Technical Approach for Migrating VB 6.0 Application to VB .NET

If you upgrade a Visual Basic 6.0 project group or an n-tier application to Visual Basic .NET, you must upgrade one project or tier at a time.

If your three-tier application includes a client component, a business component, and a data access component, you should upgrade the application in the following order:

1. Client component, Business component, Data access component
2. Business component, Data access component
3. Data access component

In an n-tier application, always upgrade the client tier first, and then upgrade other tiers on the dependency tree. You should follow this order for two reasons:

• This approach allows you to keep the application working. When you upgrade the client, you break and work with only one component of the application. All of the other components continue to work the same way that they did previously. With this approach, you isolate the work area. Alternately, if you upgrade the data tier first, suddenly you break the data tier and the components that depend on the data tier.

• Visual Basic 6.0 locks type libraries (TypeLibs). This creates a problem if you need to rebuild the TypeLib or recompile the underlying dynamic-link library (DLL). If you upgrade the business tier first and then upgrade the client, you must continually stop and restart Visual Basic 6.0 every time you change the middle tier. Consider the following workflow:

1. Upgrade the middle tier. Change the Visual Basic 6.0 client to access the middle tier. Run the middle tier.
2. Change the Visual Basic 6.0 client to access the middle tier. Run the middle tier.
3. Run the middle tier.

If you want to change the .NET DLL, you must then close Visual Basic 6.0, recompile in .NET, restart Visual Basic 6.0, and so on. You can avoid this problem if you upgrade the client first and then upgrade the middle tier.

To upgrade each Visual Basic 6.0 application, use the Upgrade tool that is included with Visual Basic .NET. The Upgrade tool is started when you use Visual Basic .NET to open a Visual Basic 6.0 project. When you use the Upgrade tool, the Visual Basic 6.0 project is not changed, and a new Visual Basic .NET project is created. Before you upgrade a Visual Basic 6.0 project, it is best to prepare it for upgrade.