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We've recently started to support a PowerBuilder 10.5 application and the question has come whether or not we should think about alternatives or keep the app running in PB 10.5. It is a classic PB app; an administrative software, build upon an Oracle DB.
Right now, the app works great, but there are two reasons why we reconsider:
The sole developer of this app is about to retire. He's the only one
who has all the PB-knowledge to support this app.
We might want to improve the offered services of the app. So integrations with other tools are right around the corner.
I'm not very familiar with PB, but I've read it (only the newest versions) is now supported by Appeon. The latest version is now 2017 R3, with a 2019 version coming up.
I'm wondering what the pro's & con's are of trying to update the current 10.5 version to the latest version. Is it worth it to update? Or what are the pro's & con's of sticking to the 10.5 version?
Or should we consider moving to a newer technology, since so few Powerbuilders are to be found nowadays? And if so, what technology would you advise?
Rather than just differences between the older and newer PB-versions, I'm looking for motivations to upgrade/migrate/do nothing at all.
Thanks.
So, there's no clear cut answer, but we can throw around some ideas on the non-technical bullet points (as requested).
Staying on 10.5: There's a lot to be said for "if it ain't broke, don't fix it." If it works and you're happy with what it does, don't move it.
However, since you've said that you're planning on moving it forward, you might want to consider that 10.5 doesn't support current operating systems (within a year, Windows systems currently supported by MS will be only Win8 and Win10), which were nothing but figments of imagination when 10.5 was out. Your 10.5 app may work on Win10 now, but that's solely because of MS's work on backward compatibility for apps, and that you haven't leveraged an area in PB that had a problem in a then-future version of Windows. If you need to add functionality, being on a version that at least suggests that it works on your operating system could be helpful.
Parallel argument for databases, the exception being that if your app uses SQL Anywhere, the database that used to come for free in several PB packages. It is now something you'd have to purchase separately.
One thing to remember about trying to move forward with an old version of anything is support. If you get stuck, the vendor will basically not talk to you, and the peer community has been shrinking, so you've got less chance of getting into a dialog with fellow developers.
Upgrading: Upgrading is usually a minor effort. The most frequent reasons I've seen exceptions to this: deprecated functionality, and coding that depends on behaviours that didn't stay consistent between versions (some behaviours are promised to stay consistent, but not all). Run a migration test with a trail version with your PB expert to get that question off the table.
One thing to keep in mind when upgrading is that the licensing model has changed. PB used to have a perpetual model (buy once, use forever), but it's now a subscription model. Whether this is an improvement for you or not is up to you to figure out.
Whether it is "worth it" to upgrade, in my mind it usually boils down to
OS support
DB support
vendor support
peer support
deprecated features, and whether I use them
new features, and whether I would use them (and you asked us not to discuss these last two items, which need to be weighed very individually anyway, and are well documented on Appeon's site)
"Migrating": I've put "migrating" in quotes, because I don't believe there's a technology that lets you "migrate" in the sense of a code translation. (I'll let you read one of my old tirades about wanting to "migrate" off PB.) What I'll talk about here is rewriting in a new technology. Both pulling business rules out of an old PB system and redesigning/rewriting in another technology is a big effort.
The biggest argument in favour these days is getting and keeping PowerBuilder talent. Getting people with PB under their belt is hard, and judging legitimate talent is challenging, even with someone with PB on your side of the interview table. (Leverage your retiring guy if you want to move forward with PB.) Training someone with PB is no small task either. Someone once asked me, not an educator, if I could come up with a course and train his team in a week. I laughed. After a two week course designed and given by professional educators from the then-vendor Powersoft, I came home and wrote incredibly embarrassing code. I also needed lots of time practicing, and getting feedback from my peers. If you can get someone or train someone, if they are only doing PB work a couple of weeks per year, those PB "muscles" will atrophy. No matter the technical arguments of PB vs something else, if you can't get PB talent to maintain it, PB is a dead end.
I'm afraid I'm not one to suggest an alternative technology. It used to be that, in terms of of rich client apps, you couldn't go wrong with choosing Microsoft, but since then, MS has sent the development community on some wild goose chases, that have ended in deprecated technologies. I wouldn't want to be the guy looking into the future to guess.
Good luck.
I would recommend migrating.
You will find several companies that offer migration to both java and .net which are the leading platforms.
In terms of UI for me currently the only option is web. Using other technologies does not make a lot of sense.
If your company uses a lot of MS stack, like MS OS, SQL server. Exchange, Sharepoint etc I will recommend migrating to C# otherwise migrating to Java makes more sense
Terry's answer is quite good but the point about migration was not addressed with respect to the new features in PowerBuilder 2019.
One major feature of PowerBuilder 2019 is a C# DataStore (compatible with .NET Core) and DataWindow object migration utility. The C# DataStore has the same APIs and transaction mechanism as the PowerScript DataStore. It is documented in detail on the Appeon Website: https://www.appeon.com/support/documents/appeon_online_help/powerbuilder/api_reference/PowerBuilder.Data/DataStore/IDataStore/IDataStore.html
Should you decide C# is the way to go, this feature of PowerBuilder 2019 makes the migration effort a "port" of the PowerScript non-visual code rather than a rewrite (for the reasons mentioned above).
Here is example PowerScript code:
public function datastore of_retrieve (date ad_start, date ad_end, decimal adec_amt);
Datastore lds
lds = Create Datastore
lds.dataobject = "d_order_customer"
lds.SetTransObject(SQLCA)
lds.Retrieve(ad_start, ad_end, adec_amt)
Return lds
end function
Here is the same example in C# using the C# DataStore:
public IDataStore GetOrderCustomerInfo(DateTime startDate, DateTime endDate, decimal amount)
{
IDataStore dataStore = new DataStore("d_order_customer", _context);
dataStore.Retrieve(startDate, endDate, amount);
return dataStore;
}
Seaside just released a release candidate for the upcoming 3.0 version, so it appeared on my radar again. As I'm currently pondering what web framework to use for a future project, I wonder whether it's something to consider. Alas, most of the publicity for Seaside is from '07, which is probably one or two generations for the web. So I'm hoping that the community here can answer some questions
Continuation-based frameworks were pretty great when most of your workflow was mostly in HTML, e.g. form submits. For today's JavaScript-heavy environments, that hardly seems worthwhile anymore.
Is Squeak able to handle a reasonable workload? From other questions here and elsewhere, it seems that for proper scaling another implementation (Gemstone etc.) would probably fare better in the long run, but I don't have a proper idea how far away that is. Sessions seem to be rather expensive.
I know that comparisons are hard, but most of the articles you find on the net set Seaside and Rails side by side. How would combinations like Scala/Lift, Clojure/Compojure or Erlang/Nitrogen do instead?
I have answers to question one and two:
This is true. However since version 2.8 Seaside is not a strictly "continuation-based" framework anymore. Seaside uses continuations in the flow module only. Since Seaside 3.0 the flow module is even optional. Also note that Seaside has strong Javascript support since 2005, this is long before mainstream frameworks started to add Javascript functionality. Today Seaside comes with JQuery and JQueryUI support built-in.
Of course that depends on what you store within your session objects, but typically sessions are small (less than 20 KiB). Use the memory profiler in your application to determine the exact memory consumption.
And there is a new seaside book: http://book.seaside.st/book
I find the productivity of working in a Smalltalk IDE with a good set of abstractions outweights all other issues in engineering dominated projects. It works well as an enterprise system for a small company with about 100 (simultaneous, but not heavy) users on a single server (without going to SSD). Since 2007:
Seaside has shown to be able to make the switch from html workflows to javascript ones;
Seaside has been ported to a lot of different Smalltalks;
Has seen Gemstone release GLASS;
The new 'cog' vm with much improved performance has been released a few weeks ago and shows great promise for improved performance.
In Smalltalk we have now three web frameworks to consider, besides Seaside also
Aida/Web and
Iliad.
Both later effectively solve three-like control flow, but without needing continuations. Both also have a very strong Ajax integration, actually you don't realize anymore that you are working with Ajax.
Both also scale in memory consumption well. 10.000 sessions spend 220MB in Aida/Web, that is about 23KB per session, which can be further optimized down to mere 400B per session. This means, that you can run not only but many websites from the single Smalltalk image. Of course you can always upgrade to load balancing solution, when you really need. Which is from my experience very rarely needed.
Comparing to Ruby on Rails, a friend of mine complained that he needs 50MB of memory initially for every webshop site he is selling. He then turned to the Aida/Web solution where he needs about the same MB for the image, but then just few KB for every additional webshop site.
Avi Bryant, the developer of Seaside, said that AJAX triumphs continuations in almost all situations. Nevertheless, you can build reasonably powerful applications with Seaside and AJAX, too.
The Application part of a Web-App can be done in other frameworks quite well using Ajax.
I think a Seaside integrated Smalltalk-to-Javascript Framework like Cappuccino-for-Clamato is missing, currently. I'd like to be able to build real Javascript-Apps using Smalltalk.
Javascript is awesome but being capable of dealing with complicated workflow in a clean cheap way in the server side (as Seaside allows you to) is preventing it to become obsolete. Economy and utility are things that gives return in the short and long run. But telling this in the abstract has no value at all. You should be talking about a precise application and deciding if seaside is part of your bunch of competitive advantages to form an equation that rocks (and knowing why).
About scaling workload with Seaside, the short answer is that you can scale it like hell yah (for the long answer check my answer here: Does Seaside scale?).
too unanswerable man :) rty a variation of what you're really trying to ask
I think the best thing you can do is a prototype of something in a weekend.
If you can do a prototype in two days and you can capture some attention and you enjoyed the developing experience of doing it with seaside then you'll have the foundation of your next thing.
It costs only your time (you can publish in an amazon server).
BTW, this week I've heard about a startup that made its prototype by hand (was everything static and they processed stuff manually). Pretty amazing and crazy and cheap. When they felt that they had enough traction on the idea (which the had) they implemented the app (with whatever tech, I'm sure is no challenge for a seaside developer)
I have the following dilema: My clients (mom-n-pop pawnshops) have been using my mgmt. system, developed with ISQL, for over 20 years. Throughout these two decades, I have customized the app to each clients desire, or when changes in Laws/Regulations have required it. Most clients are single-user sites. Some have multiple stores, but have never wanted a distributed db, don't trust the reliability or security of the internet or any other type of networking. So, they all use Standard Engines. I've been able to work around some SE limitations and done some clever tricks with ISQL and SE, but sooner or later, new laws may require images of pawnshop customers, merchandise, electronic transmision, etc. and then it will be time to upgrade to IDS, re-write the app in 4GL or change to another RDBMS. The logical and easiest route would be IDS/4GL, however, when I mentioned Linux or Unix-like platforms to my clients, they reacted negatively and demanded a Windows platform, so the easiest solution could be 4Js, Querix, etc.?.. or Access, Visual FoxPro or ???.. anyone have suggestions?
This whole issue probably comes down to a couple of issues that you'll have to deal with.
The first thing is what application programming and development language Are you willing to learn and work with?
The other thing is what kind of Internet capabilities to you want?
So for example while looking at a report do you want to be able to click on a button and have the report converted to a PDF document, and then launch the e-mail client with that PDF attached?
What about after they enter all the information data into the system, perhaps each store would like their own miniature web site in which people in town could go there to check what they've have place of having to phone up the store and ask if they have a $3 used lighter (the labor of phone and checking for these cheap items is MORE than the cost of selling the item – so web really great for this type of scenario).
The other issue is what kind of interface do you want? I assume you currently have some type of green screen or text based interface? Or perhaps over the years you did convert over to a GUI (graphical user interface).
If still green screen (text based) you now you have to sit down and give a considerable amount of effort and time into the layout and how you of screens will work with a graphical based system. I can remember when going from green screens to color, all of a sudden now the choices and effort of having to choose correct colors and layouts for that screen actually increased the workload by quite a bit. And then I went from color test screens to that of a graphical interface, then again all of a sudden now we're presented with a large number of new controls, colors, and in addition to that we have large choices in terms of different fonts and sizes.
And then now with the web, not only do you deal at different kinds a button styles (round, oval, shading, shadows, glow effects), but in addition to all those hover effects and shading effects etc, you now have to get down to some pretty serious issues in terms of what kind of colors (theme) your software will adopt for the whole web site.
This really comes down to how much learning and time you are willing to invest into new tools and how much software you can and will produce for given amount of time and effort.
I quite partial to RAD tools when you get down into the smaller business marketplace. Most of the smaller businesses can not afford rates for a .net developer (it not so much the rate, as the time to build an application). So, using ms-access is a good choice in the smaller business market place. Access is still a good 3 to 5 times many of the other tools in the marketplace. So quote by .net developer to develop something might be 12,000 bucks, and the same thing in Access might be $3000. I mean that small business can not afford to pay you to write unit testing code. This type of extra cost is just not going to happen on the smaller scale projects.
The other big issue you have to deal is what kind of report writing system are you going to build into the system? This is another reason why I like for the smaller business applications is access is because the report writer is really fantastic. Access reports have a whole bunch of abilities to bake connections in from forms and queries and pass filters and parameters into those reports. And, often the forms and queries that you spend time building already can talk to reports with parameters and pass values in a way that again really reduces the workload (development costs).
I think the number one issue that you'll have to address here however is what you're going to do for your web based strategy? You absolutely have to have one. Even if you build the front end part in access, you might still want to use a free edition of SQL server for the back end part. There are several reasons for this, but one reason is then it makes it easy to connect multiple stores up over the Internet.
Another advantage of putting your data in some type of server based system, is now you can set up some type of web server for all the stores to use, and build a tiny little customize system that allows each store to have their products and listings online (but, they use YOUR web server, or one that you paying $15 per month to host all of those customers). This web part could be an optional component that maybe perhaps all customers don't necessarily want. It would work off of the data they have to enter into the system anyway.
One great advantage of adopting these web based systems is not only does it allow these stores to serve their customers far better, but it also opens up the doors for you to convert your software into a monthly fee based system, or at least some part of it such as the optional web hosting part you offer.
When I converted so my longer time applications from green screen mainframe type software into windows desktop based applications it opened up large markets for me. With remote desktop, downloading software, issuing updates from a web site, then these new software systems make all of these nuts and bolts part of delivering software very easy now and especially so for supporting customers in different cities that you've never met face to face.
So, if you talking still primarily single user and one location, Access will reduce your development costs by a lot. It really depends on how complex and rich of an application you are talking about. If the size and scope of the project is beyond one developer, then you talking more about developer scaling (source code control, object development methodology, unit testing, cost and time of setting up a server based database system like SQL server etc). So they're certainly tipping point here when you go beyond that tipping point of cost time in complex city, then I actually don't recommend access. So this all comes down to the right horse for the right course.
Perhaps that the end of the day, it really comes down to what application development system are you willing to invest the time to learn?
Look at Aubit4GL - that is, I believe, available (or can be compiled on) Windows.
Yes, IDS is verging on overkill for a single-user system, but if SE doesn't provide all the features you need, or anticipate needing in the near future, it is a perfectly sensible choice. However, with a modicum of care, it can be set up to be (essentially) completely invisible to the user. And for a non-stressful application like this, the configuration is not complicated. You, as the supplier, would need to be fairly savvy about it. But there are features like silent install such that you could have your own installer run the IDS installer to get the software onto the customer's machine without extra ado. The total size of the system would go up - IDS is a lot bigger on disk than SE is (but you get a lot more functionality). There are also mechanisms to strip out the bigger chunks of code that you won't be using - in all probability. For example, you'd probably use ON-Tape for the backups; you would therefore omit ON-Bar and ISM from what you ship to customers.
IDS is used in embedded systems where there are no users and no managers working with the system. The hardware sits in the cupboard (closet) and works, communicating over the network.
It's good to see folks still getting value out of "old school" Informix Tools. I was never adept at Perform, but the ACE report writer always suited me. We skipped Perform and went straight for FourGen, and I lament that I've never been as productive as I was with FourGen. It had it own kind of elegance from its code generators to it funky, but actually quit powerful, stand alone menu system.
I appreciate the modern UI dynamics, but, damn, is it hard to write applications today. Not just tools, but simply industry requirements et al (such as you may be experiencing in your domain). And the Web is just flat out murder.
I guess part of it is that since most "green screen" apps look the same, it's hard to make one that looks bad! With GUIs and the Web etc., you can't simply get away with a good field order and the labels lining up.
But, alas, such as it is, that is what we have.
I have not used it in, what now, 15 years, but you may also want to look at Alpha 5. It was a pretty powerful, but not overly complicated, database development package, and (apparently) still going strong.
I wouldn't be too afraid of IDS. It runs pretty simply. Out of the box with zero or little tweaking, the DB works and is efficient, and it used to be pretty trivial to install. It was no SE, in that SE's access was tied to the application (using a library) vs an independent server that is IDS. But, operationally, it's really straightforward -- especially for an app like what you're talking about. I appreciate that it might be overkill, but even today, the resource requirements won't necessarily be insane. There's a lot of functionality, of course, and flexibility that you won't use. But frankly, beyond "flat file" DBase style databases, pretty much ALL of the server based SQL databases are very powerful and capable and potentially complicated. But they don't have to be. They can still be used "simply" and easily (well, save for Oracle -- Oracle can't do anything "simply").
As far as exploring other solutions, don't be too afraid of the "OOP" stuff, as most applications, while they leverage OOP libraries, aren't really OOP themselves (they can be, they just typically aren't, they simply don't need to be). The biggest issue with many of the OOPs systems, is they're simply to finely structured. Dealing with events at far too low of a level. While many programs need to access to that fine level of control, most applications, particularly the ones much like yours, do not. So, the extra flexibility simply gets in the way or creates more boiler plate.
That said, you shouldn't be frightened away from them per se, citing lacking of expertise. They can be picked up reasonably quickly. But I would certainly exhaust the more specialized tools (like Alpha 5, or Access, etc.) first to see if they don't offer what you want.
In terms of Visual FoxPro, was and remains a peerless tool (despite flak from people who know little about it). It has a fast, native database engine, built-in SQL and powerful report designer and so on. But you also have to consider that Microsoft support will be dropped for it in 2014, there will never be a 64-bit version, and so on. And the file locking method it uses will be increasingly flaky on future versions of Windows IMO.
In 2002 I did a pretty large VB6 app for a client. It used a lot of UserControls and a 3rd party menu control (for putting icons next to menu names). It had dynamically "splittable" panels, TreeViews with multi-state checkboxes, etc. A very rich UI. My total time on the project was about 500 hours, which the client graciously let me spread over a whole month. (Yeah, it was that kind of job.) They were very happy, though, and they paid the bill on time with no argument.
So after having no contact with them for years, they suddenly call and wonder if I can update the app to .Net for them. My initial reaction is just to decline, since I don't use VB.Net. And having read a bunch of posts on SO about the difficulties of porting, etc., etc., I'm even more inclined to decline, so to speak.
Still, before I tell them no I am interested in roughly quantifying the effort it would take. I would love to hear from anyone who has done this kind of thing and has a feel for how much work it is. Was it:
Significantly less than the effort you used on the original?
Somewhat less than the effort you used on the original?
The same as the effort you used on the original?
More?
A lot more?
Please only respond if you have actually done this kind of port. And the answer doesn't have to be exact, since I really am only trying to ballpark this. My feeling is that the effort will be at least as much as it took for the original, if not more. But I could be wrong. Thanks for any help.
I have done what you've been asked. In my case it was an (amateur) bowling tournament mgmt system: Member database mgmt (personal info, IRS/Winnings info, mailing lists etc etc etc) , tournament mgmt (player assignments, scoring, lane ticket generation, check register for winnings and side-pots, etc etc etc) as well as IRS EDI generation for all winners in a given year. Plus about a billion little items scattered across ~ 50 screens/sub-screens.
The key is CLIENT UNDERSTANDING -- You must be clear that they understand that this is not trivial; this is a new adventure for BOTH of you (particularly if you are a new-comer to VB.Net) If they liked your work before then they may very well give you the leeway/freedom to learn VB.Net on their nickel.
Reading some of the previous answers let me make a few suggestions (based upon 30 years experience as a software developer, the last 20 of which as a consultant....)
TAKE THEIR MONEY IF THEY OFFER IT. You need to bring your skillset into the 20th century; let them pay for it (again, if they agree.) They may be Magazine surfing and want "the latest stuff" for NO GOOD REASON -- but maybe they realize that they can extend the life of (your) application by this port. In essence they may have all kinds of goofy reasons for doing it... if they are NOT hiring you to DECIDE whether or not this port is a good idea. Then you may express any dismay you have about the decision PROCESS out of good client relationship building; BUT if they want to do this then it might as well be your job.
Take all this mumbo-jumbo re: C# vs VB.Net with a grain of salt. I have worked EXCLUSIVELY in VB.Net / ASPX.net (vs C#) since its inception and have yet to come across ANY functionality NOT attainable in VB.Net. There are some 'purists' out there that just view VB.Net as a toy. Well, I came from the days of writing in Assembler, then C, then C++ (And you can throw in Fortran, PL1 for good measure) then VB5, then 6, then VB.Net ... and NOW JAVA for Android. Its ALL GOOD FUN... and each has it merits and drawbacks. Remember that C# and VB.Net are essentially just GUIs to achieve a meta-language intermediate. You can write a TERRIBLE (as measured by efficiency or memory use or whatever metric you choose!) Program in C# and a great one in VB.Net (and vice-versa.) DO NOT EQUATE GOOD PROGRAMMING WITH LANGUAGE SYNTAX. (... C# is "superior" ???? Gimme a break.)
I chose to allow the Visual Studio do most of the heavy lifting for the first pass. Then you go through the gazzillion errors and clean it up. It goes pretty fast.
BUT you need to decide whether or not to take advantage of any framework benefits that you had hand-coded in VB6. E.G. looping through a string to locate a specific character(s) is now as simple as The_String.IndexOf("c") I found that in my case I went through the code several times and took better and better advantage of the Class (i.e. object orientation) as well as framework goodies as I became comfortable... this adds to your development time (see CLIENT UNDERSTANDING mantra) BUT your code WILL BE MORE Efficient then it ever could have been in VB6. You could simply port to get the errors out and not take ANY advantage necessarily from the framework.
I have not found any issue with 3rd party active-x controls. You can add a reference to FRAMEWORK objects, COM objects, etc. It may even be likely that the control vendor has a .Net (managed code) version... OR there may be suitable alternatives since you wrote the thing in VB6. (See CLIENT UNDERSTANDING mantra)
So if your still reading, then now I will finally tell you that the second attempt at my application in VB.Net CONVERTED/PORTED from VB6 was ~ 1/3 of the original time to get to a working model... and I was learning the framework as well. (If your confident in your skill set, have learned a few languages through the years you will get the gist of VB.Net quickly --- its the SUBTLETIES that take awhile.)
I must caution you that the thing that can REALLY kill you if you do not preach the CLIENT UNDERSTANDING well enough is if they want to make changes WHILE you're porting (and this is VERY LIKELY since they've been using it for awhile... I was very true in my case as well.
There is no hard and fast rule here. It could be that changes will actually HELP YOU get to a better understanding the framework faster OR changes could be a real pain. Only you can determine which flavor they might be. AND if they look to be the PAINFUL type -- you might ask to do the conversion first so that you have reliably reproduced the functionality -- THEN go back and review the code to make changes and take advantage of the framework as necessary. But, as I said, there is NO Hard and Fast rules here -- and don't let the purists tell you differently --- remember they are probably the same guys that said that PASCAL was going to take over the world!
So after having no contact with them for years, they suddenly call and wonder if I can update the app to .Net for them.
You need to ask why they want this done.
It's a Bad Plan(tm) for clients to be making technical decisions on a whim. Before applying any solution, thoroughly understand their needs and their problem. Only after you understand the problem as they do should you make recommendations.
It could be that they're infatuated with a buzzword and want to be using the latest thing, or it could be any of a million other things. The solution to their problem could be something really easy, but if you don't find out what their problem is, you will never know the best way to solve that problem.
I would plan on it taking about 50% of the time and effort it took to create it in the first place.
I have done EXACTLY what you are asking about for a commercial software product that consisted of roughly 500 kloc. We balanced the desire for refactoring with the desire to get something working and released as quickly as possible.
It took nearly the entire team about 1 full year to get it done...for a product that took 4 years to create in the first place. It was a gigantic undertaking..not to be underestimated.
We are doing that exact thing right now, however it's slightly different. Instead of one huge application, there are many smaller ones. However, there are a few bigger ones in the list. What we found out is that it was significantly less work than we originally thought. BUT...the biggest unknowns had to do with third-party controls we had. If you have a lot of those to basically redesign, you will probably be looking at more work.
The one good suggestion I have is to use Visual Studio 2008 for the conversion (not VS 2005). There were far less problems when using the built-in converter in VS 2008 than there was with VS 2005. Not sure why, it just was that way.
So, I can't say that you won't spend 500 hours again, but most likely not. Most of your time should be spent on testing to verify no functionality was lost.
In my opinion,
Visual Basic 6 and Visual Basic .NET are so different that you should forget about their coincidence in their name, and treat this as a migration to .NET =P
I think that you have one big advantage and one disadvantage:
Advantage: you allready know what the application has to do. Probably you´ll have some meetings with your clients, in order to add some modifications or new features, but you have the major part of the requirements very clear. You´ll even have the old working application to see while you, or your team, are coding.
Disadvantage: you´ll have to learn a new language. Saying how much time is going to take, is very subjective. I´ve allways thought that learning a new language is not the problem, the big problem would be if you have to learn to program!. But you do know to program. You´ll know what you want to do, and you´ll have only to search how to do it in the new languaje.
Since you have to learn .NET, If you have to estimate, I think that you should suppose that it will take you at least the same as the previous application, even more.
My advice is too take the chance to learn this new language. And if you are more inclined to decline the project, let me give you another idea... Maybe you could estimate a little module of your application, and tell the client that you´ll try to do that module, to see how much it takes to you. The client should pay this little module, even if you decide not to continue.
You can´t tell the client that you know all the requirements, so they are saving money with you, in some way. And that they should take this little risk (we are talking about a little module), because who better than you to make the project, you know the old application perfectly!
If the client is right with this, you´ll can take the decision with more arguments. And If finally you decide to continue, with this little module done, you can estimate the whole project better than before.
Sorry, my English isn´t very good, and probably I´ve made many gramatical errors.
EDIT grammar mistakes... =P
This is a huge topic.
You should take a look at MS' Free Book - Upgrading Microsoft Visual Basic 6.0 to Microsoft Visual Basic .NET.
Were you to rewrite from scratch, rather than simply try and port the previous application, it shouldn't take too long. VB.Net has some features that will make the new application build take shorter. As it seems that you don't know VB.Net already, what will take you the most time is learn how to do things the new way(tm).
I'd go for two months this time.
But seriously, that's probably not far off — or even low — because the client will have some reason for needing the port done. It won't be just a straight port - they'll want some "small little enhancement" that will blow the whole project up.
Additionally, I'd worry a bit about the third party controls. Most of the rest of it should convert okay, but there's sometimes an issue finding a good analog to a 3rd party control in .Net.
I'm also concerned that you don't use vb.net. If you're a c# user and wanted to re-implement that way you'd be fine, but otherwise this is a non-starter. VB.Net is just different enough from vb6 for you to get yourself into trouble.
Aside from all that, I would expect it to be less work this time than last, because you have a very explicit design laid out in front of you that you merely need to follow.
I am looking at minimizing the future impact on a yet to be written application. I am trying to avoid any 3rd party products, and even avoid operating system specific calls. Can anybody suggest other ways of future proofing the application. The idea would be not having to rewrite major portions in 10 or 20 years, and that only maintenance (bug fixes) would ever need to be done.
If you want your program to keep running (on modern OSes) for that kind of time period, you'll probably end up having to write it in only pure ANSI C (or C++). Anything else is likely to need some kind of tweaking over the years - and nobody really knows what'll happen over the next 10-20 years.
That said, here are a few tips to minimize these kinds of problems:
Avoid weird dependencies. If you're going to depend on some library, make sure it is very well-established (and thus likely to survive at least 5 of those 10-20 years), or at least open-source so you can fork it yourself if need be.
Avoid OS-specific calls. This will be a balancing act with 1. - you can use a wrapper library like boost or Qt or glib or what-have-you - but that would increase the chance for compatibility issues on that front.
Document everything. Fact is, no matter how hard you try, this program will need compatibility fixes and bug fixes, and probably feature additions too. So make life easier on that poor maintenance programmer who comes along 15 years later. :)
Dig out some 10 and 20 year old programs that still run on todays machines and see why they still run and why they are of value. I see a number of computation intensive console based apps still in occasional use, mostly written in C and FORTRAN, called by other apps. If your app has a lot of GUI, are you sure it will still have any value in a few decades time? Perhaps consider seperating the user interface from the core functionality, in such a way that the UI can be replaced in the future as UI paradigms change and evolve. If you write your system in a very modular fashion, modules that still provide value can be kept while those that are clearly obsolete can be replaced.
Write with 64-bit in mind. We're discovering now that many of our third-party dependencies don't support 64-bit, and so we have issues.
The best way to build an application that is still functioning with little-to-no maintenance in 10 years is to look at the systems that were built and are still running from 10 years ago.
From my experience, most of those systems, which did not need major upgrades are doing so by running on the same or similar hardware that they were deployed to 10 years ago and use the same interface.
The maintainers chose to trade away the performance improvements due to Moore's law or usability improvements in favor of little to no maintenance over the years.
Automate Testing can also help.
Not that your application will be "proofed", but I you'll have some idea of what has to be fixed when things changed.
A lot of the applications I develop run on a cycle (example: yearly). The most important thing I do to ensure they continue to work is not hard-coding dates or date ranges. Example:
year(now()) for the year
DateSubmitted BETWEEN year(now()) AND DATEADD(year,1,year(now())) for a range
Of course, these are just examples.