Fullstack .NET Developer

Job Title: Fullstack .NET Developer

Client: Agency of the United Nations

Location: Vienna, Austria (On-site – no remote work allowed)

Type of Engagement: Freelance / T&M consultancy – initial 6 months, renewable (long-term engagement)

Rate: Max 400 euro per day all inclusive

Start: ASAP

Relocation: no relocation support or visa sponsorship provided For an Agency of the United Nations, we are looking for a Fullstack .NET Developer to design, develop and maintain secure integration components and plugins within an enterprise system environment. The role focuses on enterprise-grade .NET (C#) development, system interoperability and secure data exchange with external governmental and financial intelligence systems.

Main Responsibilities:

Analyze business and technical requirements provided by stakeholders and system architects
Review external system interfaces (APIs, databases, XML feeds) to determine the integration approach
Design integration solutions using .NET (C#) aligned with the system architecture
Define the data flow between the platform and external systems, object models (POCOs) and mapping strategies, and the authentication and security approach (tokens, certificates, etc.)

Hard Skills:

Minimum 5 years of professional experience in software development, including at least 3 years of hands-on C# (.NET) development
Minimum 2–3 years of hands-on DevOps experience in enterprise environments
Experience in secure software development for sensitive data environments
Object-oriented design and POCO modelling: DTO ↔ domain model separation, layered architecture, SOLID principles and clean code; POCO-to-POCO mapping, both manual and with frameworks such as AutoMapper; schema-driven models
HTTP-based network programming in .NET: HttpClient, HttpRequestMessage / HttpResponseMessage lifecycle, synchronous and asynchronous programming (async/await)
Integration with RESTful APIs and SOAP services, including retry policies, timeout handling, connection resilience and proxy configuration in secure government environments
Secure authentication in .NET: OAuth 2.0, JWT handling and validation, API key authentication, token lifecycle management, secure storage of secrets, bearer tokens on outbound requests
Secure communication: TLS/HTTPS enforcement, certificate validation and handling
JSON and XML serialization/deserialization, mapping between external payloads and internal POCO models, validation of structured data before transmission
Familiarity with logging, auditing and traceability in integration systems (preferred)
University degree (Bachelor's or equivalent) in computer science, information systems or a related field

Soft skills:

Fluency in English is required; knowledge of another UN official language is desirable
Ability to analyze requirements and work closely with stakeholders and system architects

The ideal candidate is an experienced C# / .NET developer with a strong background in secure system integrations, available to work onsite in Vienna.

What do we expect from you?

We expect enthusiasm and creative solutions. Active contribution to nurture the Community with research, scouting and concept of new solutions will be required.

OUR Client

Agency of the United Nations based in Vienna.

Stellenbeschreibung