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The biggest question now is whether or not to migrate. The answer to this question will depend on various factors, but keep in mind the old saying “If it ain’t broke, don’t fix it.”
Does the application lack certain desirable features that .NET can bring? If so, the application is a candidate for migration; however, this will still depend on other factors.
Are the application layers well-partitioned? In other words, are the presentation layer, business layer, and data layer well-partitioned? If the application is not well-designed, migration will be more difficult. It might be better to design a new system from scratch than to migrate what in effect is a badly designed system. This has to be weighed against cost and time.
Is data communication with other platforms or over the Internet with other applications required? XML and disconnected data connection is a breeze with ADO .NET.
Is the current client load overwhelming or will it soon overwhelm the current system? This is particularly true with e-commerce and business-to-business applications. .NET has various features that allow developers to build distributed systems that lend themselves very well to scaling and load balancing. Furthermore, ASP .NET has many performance enhancements over ASP.
Do you have the required support staff? .NET is relatively new, and even if you are an old hand with Visual Basic, there are many new things in VB .NET that you will need to learn and master. It might be worthwhile to delay the migration until your staff has acquired the required level of expertise and experience. This book will go some way in helping you along that track.
Once you have considered the various factors, the next stage is to plan the migration.
The prime candidates for migration are usually ASP web applications. This is because ASP .NET is a great leap forward in both design and run-time implementation of web applications. It provides various tools and enhancements that many web application developers wanted in the past. So from this point of view, we will consider mainly ASP applications, but the steps mentioned will also apply to Visual Basic.
Migrating the clients or presentation layer is probably the easiest step. This will allow you to build up experience and get most of the mundane work out of the way. The client can include both ASP pages and Windows Forms. Access to the business layer, which previously was done through COM, can still be maintained by using the InterOp services.
Once the new clients are implemented and tested, the next step is to create .NET wrapper classes to the COM components. These are simply .NET classes that act as an interface to the COM components. The logic is still implemented in COM, but the clients now need to be changed so that they only access the .NET classes. At this stage the clients should be fully .NET.
Now you can go ahead and migrate all the COM components to .NET classes. At this stage, changes to the application will not affect the clients. Once this step is over, the application will be fully .NET.
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