Help with Conversion to Byte

[harrisonhjones]

I have a application I'm working on that requires the calculation of Checksum. The checksum can be calculated by adding up all the bytes of the data and subtraction that from 255(ie, 255 - sum of data). As one would except, the value of the sum of the data is, at times, higher than 255. This gives me a negative value to convert into a byte. It seems that VB.net won't do that.

I've previously done the calculation manually via Excel. Here's an example from excel:

The checksum ends up being: -1465
when converted to hex I get: FFFFFFFA47
The value I need for the checksum is the 47

Anybody have a suggestion?

-Regards

[Shaggy Hiker]

Well, normally, I would say add everything up and mask off the high bytes by ANDing with &H000000FF, but that would be because you were only looking at the low byte. If you are content with the numbers going negative, that technique won't work because a negative number is represented by the twos complement.

Well, actually, it should still work, but it makes me queasy. If you perform that AND operation, the three high bytes will be cleared, and all you will have left is the contents of the low byte, which is the 47. Therefore, something like:

check = CByte((255 - sum) AND &H000000FF)

should do. The thing is that many people would put the parenthesis in a different place:

check = CByte(255-(sum AND & H000000FF))

which will give a much different answer. As long as you know which one you want, give it a try. The latter will keep the value from ever going negative, so you never see the twos complement issue, but you end up with a different checksum value.

[dbasnett]

        Dim cksum As Byte
        Dim bi As Integer = 0
        Dim r As New Random
        For x = 1 To 100
            bi += r.Next(0, 255)
            bi = bi And 255
        Next
        cksum = CByte(255 - bi)

Are the bytes added or XOR'ed?

[Lord Orwell]

        Dim cksum As Byte
        Dim bi As Integer = 0
        Dim r As New Random
        For x = 1 To 100
            bi += r.Next(0, 255)
            bi = bi And 255
        Next
        cksum = CByte(255 - bi)

Are the bytes added or XOR'ed?

good point. If you are XORing them, they should never get above 255.