Taux journalier (TJM): 430

Backend Developer C# / .NET

Mission Context

As part of the continued growth of its technical teams, a B2B SaaS software company is looking for several Senior Backend Developers with strong C# and .NET expertise.

The assignment takes place within a complex product environment built on a large existing codebase and a set of APIs used by multiple internal systems and external integration partners.

The consultant will mainly work on features related to the integration, transformation, and storage of customer product data. Particular attention will be paid to data quality, software reliability, and the prevention of regressions affecting existing API contracts.

The consultant will join an established technical environment and contribute to a codebase owned by another product team. The ability to integrate quickly, follow existing conventions, and work constructively with the owning team will therefore be essential.

Mission Objectives

The main objective is to contribute to the development and reliability of backend services used to process and store customer product data.

The consultant will be expected to:

• Develop new features in accordance with published API contracts.

• Ensure the integrity and quality of stored data.

• Prevent regressions that could affect external integrators or consuming systems.

• Strengthen automated test coverage.

• Contribute effectively to an established codebase and development framework.

• Deliver reliable and maintainable backend components.

Key Responsibilities

Backend Development in C# / .NET

• Design and develop backend components using C# and ASP.NET Core.

• Work within a large and mature existing codebase.

• Implement new features while following the architecture and technical conventions already in place.

• Develop commands, handlers, and processing workflows based on CQRS principles.

• Participate in code reviews and incorporate feedback from the team that owns the codebase.

• Maintain a high standard of code quality, readability, and maintainability.

API Contract Implementation

• Implement features in accordance with published API contracts and technical specifications.

• Understand the impact of changes on external consumers and integration partners.

• Preserve backward compatibility whenever required.

• Identify changes that could break an existing integration.

• Strictly respect expected data formats, validation rules, and API behaviours.

• Raise potential compatibility risks before implementation.

Product Data Processing and Transformation

• Develop mapping logic between incoming customer data and internal domain models.

• Implement business transformation and validation rules.

• Store and retrieve data using a document-oriented database.

• Identify and prevent silent data corruption or incorrect mappings.

• Ensure data consistency throughout the processing lifecycle.

• Investigate discrepancies between expected and stored product data.

Testing and Software Quality

• Develop integration tests covering key business and technical scenarios.

• Implement round-trip tests to verify that data remains consistent after being written and retrieved.

• Test mappings, transformations, validation rules, and persistence logic.

• Detect functional defects that may not generate visible technical errors.

• Treat automated testing as an integral part of delivery.

• Contribute to the continuous improvement of the testing strategy.

• Prevent regressions through reliable and meaningful test coverage.

Collaboration with Existing Teams

• Integrate effectively into an established engineering environment.

• Follow existing development, architecture, naming, and code review conventions.

• Work closely with the team responsible for the codebase.

• Accept and incorporate technical feedback constructively.

• Avoid unnecessary changes to established patterns and practices.

• Document developments where necessary to support long-term maintainability.

• Communicate clearly about progress, technical risks, and implementation decisions.

AI-Assisted Development

• Use AI-assisted development tools effectively and responsibly.

• Work with tools such as Claude Code, GitHub Copilot, or equivalent solutions.

• Use AI tools to support code analysis, test generation, refactoring, debugging, and documentation.

• Systematically review and validate AI-generated code.

• Remain fully accountable for the quality, security, and correctness of all delivered work.

Technical Environment

• C#

• .NET

• ASP.NET Core

• CQRS

• MediatR or similar command-handler patterns

• REST APIs

• Document-oriented databases

• RavenDB

• MongoDB or Azure Cosmos DB

• Integration testing

• Round-trip testing

• Git

• Code review workflows

• Claude Code

• GitHub Copilot

Required Profile

• Strong professional experience in backend development using C# and .NET.

• Advanced knowledge of ASP.NET Core.

• Proven experience working within large or complex existing codebases.

• Good understanding of CQRS architectures.

• Experience with MediatR or similar command-handler patterns.

• Hands-on experience with a document-oriented database.

• RavenDB experience would be highly valuable.

• Experience with MongoDB or Azure Cosmos DB is also relevant and transferable.

• Proven experience implementing APIs from published contracts or specifications.

• Ability to anticipate the impact of API changes on external integrators.

• Strong automated testing mindset.

• Experience with integration testing and data quality issues.

• Ability to detect silent errors in mapping, transformation, and persistence logic.

• Regular use of AI-assisted development tools.

• Ability to work independently while respecting the standards of an existing team.

Personal Qualities

• Strong attention to detail.

• High level of technical discipline.

• Strong awareness of data quality and regression risks.

• Analytical and problem-solving mindset.

• Ability to understand complex functional and technical requirements.

• Constructive attitude towards code review feedback.

• Clear communication with technical stakeholders.

• Pragmatic, autonomous, and delivery-oriented approach.

• Ability to contribute effectively to a codebase owned by another team.

Mission Details

• Contract type: Freelance

• Start date: ASAP

• Duration: Until the end of December 2026

• Location: Fully remote

• Workload: Full-time

• Working environment: International and English-speaking