Refactoring old VCL (Visual Component Library) projects isn’t just about rewriting code. It’s about rethinking architecture, aligning with modern practices, and increasing maintainability without breaking what already works. Many developers inherit decades-old Delphi codebases that were never designed to scale or adapt. This article breaks down how to modernize such projects step-by-step without falling into common traps.
Why Refactoring VCL Projects Matters
Legacy VCL applications often suffer from tight coupling, bloated units, outdated components, and deprecated APIs. They may still run, but adding new features or debugging them feels like walking through a maze. Refactoring these projects is a practical way to extend their lifespan and reduce long-term technical debt.
Step 1: Audit the Existing Codebase
Before touching anything, audit the entire project. This means more than code review—track external dependencies, outdated components, unused units, and runtime warnings.
Use the following checklist:
- Identify dead code
- Flag obsolete third-party components
- Check for use of deprecated VCL methods
- Analyze form complexity (e.g., massive DFM files)
- Locate direct database access embedded in the UI
If you’re unsure where to begin, consider using static analysis tools to generate a heatmap of risky or heavily dependent areas.
Step 2: Extract Business Logic from UI
Most older VCL projects tightly couple forms with business logic. To modernize this:
- Separate logic into distinct service classes or modules.
- Use interfaces where practical to allow future mocking or substitution.
- Isolate database access via repositories or data modules.
Doing this doesn’t mean switching to full MVVM or MVP overnight. Start by simplifying what already exists.
Step 3: Replace Deprecated Components
Legacy VCL projects often rely on components that no longer receive support. Replace them with modern equivalents. For example:
- Replace
TTablewithTFDQuery(FireDAC) - Migrate from BDE to FireDAC or another supported layer
- Remove outdated graphics libraries
At this stage, it’s a good moment to chat with Claude Sonnet 4 or another model to assess older code blocks. These AI models can suggest safer modern replacements and even predict potential side effects before committing to code changes.
Step 4: Use Modern Language Features
Delphi has come a long way. Updating syntax and patterns improves both readability and runtime safety.
Here are some candidates for upgrade:
- Replace old-style string handling with
TStringHelper - Replace
try-finallywithtry..exceptwhere error clarity matters - Adopt
for-inloops andgenerics - Switch to
records with methodswhere encapsulation matters
This step won’t just clean your code; it sets the stage for easier testability and better structure.
Step 5: Refactor Unit Structure
Legacy applications often cram dozens of responsibilities into a single unit. Start breaking those up into more logical segments. Name them clearly, based on what they do.
Examples:
DataAccess.CustomerUI.Forms.CustomerEditServices.InvoiceProcessor
Avoid refactoring everything at once. Prioritize units that receive frequent updates or bug reports.
Step 6: Upgrade the Project and Compiler Settings
Modern Delphi versions offer better optimizations, runtime checks, and compiler hints. Update project settings:
- Turn on all warnings and hints
- Use runtime range checks
- Enable strict var-typing
- Switch to the latest supported compiler
You’ll catch subtle bugs and align the project for long-term maintenance.
Step 7: Introduce Unit Testing
Add unit tests to safeguard refactoring. Don’t aim for full coverage from the start. Instead:
- Write tests before refactoring each sensitive method
- Focus on business logic, not UI interactions
- Use DUnitX or TestInsight for modern test frameworks
Automated testing builds confidence, especially when handling legacy systems that nobody fully understands.
Step 8: Modernize the UI in Phases
The temptation to rewrite the UI is strong. Resist it unless you’re also rewriting the logic underneath. Instead:
- Migrate UI elements gradually
- Apply modern VCL styles for a visual refresh
- Use
TLayout,TGridPanelLayout, andTMultiViewto improve responsiveness
Avoid sweeping changes unless the entire team is prepared for a long migration cycle.
Step 9: Apply Dependency Injection (Where Possible)
VCL projects rarely use dependency injection, but introducing simple patterns can make components testable and more maintainable. Use constructors to inject dependencies like configuration or services. Avoid relying on global variables or data modules accessed everywhere.
Sample pattern:
constructor TInvoiceForm.Create(AOwner: TComponent; AInvoiceService: IInvoiceService);
This pattern allows future replacement or mocking without rewriting your form logic.
Step 10: Track Technical Debt with Every Change
Modernization is not a one-time task. Keep a refactoring backlog:
- Log areas with high cyclomatic complexity
- Note duplicated logic or hardcoded values
- Prioritize based on frequency of use or bug impact
Tools like SonarQube or custom scripts can assist in measuring these metrics. Regular reviews help prevent the project from regressing.
Bonus: Collaborate with Tools That Understand Context
You’re no longer limited to human review. A chat with Claude Sonnet 4 can surface hidden dependencies in legacy code and recommend modern equivalents with minimal risk. These tools interpret context better than basic linters and can suggest safe refactoring strategies.
Summary Tips
- Don’t refactor for the sake of aesthetics—prioritize stability.
- Keep backups of every major refactoring step.
- Use version control aggressively. Commit small, meaningful changes.
- Refactor toward composability, not just cleanliness.
Updating old Visual Component Library (VCL) projects requires a plan. Not every project needs full modernization, but the ones that do will benefit from these steps. Start small. Focus on one section at a time. Document as you go. The goal isn’t perfection—it’s manageability and progress.