Senior .NET Developer

Role: Lead C#/.NET Engineer, Entity Framework Core
Location: Hamilton, NJ
Work arrangement: Onsite full time for the first 30 days, then hybrid (3 days onsite, 2 days remote)
Job type: Contract
Experience: 12 to 15 years of professional C#/.NET engineering

About the Role

We are looking for a Lead/Principal-level C#/.NET engineer with expert Entity Framework Core experience to lead the modernization of a mission-critical legacy benefits system hosted on AWS.

The programme is converting a large mainframe codebase written in COBOL and JCL to C#/.NET, migrating a hierarchical database to PostgreSQL and deploying on AWS. An automated modernization tool delivers the converted code “as is”. Your job is to turn that converted code into clean, maintainable, high-performance, production-grade C#/.NET. You will define the data-access architecture using Entity Framework Core on PostgreSQL, set engineering standards for the team and protect functional equivalence with the legacy system.

Correctness matters here. Benefit calculations must stay accurate, nightly batch processing must finish within its window and the code must be maintainable for years to come.

C#/.NET Engineering

• Refactor machine-converted, COBOL-derived C# into idiomatic, maintainable .NET with meaningful naming, proper object-oriented and domain design, separation of concerns and dependency injection
• Work in the latest .NET and apply modern C# features such as LINQ, async/await, records, pattern matching, nullable reference types and generics
• Design and build service layers, domain models and ASP.NET Core Web APIs that support an Angular front end
• Refactor legacy batch and procedural logic into maintainable, testable .NET, handling exact decimal and financial arithmetic and rounding rules so benefit calculations stay correct
• Tune performance to meet the nightly batch window through profiling, memory management, async I/O and safe parallelism

Entity Framework Core and Data Layer

• Own the data-access architecture using Entity Framework Core with PostgreSQL (Npgsql provider)
• Refactor the auto-generated data-access layer into a clean, performant EF Core model
• Design entity models, DbContext configuration, Fluent API mappings, relationships, keys and indexes that reflect the migrated schema
• Optimize EF Core queries (eager, lazy and explicit loading, projection, AsNoTracking, compiled queries and batching) to avoid N+1 problems and meet performance targets on high-volume batch work
• Manage EF Core migrations, concurrency handling, transactions and connection resiliency
• Map legacy data types and semantics (packed decimals, fixed-width fields, EBCDIC-origin data and condition codes) to PostgreSQL and EF types without changing behaviour
• Contribute to the data dictionary that links legacy fields to PostgreSQL columns and business terms

Technical Leadership

• Define and enforce C#/.NET coding standards, architecture patterns and refactoring guidelines
• Act as the senior technical authority on the converted codebase and make cross-cutting design decisions
• Mentor mid-level and junior C# developers, run code reviews and raise engineering quality
• Partner with the Technical Manager and Architect on target architecture and refactoring strategy

Quality, Testing and Delivery

• Prove functional equivalence by building unit and regression tests (xUnit or NUnit, Moq) against the legacy baseline, and fix defects found in equivalence testing
• Work with CI/CD on AWS CodeBuild and CodePipeline, and enforce quality and security gates such as SonarQube, static analysis and OWASP secure coding
• Work with business subject matter experts to preserve business logic for eligibility, benefit calculation and payments
• Deliver incrementally within Scrum and document designs and decisions

What You Bring

• 12 to 15 years of professional C#/.NET engineering, including senior or lead responsibilities
• Deep expertise in C# and .NET (.NET Framework and .NET Core / .NET 6 and 8+), including async programming, memory and performance optimization and modern language features
• Expert-level Entity Framework / EF Core: entity modeling, Fluent API, migrations, query optimization, transactions, concurrency and provider-specific behaviour
• Strong relational database skills. PostgreSQL strongly preferred, Oracle or SQL Server acceptable: schema design, indexing, query tuning and stored procedures
• ASP.NET Core Web API (REST) design for front-end integration
• Solid software design: SOLID, design patterns, domain modeling, layered or clean architecture and dependency injection
• Testing discipline across unit, integration and regression testing, TDD and mocking
• Git, CI/CD and static code analysis (SonarQube)
• Proven ability to read, reason about and safely refactor large, unfamiliar codebases
• Exceptional attention to correctness in financial and benefit calculations

Nice to Have

• Legacy modernization experience: COBOL to C# or Java conversion, mainframe migration or automated transformation tooling
• Understanding of mainframe concepts such as COBOL, JCL, batch processing and hierarchical or CODASYL databases (IDS-II, IMS)
• High-volume batch performance tuning
• AWS experience: ECS/EKS, Lambda, RDS for PostgreSQL, S3, CloudWatch and CloudFormation
• Experience with enterprise job schedulers such as Stonebranch, Control-M or CA7
• Government, benefits, healthcare or other regulated, high-correctness domains
• Npgsql specifics, EF Core performance internals or ORM migration experience

Frequently Asked Questions

1. Where is this job located?

Hamilton, New Jersey.

2. Is the role remote?

No. It is fully onsite for the first 30 days, then hybrid with 3 days onsite and 2 days remote.

3. What type of job is it?

A contract role.

4. How much experience is required?

12 to 15 years of professional C#/.NET engineering.

5. What level is this role?

Lead/Principal level, with technical leadership and mentoring.

6. What is the main focus?

Refactoring machine-converted legacy code into clean, production-grade C#/.NET.

7. Is Entity Framework Core required?

Yes, at an expert level.

8. Which database is used?

PostgreSQL. Oracle or SQL Server experience is acceptable.

9. Which cloud platform is involved?

AWS.

10. Do I need COBOL or mainframe experience?

No. It is a nice-to-have that helps you understand the source system.

11. Will I build APIs?

Yes. ASP.NET Core Web APIs that support an Angular front end.

12. Which testing tools are mentioned?

xUnit or NUnit and Moq.

13. Why does correctness matter so much?

The system handles benefit calculations and payments, so results must match the legacy system exactly.

14. Does the role include mentoring?

Yes. You will mentor mid-level and junior developers and lead code reviews.

15. How do I apply?

Email your resume to View email address on click.appcast.io.