Jason Kerney Workmood Legacy Code Rescue

Legacy Code Rescue Skill

When to Use This Skill

Use this skill when:

• You need to change or refactor code with little or no test coverage
• The code is hard to understand, risky, or tightly coupled
• You want to add tests to legacy code safely
• You need to introduce seams for dependency injection or testability

Rescue Approach

• Characterization Tests: Write tests that capture current behavior before changing anything.
• Introduce Seams: Add interfaces, delegates, or factories to break dependencies and enable testing.
• Peel and Slicing Technique: Gradually peel away or slice off small, testable pieces of logic from the legacy code. Move these into new, well-tested methods or classes, leaving the original code smaller and easier to understand. Repeat until the legacy code is fully replaced or manageable.
• Incremental Refactoring: Make one small, safe change at a time; verify tests after each.
• Safe Commits: Commit after every passing test and refactor step.
• Risk Management: Avoid large rewrites; prefer gradual improvement.

Rescue Patterns

• Characterization Test Example:

  [Fact]
  public void ShouldBehaveAsBefore_WhenInputIsX()
  {
      // Arrange: set up legacy object
      // Act: call legacy method
      // Assert: verify current (even if odd) behavior
  }
  

• Seam Introduction: Use interfaces or delegates to inject dependencies.
• Peel and Slicing Example: Identify a small, self-contained piece of logic in a large method. Extract it into a new method or class, write tests for it, and replace the original code with a call to the new method. Repeat this process to gradually reduce legacy complexity.
• Sprout Method/Class: Add new code alongside old, then migrate usage.
• Wrap & Delegate: Use adapter or wrapper to intercept and test legacy logic.

Example Prompts

• "/legacy-code-rescue [file.cs] Add characterization tests before refactoring."
• "/legacy-code-rescue [method] How do I introduce a seam for DI?"
• "/legacy-code-rescue [class] What’s the safest way to start refactoring?"

Verification Steps

• Build: dotnet build --framework net9.0-windows10.0.19041.0
• Test: dotnet test (ensure new and old behavior is covered)
• Commit: After every passing test and refactor step

Resources

• Refactoring Skill
• TDD Skill
• Code Smells Detection
• Working Effectively with Legacy Code (Feathers)

Maintenance

• Update as new rescue patterns or project-specific strategies emerge.
• Ensure examples and checklists match current best practices.

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