01 / The context
The problem behind the project.
An invoice-generation application had to move from .NET into SQL without silently changing established results. Reading the legacy implementation was necessary, but not sufficient: the replacement needed a repeatable way to demonstrate equivalent behavior.
02 / The contribution
How I approached it.
- Traced the existing generator line by line and recorded its behavior in the form of SQL before completing the replacement.
- Placed the legacy .NET classes inside a Windows harness so the original and replacement implementations could run side by side.
- Investigated and resolved output differences, including the final text comparisons, then spent a dedicated week exercising leap-year February pay periods on a large sample.
- Deployed with rollback testing and compared production and QA stored procedures after the release.
03 / The result
What the work made possible.
An eleven-week conversion supported by executable comparison evidence, with differences reconciled to zero for the tested outputs. The case illustrates a recurring method: make equivalence observable, exercise calendar edge cases, and preserve a recovery path through deployment.
11 weeksdocumented conversion
Zero differencetested output reconciliation
Leap-yearpay-period edge-case tests
