Packet data using the checksum?

Question

I have packet data, which I am storing as a public static List<byte> packet = new List<byte>();

The last byte of this is a checksum, and I am trying to figure out how to use this to check the packet data.

When I have a full packet, I run:

//get checksum                                    
byte chcksm = packet[packet.Count - 1];
int sum = chcksm;

to get the value of the checksum byte.

Then I run:

//checksum calculation
public static byte ComputeAdditionChecksum(List<byte> data)
{
    byte sum = 0;
    unchecked // Let overflow occur without exceptions
    {
        foreach (byte b in data)
        {
            sum += b;
        }
    }
    return sum;
}

on packet to get the value of the whole packet, and save it in an int called packetSum.

Comparing these two vales, on a test packet,packetSum, the value of the whole packet, returns 76. And the value of the checksum byte returns 103.

Am I doing this wrong?

//edit, in thedocs, it says this about the checksum..

1 Byte checksum expressed as:

unsigned char CalcChkSum(unsigned char *MsgBlock,
int Length)
{
    unsigned char Total = 0x40;
    int i;

    for (i = 0; i < Length-1; i++)
    {
        Total -= MsgBlock[i];
    }
    return Total;
}

Answer 1

It looks like you're including the checksum in your calculation. Without seeing the underlying data with expected and actual results I'd say changing the loop as follows should fix the issue:

public static byte ComputeAdditionChecksum(List<byte> data)
{
    byte sum = 0x40;
    unchecked // Let overflow occur without exceptions
    {
        for (var i = 0; i < data.Count - 1;  i++)
        {
            sum -= data[i];
        }
    }
    return sum;
}