Principal C#/.NET Engineer, Mainframe Modernization & EF Core

Rose International; Trenton (NJ); On-site; USD 160,000 - 210,000; Full time

Job summary

Rose International in Hamilton, NJ seeks a Principal C#/.NET Engineer to lead modernization of a large mainframe-based benefits system. You will refactor COBOL-derived code into clean .NET, define EF Core data access against PostgreSQL, and set engineering standards while ensuring functional equivalence with the legacy system.

Onsite in Hamilton, NJ is required. The role emphasizes high-performance, nightly batch window optimization, and mentoring junior developers within a Scrum environment.

Qualifications

• 12-15 years of professional C#/.NET software engineering, including senior/lead responsibilities.
• C# Programming Experience (10 Years)
• .NET Core / .NET 6/7/8/10 Experience (10 Years)
• .NET REST/WEB API Development Experience (5 Years)
• Entity Framework Core and LINQ and async/await patterns Experience (10 Years)
• Solid software design: SOLID, design patterns, domain modeling, layered/clean architecture, dependency injection (5 Years)
• Angular 17+, Typescript and Node.js experience (5 Years)
• Strong relational database skills; PostgreSQL strongly preferred (Oracle/SQL Server acceptable) — schema design, indexing, query tuning, stored proc (5 Years)
• unit/integration/regression testing frameworks; TDD/mocking (5 Years)
• Git and static code analysis (SonarQube) Required 5 Years
• Docker containerization and CI/CD pipeline experience (AWS CODEPIPELINE OR GitHub Actions) (3 Years)
• AWS cloud native applications (S3, SNS, SQS, ECS, RDS, EC2, Lambda) (3 Years)

Responsibilities

• Refactor COBOL-derived C# into idiomatic .NET with strong OO/domain design and DI.
• Work in .NET Core / latest version; use LINQ, async/await, records, pattern matching, nullable types, generics.
• Design and implement service layers, domain models, and ASP.NET Core Web APIs backing the Angular UI.
• Refactor legacy batch/procedural logic into maintainable code; preserve decimal/financial accuracy.
• Optimize performance to meet nightly batch window; profiling, memory management, async I/O, parallelism where safe.

Skills

C# / .NET

Senior / Lead

EF Core

ASP.NET Core Web API

PostgreSQL

SQL Server

LINQ / async

Angular / TS

Docker / CI

AWS

Tools

Git

SonarQube

AWS CodePipeline

Docker

Npgsql

Job description

***Only qualified Principal C#/.NET Engineer – Mainframe Modernization & EF Core candidates located at the Hamilton, NJ area to be considered due to the position requiring an onsite presence***

Required Skills

• 12-15 years of professional C#/.NET software engineering, including senior/lead responsibilities
• C# Programming Experience (10 Years)
• .NET Core / .NET 6/7/8/10 Experience (10 Years)
• .NET REST/WEB API Development Experience (5 Years)
• Entity Framework Core and LINQ and async/await patterns Experience (10 Years)
• Solid software design: SOLID, design patterns, domain modeling, layered/clean architecture, dependency injection (5 Years)
• Angular 17+, Typescript and Node.js experience (5 Years)
• Strong relational database skills; PostgreSQL strongly preferred (Oracle/SQL Server acceptable) — schema design, indexing, query tuning, stored proc (5 Years)
• unit/integration/regression testing frameworks; TDD/mocking (5 Years)
• Git and static code analysis (SonarQube) Required 5 Years
• Docker containerization and CI/CD pipeline experience (AWS CODEPIPELINE OR GitHub Actions) (3 Years)
• AWS cloud native applications (S3, SNS, SQS, ECS, RDS, EC2, Lambda) (3 Years)

Preferred Skills

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

Summary: We are modernizing a large, mission-critical Honeywell Bull mainframe benefits system (FAMIS) — converting ~1.5M lines of COBOL and ~160K lines of JCL to C#/.NET, migrating an IDS-II hierarchical database to PostgreSQL, and deploying on AWS. Converted code is delivered by an automated modernization vendor in an "as-is" state. This Lead/Principal role owns the refactoring of that converted code into clean, maintainable, high-performance, production-grade C#/.NET, defines the data-access architecture using Entity Framework Core against PostgreSQL, sets engineering standards for the team, and safeguards functional equivalence to the legacy system. This is a benefits system serving NJ families — correctness, performance within the nightly batch window, and maintainability are paramount.

Responsibilities

C# / .NET Engineering

• Refactor machine-converted, COBOL-derived C# into idiomatic, maintainable .NET — meaningful naming, proper OO/domain design, separation of concerns, dependency injection
• Work in .NET (Core) / .NET (latest version); apply modern C# language features (LINQ, async/await, records, pattern matching, nullable reference types, generics)
• Design and implement service layers, domain models, and APIs (ASP.NET Core Web API) that back the Angular UI
• Refactor legacy batch/procedural logic (converted from COBOL/JCL) into maintainable, testable .NET; handle exact decimal/financial arithmetic (decimal, rounding rules) to preserve benefit-calculation correctness
• Optimize performance to meet the nightly batch window (profiling, memory management, async I/O, parallelism where safe)

Entity Framework / Data Layer

• Own the data-access architecture using Entity Framework Core against PostgreSQL (Npgsql provider)
• Refactor the auto-generated/converted data-access layer into a clean, performant EF Core model
• Design entity models, DbContext configuration, Fluent API mappings, relationships, keys, and indexes reflecting the IDS-II ? PostgreSQL schema (168 tables, 4,024+ fields)
• Optimize EF Core queries (eager/lazy/explicit loading, projection, AsNoTracking, compiled queries, batching) to avoid N+1 and meet performance SLAs on high-volume batch operations
• Manage EF Core migrations, concurrency handling, transactions, and connection resiliency
• Map legacy data types and semantics correctly (packed decimals, fixed-width, EBCDIC-origin data, condition codes) to PostgreSQL/EF types without behavioral drift
• Contribute to the COBOL-field ? PostgreSQL-column ? business-term data dictionary

Technical Leadership

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

Quality, Testing & Delivery

• Ensure functional equivalence: build unit and regression tests (xUnit/NUnit, Moq) to validate refactored code against the legacy baseline; fix FET-surfaced defects
• Integrate with CI/CD (AWS CodeBuild/CodePipeline), enforce quality/security gates (SonarQube, static analysis, secure coding/OWASP)
• Collaborate with FAMIS SMEs to preserve business logic (eligibility, benefit calculation, payments, EBT)
• Deliver incrementally within Scrum; document design and decisions

***In person interview required***

• **Only those lawfully authorized to work in the designated country associated with the position will be considered.**
• **Please note that all Position start dates and duration are estimates and may be reduced or lengthened based upon a client’s business needs and requirements.**

About Rose

• Founded in 1993
• Office Locations Across the U.S.
• 150+ Clients: Corporations and Government Agencies
• Employee Oriented Company
• Challenging Assignments Across the U.S.
• Continuous Professional Development

EMPLOYEE COMMENTS

• We want you to work with us, but don't take our word for it. Take a look at this sampling of employee comments. They speak for themselves.