Senior .NET Modernization Engineer

What You'll Do7

• Dig through the existing codebase to establish what the system does, then ask the questions that pull hidden behavior and edge cases into view.
• Break the upgrade into small, releasable steps, decide which parts stay exactly as they are and which get rethought, and state the trade-offs in plain terms.
• Write and review production C# daily, directing AI coding agents and checking their output against evidence.
• Build regression safety as you go, using characterization tests and replaying production-like data through old and new versions to compare results.
• Ship each change to production with a rollback plan attached, since a wrong calculation means someone gets paid incorrectly.
• Work alongside engineers, product, and leadership on the client side, surfacing risk early and keeping communication steady.
• Pick up the leave and absence domain as you go, leaning on the people who already know it well.

Requirements6

• Roughly 8 to 10+ years of professional C#/.NET work, with genuine depth in .NET Framework 4.x.
• Hands-on leadership or substantial delivery on a large .NET Framework to modern .NET migration where the system stayed live in production throughout. A greenfield rewrite or a small app port does not compare.
• Solid MongoDB background that includes carrying driver and server versions through upgrades.
• Track record in large legacy codebases with thin test coverage, and a method for adding regression safety before behavior changes.
• Daily use of AI coding tools such as Claude Code, Cursor, or Copilot, with tests, review, and evidence backing the output.
• Clear communication skills and the ability to work independently inside a product team, gather requirements, and explain technical risk to people outside engineering.

Preferred Qualifications6

• .NET Standard 2.0 or multi-targeting strategies for shared libraries during incremental migrations
• HR tech, benefits, payroll, or leave and absence domains with heavy business rules
• Multi-tenant SaaS work with per-customer data isolation
• AWS, Docker, and CI/CD with GitHub Actions or Jenkins
• Structured logging and observability, for example Datadog
• A second modern stack such as TypeScript or Node.js

Who Should Apply

A senior .NET engineer who has kept a legacy system shipping through a Framework-to-modern migration, not just rewritten one from scratch. You know older ASP.NET patterns and MongoDB drivers well enough to reason through them yourself, and you treat AI agents as tools to direct and verify. Comfort with US Pacific hours and a nearshore or South America base helps.

Salary Insight