Backend Developer

SIENN Poland is working on an international project in the financial sector for a Dutch client specializing in audits, accounting, payroll, HR, and tax advisory. The project involves supporting the development and modernization of an existing accounting application.

• Strong experience with C# and ASP.NET Core
• Commercial experience with .NET (preferably .NET 8+)
• Experience in modernizing legacy applications (ASP.NET Framework 4.8 / WebForms / WCF) and migrating them to modern architectures (.NET 10)
• Solid experience in designing and building REST APIs and service-based architectures (API-first, domain-driven design)
• Good knowledge of MS SQL Server / Azure SQL
• Hands-on experience with Microsoft Azure, especially Web Apps, Container Apps, Service Bus, Key Vault
• Strong analytical skills and ability to work independently
• Team player with experience working in an Agile / Scrum environment
• Proactive mindset with a focus on continuous improvement and system modernization
• Fluent English (spoken and written)

Nice to have:

• Experience with Docker (containerization)
• Experience in upgrading legacy systems
• React.js and experience in ERP environments

• Build and modernize backend services using ASP .NET Core MVC and .NET 8+
• Participate in migration to Microsoft Azure
• Ensure code quality and scalability
• Collaborate with international teams