Visual Basic 6 dates back to 1998, and its development environment has been unsupported for years. Yet millions of VB6 applications still run in businesses. Sooner or later the question comes up: should we migrate to .NET? And if so, how do we do it without breaking an application that works?

Why migrate

  • Longevity: hiring a VB6 developer is getting hard, and the runtime depends on Microsoft's continued compatibility goodwill;
  • Security: native VB6 code doesn't benefit from modern memory protections;
  • Ecosystem: access to .NET libraries, the web, the cloud, and today's testing and CI/CD tools;
  • Integration: interoperating with modern systems is far simpler from .NET.

The often-forgotten prerequisite: having the source code

Most migration guides assume you have the VB6 project at hand. In reality, that's not always the case: the source code may have been lost over the years. And you can't migrate what you don't have. This is where decompilation comes in: reconstructing a usable VB6 project from the executable is the mandatory starting point for any migration when the sources are missing. We cover that step in our article on recovering lost source code.

Migration strategies

Full rewrite. Start from scratch in modern .NET. It's the cleanest, but also the longest and riskiest — years of accumulated business logic must be faithfully reproduced.

Assisted migration. Tools automatically convert part of the VB6 code to VB.NET or C#. The result always needs manual fixes, but it accelerates the start.

Progressive migration. Wrap the existing app and replace modules one by one. The safest approach for critical applications that can't stop.

Classic pitfalls

  • Third-party ActiveX and OCX controls with no direct .NET equivalent;
  • The behavior of data types (Variant, Currency) that differs subtly;
  • Error handling (On Error GoTo) to rethink as structured exceptions;
  • Win32 API calls via Declare to rewrite cleanly.

Where to actually start

Begin with an inventory: what does the application do, which components does it depend on, and do you have the sources? If yes, you can evaluate conversion tools. If not, the first building block is recovering the code from the binary — VBReFormer reconstructs UI, resources and logic, giving you a clean base before starting modernization. You can test recovery for free with the free edition.