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

Job Description: Lead/Principal C#/.NET Engineer with expert-level Entity Framework Core experience to lead the modernization of a mission-critical legacy benefits system hosted on AWS Cloud .

Position 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.







Key Responsibilities:







C# / .NET Engineering : Refactor machine-converted, COBOL-derived C# into idiomatic, maintainable .NET - meaningful naming, proper OO/domain design, separation of concern s, 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.

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. Solid software design: SOLID, design patterns, domain modeling, layered/clean architecture, dependency injection. 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, inc