Question

How to correctly convert the following C++ into C#?

//--------------------------------------------------------------------
// Calculate a 16-bit Cycle Redundency Check (CRC) value on a block of //data.
//
// Params:
// pData : Pointer to data to calculate CRC.
// dwSize : Size of data in bytes.
//
// Return:
// 16-bit CRC value.
//
// Notes:
//--------------------------------------------------------------------
private WORD i_pwCalculateCRC16Continuation(PBYTE pData, WORD dwSize, WORD wCRC)
{
    // high byte of CRC initialized
    BYTE cCRCHi = (BYTE) ((wCRC >> 8) & 0xFF);
    // low byte of CRC initialized
    BYTE cCRCLo = (BYTE) (wCRC & 0xFF);
    // will index into CRC lookup table
    BYTE cIndex;    
    while (dwSize--) // step through each byte of data
    {
        cIndex = cCRCHi ^ *pData++; // calculate the CRC
        cCRCHi = cCRCLo ^ m_cCRCHiArray[cIndex];
        cCRCLo = m_cCRCLoArray[cIndex];
    }
    return (cCRCHi << 8) + cCRCLo;
}

I found a tool that converted it but it were not perfect and it does not understand the context.

private ushort i_pwCalculateCRC16Continuation(ref byte pData, ushort dwSize, ushort wCRC)
{
    byte cCRCHi = (byte) ((wCRC >> 8) & 0xFF);
    byte cCRCLo = (byte) (wCRC & 0xFF);
    byte cIndex;

    while (dwSize-- > 0)
    {
        // Cannot convert source type 'int' to target type 'byte'
        cIndex = cCRCHi ^ pData++;
        // Cannot apply indexing to an expression of type 'byte'
        cCRCHi = cCRCLo ^ m_cCRCHiArray[cIndex];
        // Cannot apply indexing to an expression of type 'byte'
        cCRCLo = m_cCRCLoArray[cIndex];
    }
    // Cannot convert expression type 'int' to return type 'ushort'
    return (cCRCHi << 8) + cCRCLo;
}

The CRC concept and bit wise operations are little bit unfamiliar for me and I do not understand the above code well. Hence I do not know how to “fix” it.

Thanks

EDIT:
Missed the following variables.

private static byte[] m_cCRCHiArray = {
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,  0x81, 0x40
};

private static byte[] m_cCRCLoArray = {
    0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0x80, 0x40
};

Answer 1

Try this:

private static byte[] m_cCRCHiArray = { 
    0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41,  0x81, 0x40 
};

private static byte[] m_cCRCLoArray = { 
    0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0x80, 0x40 
};

private ushort i_pwCalculateCRC16Continuation(byte[] data, ushort wCRC)
{
    byte cCRCHi = (byte)((wCRC >> 8) & 0xFF);
    byte cCRCLo = (byte)(wCRC & 0xFF);

    byte cIndex;

    for (int i=0; i < data.Length; i++)
    {
        cIndex = (byte)(cCRCHi ^ data[i]);
        cCRCHi = (byte)(cCRCLo ^ m_cCRCHiArray[cIndex]);
        cCRCLo = m_cCRCLoArray[cIndex];
    }

    return (byte)((cCRCHi << 8) + cCRCLo);
}

It isn’t obviously wrong at first glance 😉

I’ve not altered any of the hungarian notation, except for changing pData to data