Converting CRC16 calculation from C to C#

Question

I have to translate a CRC16 calculation from C to C# but get the message that I cannot implicitly convert type 'int' to 'bool' on the (crc & 0x8000) and on return (crc & 0xFFFF)

The code so far:

public unsafe short Crc16(string str)
{
    short crc = 0;

    for(int i = 0; i < str.Length; i++)
    {
        crc = (crc << 1) ^ str[i] ^ ((crc & 0x8000) ? 0x1021 : 0);
    }
    return (crc & 0xFFFF);
}

EDIT: Changed char parameter to string

Original C code

short Crc16( char *str )
{
    short crc=0;
    unsigned int i;
    for (i=0; i<strlen(str); i++)
        crc = (crc << 1) ^ *(str+i) ^ ((crc & 0x8000)? 0x1021:0 );
    return (crc & 0xffff);
}

Answer 1

In C, 0 and FALSE are synonymous, and any number that is not 0 is true. (reference)

To make the conversion you would do it something like this:

public short CalcCrc16(string str)
{
    short crc = 0;
    int i;

    unchecked
    {
        foreach(char c in str)
        {
            short exponent = (crc & 0x8000) != 0 ? 0x1021 : 0;
            crc = (crc << 1) ^ ((short)c) ^ exponent);
        }

        return (short)(crc & 0xFFFF);
    }
}

So now that we have the C code to work with, I changed the code sample I have here to match. Below is the explanation of the changes:

• char* is the C equivalent of a string
• The for loop is iterating over the characters *(str + i) can be rewritten as str[i], and is equivalent to C#'s str.GetChar(i)
• The ternary condition is to determine whether the exponent is 0x1021 or 0.
• I broke up the lines so you could see the algorithm a bit clearer.
• I changed for(int i = 0; i < str.Length; i++) to a foreach on the characters because it was easier to understand.