C# Developer – Experienced / Senior – Full-Stack, .NET

Koesio

. Design and develop new features in C#/.NET following Clean Architecture and Vertical Slice principles .

Posted 11/10/2026full-timeValence • FranceSenior💰 €35,000 - €45,000 per yearWebsite

Core Competencies

Role fit

Core Competencies

Use this summary to align your resume positioning with the role.

Demonstrates expertise in backend .NET development, particularly in C# and ASP.NET, while adhering to Clean Architecture principles. Proficient in delivering secure and efficient REST APIs and managing SQL Server databases within an ISO 27001-certified environment.

Highest-signal resume keywords

C# DevelopmentASP.NET ProficiencySQL Server ExpertiseREST API DevelopmentGitLab CI/CD

ATS Keywords

Tailor your resume

Applicant Tracking System Keywords

Tip: use these terms in your resume and cover letter to boost ATS matches.

Hard Skills

C#ASP.NETSQL ServerREST APIsUnit TestingIntegration TestingDependency InjectionData ModelingOpenAPI/SwaggerPDF Document Processing

Soft Skills

CollaborationAnalytical ThinkingProblem Solving

Tools & Technologies

Visual Studio 2022GitGitLab CI/CDMSTestElasticsearchNLog

Certifications & Qualifications

ISO 27001 Certification

Industry Keywords

Clean ArchitectureVertical Slice PrinciplesE-InvoicingFactur-XUBLCIIPeppolElectronic Document ManagementArchiving

Tech Stack

Tools & technologies

ASP.NETElasticSearchSQL.NET

About the role

Key responsibilities & impact

Design and develop new features in C#/.NET following Clean Architecture and Vertical Slice principles
Evolve a platform structured as a modular monolith, with business modules deployed as independent services
Work with Factur-X (PDF/A-3), UBL, CII, DGFiP XSD schemas, PDF documents, and OCR
Expose and document REST Web APIs using OpenAPI/Swagger
Analyze complex topics related to regulations, standards, and integrations with third-party platforms
Work with the Architect on design, performance, and security
Collaborate with the Product Owner on requirements
Deliver continuously through merge requests, code reviews, GitLab CI/CD, and deployments to Windows servers
Contribute to an ISO 27001-certified environment through traceability, security reviews, and change management

Requirements

What you’ll need

3–5 years of experience or more in backend .NET development
Proficiency in C# and .NET Framework 4.7.2
Proficiency in ASP.NET and Web API (OWIN)
Proficiency in SQL Server, including data modeling, queries, and stored procedures
Knowledge of dependency injection and scheduled tasks
Experience with unit and integration testing using MSTest
Proficiency in Git, merge requests, and Visual Studio 2022
Proficiency in REST APIs, data modeling, and SQL Server
Interest in software architecture, modules, domain boundaries, and design patterns
Comfortable working across the entire development lifecycle, from design through deployment
Knowledge of e-invoicing, Factur-X, UBL, CII, and Peppol is a plus
Knowledge of PDF document processing, electronic document management (EDM), and archiving is a plus
Knowledge of modern .NET (.NET 8+), GitLab CI/CD, Elasticsearch/NLog, and AI-assisted development is a plus

Benefits

Comp & perks

Profit-sharing bonus and Group employee share ownership plan
Meal vouchers
RTT (reduced working time) days
Holiday vouchers
1 day of remote work per week