.NET / C# Developer

Sergii K

Information

Available hours \ week
40 h/w
Seniority level
Middle
Years of experience
4 yrs.
Location
Ukraine, Kyiv
Nationality
Ukraine
Timezone
(GMT+02:00) Kyiv

Languages

English
Upper-Intermediate (B2)

About

Sergii is a Middle .NET / C# Developer with hands-on experience building secure, production-grade REST APIs, backend services, data-processing solutions and AI-enabled systems. He specializes in C#, .NET / .NET Core, ASP.NET Core Web API, Worker Services, Entity Framework Core, MS SQL Server, PostgreSQL, Cosmos DB, Redis, Docker, Azure and AWS. His backend work covers layered and clean architecture, microservices, JWT and ASP.NET Core Identity, secure configuration through Azure Key Vault, data separation, auditability, observability and controlled execution of business workflows. Sergii has contributed to legacy SaaS modernization: separating business logic from legacy interfaces and exposing it through API services consumed by web applications, external integrations, reporting systems and agentic AI workflows, in mixed .NET Framework and .NET Core environments and without a full rewrite. He works hands-on with AI and Agentic AI: RAG pipelines built on Claude API, OpenAI and LangChain, multi-agent orchestration across Claude Code, Codex, Gemini and local LLMs.

Experience

Multi-Agent Software Development System

.NET / AI Backend Developer

About the Project

Distributed platform of 15 AI agents working in parallel to transform detailed software specifications into production-ready applications, orchestrated across planning, implementation, testing, review and delivery stages.

  • AI

Responsibilities

- Built a distributed platform of 15 AI agents working in parallel to transform detailed software specifications into production-ready applications; - Orchestrated agents across planning, implementation, testing, code review, bug analysis, validation, and delivery stages with clearly defined responsibilities and execution boundaries; - Implemented controlled execution through staged validation, result checks, re-testing loops, and architecture review before changes could progress to delivery; - Used Azure Databricks and MLflow for model lifecycle management, experiment and execution tracking, and evaluation of AI workflows; - Used Vector Search for semantic retrieval and Model Serving for scalable model access across the agent fleet; - Created abstractions for Claude Code, Codex, Gemini, and local LLMs so providers could be selected or replaced without changing orchestration logic; - Validated the platform on two commercial projects, reducing implementation, testing, and review time;

Skills & technologies

Cox Automotive

Full-Stack Developer

About the Project

SaaS platform for the automotive industry built around ASP.NET Core Web APIs and a microservices-oriented architecture, serving internal services, frontend applications and external systems.

  • Automotive

Responsibilities

- Contributed to a SaaS platform built around ASP.NET Core Web APIs and a microservices-oriented architecture; - Designed and implemented REST endpoints consumed by internal services, frontend applications, and external systems; - Separated business functionality from presentation and integration layers, improving maintainability and allowing services to evolve independently; - Implemented data access, validation, error handling, and performance improvements using C#, ASP.NET Core, Entity Framework Core, and MySQL; - Supported secure secrets and configuration management through Azure Key Vault and contributed to CI/CD, SonarQube quality gates, automated testing, and reliable delivery; - Collaborated with the architecture team on scalability, reliability, API contracts, and technology decisions;

MediFlow Systems

Full-Stack Engineer

About the Project

Enterprise healthcare management platform supporting patient management, internal clinical workflows, and coordination between medical and administrative departments.

  • HealthTech

Responsibilities

- Contributed to an enterprise healthcare management platform supporting patient-management and internal clinical workflows; - Developed backend modules and scalable APIs consumed by frontend applications and other system components; - Implemented business rules for patient workflows, clinical operations, and coordination between medical and administrative departments; - Separated healthcare business logic from presentation, data-access, and integration concerns to improve maintainability and reuse across interfaces; - Optimized SQL Server queries and contributed to validation, error handling, secure configuration, code reviews, and production reliability;

Betting Application

.NET Full-Stack Developer

About the Project

Sports-betting and live-event SaaS platform operating in a mixed .NET Framework and .NET Core environment, gradually modernized towards API and service layers without a full rewrite.

  • Gambling

Responsibilities

- Contributed to the development and gradual modernization of a sports-betting and live-event SaaS platform operating in a mixed .NET Framework and .NET Core environment; - Developed scalable REST and GraphQL APIs consumed by web applications, content-management systems, reporting services, and internal business applications; - Exposed business capabilities through API and service layers, reducing direct dependency on legacy interfaces and supporting incremental modernization without a full rewrite; - Implemented functionality for live text commentary, sports-event data, betting information, and dynamic content management for soccer and hockey events; - Used Azure Databricks for large-scale processing and aggregation of match and betting data, with analytical datasets consumed by reporting services and business applications; - Used Cosmos DB for high-scale metadata storage and Azure Key Vault for secure management of credentials and configuration;

Erasmus MC Patient Care

.NET Developer

About the Project

European digital healthcare platform providing online consultations, diagnosis, treatment and follow-up care through web and mobile applications, undergoing gradual modernization from .NET Framework 4.8 to .NET 6.

  • HealthTech

Responsibilities

- Contributed to a European digital healthcare platform providing online consultations, diagnosis, treatment, and follow-up care through web and mobile applications; - Worked across legacy .NET Framework 4.8 components and modern .NET 6 services, supporting gradual platform modernization; - Developed and maintained backend functionality and APIs using .NET, Azure Functions, Azure App Service, Azure Storage, and Azure SQL; - Separated application business logic from user-interface and infrastructure concerns and supported APIs consumed by web and mobile clients; - Monitored production health, failures, dependencies, and performance through Azure Application Insights; investigated incidents and improved logging, error handling, and reliability;

Skills & technologies

MeetPoint

Full-Stack Developer

About the Project

Web application for identifying optimal meetup locations for groups of users, built as a clean layered monolith with real-time sessions and external geolocation integrations.

  • Management

Responsibilities

- Architected a clean monolithic web application for identifying optimal meetup locations for groups of users; - Separated business logic, data access, authentication, real-time communication, and external Google Maps integrations using a layered architecture; - Implemented REST APIs, JWT-based authentication and authorization, SignalR sessions, database schemas, caching, validation, and unit tests; - Initialized and maintained project infrastructure and participated in planning, code reviews, and team coding standards;