Developer .NET (Medior)

OneSource Consulting; Brussel Hoofdstad; On-site; EUR 60,000 - 90,000; Full time

Job summary

OneSource Consulting is seeking a Backend Developer (C# / .NET) to design, build and maintain high-performance backend services. You will work in a modular ASP.NET Core architecture, creating APIs, data models, and integrations while collaborating with frontend teams on contracts.

You will ensure secure, scalable backend solutions with EF Core, MSSQL and monitoring tools, plus CI/CD in a Kubernetes environment. Proficient Dutch (CEFR C2) is required.

Qualifications

• Proven experience in .NET-based applications.
• Proven training with certification or equivalent in C#, .NET, ASP.NET Core.
• Able to analyze, code, test and debug independently.
• Experience with Agile development methodologies.
• Dutch language at CEFR level C2.
• Strong knowledge of C# and ASP.NET Core.
• Experience with Entity Framework Core and relational databases (MSSQL).
• Experience in developing and consuming REST APIs.
• Understanding modular software architecture and shared vs domain-specific code.
• Experience with Git and version control; automated testing knowledge.

Responsibilities

• Develop and maintain backend in C# and ASP.NET Core.
• Design and maintain REST APIs and data models.
• Work with EF Core and MSSQL for data processing.
• Integrate internal/external APIs and ensure data accessibility for frontend.
• Collaborate with frontend on API contracts and data models.
• Ensure secure software development and OWASP-compliant input validation.
• Write and maintain unit/integration tests; debug and resolve issues.
• Monitor, optimize performance, stability, security and code quality.
• Document code, APIs and key technical choices.
• Contribute to modular backend architecture and reusable components.

Skills

Git

C#

ASP.NET Core

Entity Framework Core

REST APIs

SQL / MSSQL

Agile development

OWASP security

Unit/integration testing

Modular architecture

Elasticsearch

Hangfire / background jobs

Docker/Kubernetes

GitLab CI/CD

Vue.js (front-end collaboration)

Tools

Git

Elasticsearch

Elastic APM

Hangfire

Docker

Kubernetes

GitLab CI/CD

Vue.js

Entity Framework Core

SQL Server (MSSQL)

Job description

CONTRACT DURATION: ASAP - 04/11/2027+ Possible of Extension

Job description – Backend Developer (C# / .NET)

About the position

• As a Backend Developer, you will be responsible for developing, extending and maintaining high-performance, secure and scalable backend functionalities within our C# / ASP.NET Core architecture.
• You work within a modular architecture in which generic functionalities are housed in shared libraries that are reused across different applications. In addition, you develop application- and domain-specific functionalities.
• You translate functional needs into clear data models, services and APIs and ensure that data is made available on the front-end in a consistent and actionable way. You will work closely with front-end developers on API contracts and data models.

Primary responsibilities

• Developing and maintaining backend functionalities in C# and ASP.NET Core.
• Developing new features and modules within both the shared libraries and the application-specific layer. Designing, developing and maintaining REST APIs.
• Setting up and maintaining data models and data processing via Entity Framework Core and MSSQL.
• Integrate and consume internal and external APIs.
• Converting and structuring external data into usable data models for the application.
• Align with front-end developers on API contracts, data models, validations, and error handling.
• Developing robust backend functionalities that correctly deal with erroneous, incomplete or unexpected input.
• Writing and maintaining unit and integration tests.
• Testing, debugging, and resolving technical issues.
• Monitoring and optimizing performance, stability, security and code quality.
• Apply principles for secure software development, including input validation and relevant OWASP guidelines.
• Documenting code, APIs and key technical choices.

Secondary responsibilities

• Contribute to the further development and improvement of the modular back-end architecture.
• Contribute to the further standardization and reusability of common components.
• Performing and participating in code reviews.
• Thinking along about architectural and technical choices.
• Integrating Elasticsearch for search functionalities.
• Use Elastic APM for tracing and monitoring.
• Implement and maintain background processes and planned tasks, for example via Hangfire.
• Collaborate within the existing GitLab CI/CD environment and deployment to Kubernetes.

Your profile

• Proven experience in the development of .NET-based applications.
• Proven training with certification or equivalent through experience in programming languages and environments such as C#, .NET and ASP.NET Core.
• Demonstrable experience to carry out programming assignments smoothly: analyzing, coding, testing and debugging, independently if necessary.
• Demonstrable knowledge of and experience with Agile development methodologies.
• Language requirement: Dutch at European CEFRlevel C2.
• Strong knowledge of C# and ASP.NET Core.
• Experience with Entity Framework Core or a similar ORM.
• Experience with relational databases and SQL, preferably MSSQL.
• Experience in developing and consuming REST APIs and processing JSON.
• Understanding modular software architecture and the separation between shared and domain-specific functionalities.
• Experience with Git and version control.
• Experience with automated testing, including unit and integration testing.
• Attention to software security, input validation, error handling and OWASP principles.
• Able to independently analyze technical problems and arrive at a maintainable solution.
• Pros: Experience with Elastic/Elasticsearch, Elastic APM, Hangfire, GitLab CI/CD, Docker/Kubernetes and/or Vue.js.

Skills

• Proven experience working with Git.
• Proven experience in developing .NET-based applications.
• Demonstrated experience with Entity Framework (or a similar ORM) and relational databases.
• Proven experience with automated testing (unit and integration testing).
• Proven experience in setting up and consuming REST APIs.
• Demonstrable knowledge and attention to security and input validation (OWASP awareness).
• Demonstrable knowledge of C# and ASP.NET Core.
• Demonstrated knowledge of modular architecture: separation between shared and domain-specific code.
• Demonstrable training with certification or equivalent through experience in programming languages and environments such as .Net (C#), ASP.NET, .
• Language requirement: Dutch at European CEFR - level C2.
• Be able to perform demonstrable experience in programming assignments very smoothly: coding, testing and debugging. If necessary, also completely independent (e.g. maintenance).
• Proven experience with background tasks/scheduling (e.g. Hang fire).
• Proven experience with Elastic.
• Demonstrable knowledge and experience of Agile development methodology.
• Demonstrable knowledge of front-end (Vue.js), to ensure smooth collaboration with front-end developers