Lead/Principal C#/.NET Engineer

Mbi Llc; Hamilton (NJ); On-site; USD 140,000 - 190,000; Full time

Job summary

Mbi Llc is seeking a Lead/Principal C#/.NET engineer to own refactoring of a COBOL-derived codebase into clean, production-grade C#/.NET. You will define the data-access architecture using EF Core against PostgreSQL and set engineering standards for the team while ensuring functional equivalence to the legacy system.

You will lead the modernization of a large, mission-critical benefits system for NJ families, focusing on correctness, nightly batch performance, and maintainability within a

Qualifications

• 12-15 years of professional C#/.NET software engineering with senior/lead responsibilities.
• Deep expertise in C# and the .NET platform, including async programming and memory/performance optimization.
• Expert-level EF Core: modeling, migrations, queries, transactions and concurrency.
• Strong relational database skills; PostgreSQL strongly preferred; Oracle/SQL Server acceptable.

Responsibilities

• Refactor COBOL-derived C# into idiomatic, maintainable .NET with good OO design.
• Design and implement service layers, domain models, and APIs to back the UI.
• Own data-access architecture using EF Core against PostgreSQL and optimize queries.
• Define and enforce coding standards and refactoring guidelines for the team.
• Ensure functional equivalence with legacy system and build tests (unit/integration).
• Deliver iteratively within Scrum and participate in CI/CD pipelines.

Skills

C#/.NET

.NET Core / .NET 6/7/8+

EF Core / LINQ / async

REST API design

Git

CI/CD

AWS

PostgreSQL

Performance optimization

Npgsql

Tools

ASP.NET Core

PostgreSQL

Job description

- 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.

Key Responsibilities

• 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
• 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

Required Qualifications

• 12-15 years of professional C#/.NET software engineering, including senior/lead responsibilities
• Deep expertise in C# and the .NET platform (.NET Framework and .NET Core / .NET 6/8+), including async programming, memory/performance optimization, and modern language features
• Expert-level Entity Framework / EF Core: entity modeling, Fluent API, migrations, query optimization, transactions, concurrency, provider-specific behavior
• Strong relational database skills; PostgreSQL strongly preferred (Oracle/SQL Server acceptable) — schema design, indexing, query tuning, stored procedures
• ASP.NET Core Web API design (REST) for front-end integration
• Testing discipline: unit/integration/regression testing frameworks; TDD/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 — financial/benefit calculation accuracy

Preferred / Nice to Have

• 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

Success in the First 90 Days

• Establish the C#/.NET and EF Core standards, patterns, and reference implementation the team will follow
• Refactor a representative set of converted programs (including a benefit-calculation and a batch program) to clean, tested, performant C# validated against the legacy baseline
• Define the EF Core data-access approach against PostgreSQL, including query-performance guardrails for batch workloads
• Mentor the team and set the code-review/quality bar

Fill the skill matrix below:

Skill

Amount

Candidate's No. of years of experience

C# Programming Experience

Required

10

.NET Core / .NET 6/7/8/10 Experience

Required

10

Required

5

Entity Framework Core and LINQ and async/await patterns Experince

Required

10

Required

5

Angular 17+, Typescript and Node.js experience

Required

5

Strong relational database skills; PostgreSQL strongly preferred (Oracle/SQL Server acceptable) — schema design, indexing, query tuning, stored procedure

Required

5

unit/integration/regression testing frameworks; TDD/mocking

Required

5

Git and static code analysis (SonarQube)

Required

5

Docker containerization and CI/CD pipeline experience (AWS CODEPIPELINE OR GitHub Actions)

Required

3

AWS cloud native applications (S3, SNS, SQS, ECS, RDS, EC2, Lambda)

Required

3