum manually.

8=FIX.4.2|9=49|35=5|34=1|49=ARCA|52=20150916-04:14:05.306|56=TW|10=157|

The body length here is calculated:

8=FIX.4.2|9=49|35=5|34=1|49=ARCA|52=20150916-04:14:05.306|56=TW|10=157|
0        + 0  + 5  + 5  + 8     + 26                     + 5   + 0  = 49(correct)

The checksum is 157 (10=157). How to calculate it in this case?

Answer 1

static void Main(string[] args)
    {
        //10=157
        string s = "8=FIX.4.2|9=49|35=5|34=1|49=ARCA|52=20150916-04:14:05.306|56=TW|";
        byte[] bs = GetBytes(s);
        int sum=0;
        foreach (byte b in bs)
            sum = sum + b;
        int checksum = sum % 256;
    }
    //string to byte[]
    static byte[] GetBytes(string str)
    {
        byte[] bytes = new byte[str.Length * sizeof(char)];
        System.Buffer.BlockCopy(str.ToCharArray(), 0, bytes, 0, bytes.Length);
        return bytes;
    }

Answer 2

You need to sum every byte in the message up to but not including the checksum field. Then take this number modulo 256, and print it as a number of 3 characters with leading zeroes (e.g. checksum=13 would become 013).

Link from the FIX wiki: FIX checksum

An example implementation in C, taken from onixs.biz:

char *GenerateCheckSum( char *buf, long bufLen )
{
    static char tmpBuf[ 4 ];
    long idx;
    unsigned int cks;

    for( idx = 0L, cks = 0; idx < bufLen; cks += (unsigned int)buf[ idx++ ] );
    sprintf( tmpBuf, "%03d", (unsigned int)( cks % 256 ) );
    return( tmpBuf );   
}