What are some useful strategies for converting a Classic ASP application to ASP.NET

Question

We have a ten-year-old ASP application that we are considering planning an update for. We want to take advantage of the new technologies that ASP.NET has to offer, as well as the opportunity to fix some issues with the existing framework (the existing code-base is highly fragmented, nearly impossible to test, let alone debug, and the entire application appears to have been constructed according to the "Farmhouse Pattern".)

To that end, it seems that the time has come to rebuild this application. But, we are a small business, and we simply don't have the resources to either hire out the rebuild, nor to dedicate our small team of developers solely to the task of rebuilding (we've got other tasks on our plate, and can't concentrate on this one particular task for the length of time it would take to fully reconstruct the application).

What, then, are some useful strategies we can employ to help us convert this app, without having it consume all of our limited resources for the duration of the re-write?

Answer 1

Sounds like an interesting challenge. It's definitely not going to be easy, especially if you can't dedicate any resources to the project full time.

If you have a 10 year old application that is working, I would suggest not going for a complete re-write at all. I would start by sitting down and figuring out what you want your end product to be.

Is it going to be an ASP.NET MVC Web Application, an ASP.NET WebForms App, or something else? Once that decision is made, come up with a loose design for an architecture. If you do things correctly, you can build out bits and pieces of the business logic in .NET and utilize it from your Classic ASP code until you're ready to re-write the UI in .NET as well.

Answer 2

The only approach that has worked for me is to carve of areas of functionality in small slices, and rewrite. "Converting" first, then refactoring seemed like a good idea a few times, but in the end just became horrible messes of code written in ASP.NET instead of ASP - and that added no value.

If you have a site that has distinct areas of functionality, carve one off and start with that (I chose "contact us"). Write it the way you think it should be written - that is, assume your new part is fitting into the end design of your well-written app. If you have to add "hacks" to interface with the old system, make sure they are isolated and commented.

When working on an update, think "can I carve of some of the functionality here into it's own bit?" - if so, convert it then update it. I found that if you insist on keeping the NEW app clean and allow yourself to add small hacks to the OLD app for communication, you get the best results.

This does mean you'll have two separate apps (two IIS web apps) for a while, and can make cookie/url and session management a bit hairy, as well as adding one more deployment concern. To combat this, make sure that you minimize state in your web app (always a good idea anyway), and share state through something other than Session.

If you do this a piece at a time, make the pieces small enough, and have a good design up-front, this works well - at least in my experience, it's the way that works best. Note that my experience may not match reality.

Answer 3

I agree with what Justin said; if you have a working application in place, you'll need a compelling reason (i.e., money) to justify the expense in rewriting the application for a new platform.

Although ASP classic and ASP.NET share a similar-looking syntax and some common coding conventions, they are very very different from each other. If you tried to simply copy-paste classic ASP code into an ASP.NET application, you might be able to get it to work, but you'd be missing out of a lot of the advantages of ASP.NET Web Forms or ASP.NET MVC (and their respective frameworks, of course).

You can, however, extend the functionality of the existing site with .NET code through web services or COM interop. We have a 10+ year old classic ASP web site and I've used both .NET web services (.asmx) and COM-callable .NET DLLs to enhance our existing application. In both cases, I wrote all of my new business logic in the .NET component and provided a chunky interface to work with the existing ASP page. That allowed my .NET code to be very easily testable and still use our existing (huge) investment in our classic ASP site.