- Details
- Written by Chris Pyrgas
- Created: 06 April 2020

In the last days, we have been trying to (among other things) complete the VO compatibility aspect of X#, by implementing one final missing piece, behavior of numeric arithmetic that is completely compatible to Visual Objects.

The area where this becomes visible is when dealing with integral numbers (either variables or constants) with a size < 32 bits, so of the type BYTE, SHORT and WORD.

For example, consider this code:

LOCAL b1,b2 AS BYTE LOCAL n AS INT b1 := 250 b2 := 10 n := b1 + b2 ? n

Currently this prints 260 in X#, as you may (or not!) expect. But in VO, the same code returns 4; the operation is performed between 2 BYTE vars and so is the result, which first overflows from the max value of 255 that can be stored in a byte and results to that value of 4 (260 mod 256), which is then in turn assigned to the INT var.

The current behavior of X# is compatible to c# and we intend to keep it as the default one, but plan (or had planned!) to implement the VO compatible behavior optionally, when the compiler option /vo11 (Compatible arithmetic conversions) is enabled. The problem is, that the more we research the behavior of VO, in order to fully implement it, the more weird and almost random it appears to be. For example, let’s change the above example, to use constant byte values instead of variables in the expression:

LOCAL n AS INT n := BYTE(250) + BYTE(10) ? n

Well, now also VO returns 260 instead of 4! It gets weirder; let’s store the result in a USUAL this time, instead of an INT, without changing anything else:

LOCAL n AS USUAL u := BYTE(250) + BYTE(10) ? u

And now the result in VO is 4 again!!!

It gets even better. As we saw before, the following:

LOCAL n AS INT n := BYTE(250) + BYTE(10) ? n

Gives us a value of 260. Let’s add one more operand now, to increase the result by 1:

LOCAL n AS INT n := BYTE(250) + BYTE(10) + BYTE(1) ? n

Clearly, this will return 261, right? Well, no, the result that VO gives now is 5!!!

The inconsistency does not have to do only with constants; let’s try using variables again, but now in multiplication, not addition of bytes:

LOCAL b1,b2 AS BYTE LOCAL n AS INT LOCAL u AS USUAL b1 := 250 b2 := 10 n := b1 * b2 ? n// 2500u := b1 * b2 ? u// 196

So, when storing the result to an INT variable, the expression returns 2500. But when it is stored to a USUAL, it returns 196 (2500 mod 256) in VO!

And so far we have only tested behavior of expressions using BYTE vars only. When we add to the mix different numeric types (WORD, SHORT, INT and DWORD), the results become more and more inconsistent. It is certainly not the first time we are facing problems like that, when we attempt to emulate the behavior of VO on several aspects in X#, but in most cases, after some research, we are able to deduce some rules (which still may be very inconsistent at times) which fully describe the behavior of VO and then adjust the compiler and/or runtime in X# to (optionally) match it. But, in this case, there’s really no set of rules that describes this behavior, for the most part it is almost random practically. We’ve been ourselves professional VO developers for many years, and saying the above does not feel very nice, but it’s what we unfortunately see in this area.

The question is, what can we do about it? Clearly, there’s no way to fully emulate this mess in X#, because, even if we wanted to, we just can’t cover all the very different and conflicting combinations of results that VO gives, so we’ve now started to think about completely abandoning this part of VO compatibility and just continue using the universal standard .Net rules for numeric arithmetic. Another option would be to try to emulate some of this only, but we are not sure which and how far we could or should try to go with it.

We would love to hear your (VO developers’) opinion and/or thoughts though. What would you suggest/prefer us to do on this matter?

PS. Just to be completely clear, the above applies only to the VO compatibility aspect of X#, and still, that would only be **optional** behavior. So please do not be concerned about us modifying the behavior in Core, FoxPro or other dialects of X#!

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