Senior Software Engineer (.NET / Avalonia / ASP.NET Core)

Role: Senior Software Engineer (.NET / Avalonia / ASP.NET Core) Outside IR35 Location: Mostly remote for a Somerset based client Start: Within the next two weeks Overview Supporting a technology partner, we are looking for an experienced Senior Software Engineer to deliver a defined package of new software development work across a modern cross-platform application suite. The engagement will focus on delivering new functionality across desktop, backend, API, and tooling components, whilst improving maintainability, testability, and platform consistency. Essential Skills & Experience • Strong commercial C# and .NET development experience • Experience developing ASP.NET Core Web APIs and backend services • Commercial experience developing desktop applications using Avalonia UI (or extensive WPF/XAML experience with the ability to adopt Avalonia) • Strong understanding of MVVM architecture • Experience with PostgreSQL, Dapper, and database-driven applications • Experience working with JSON serialization, REST APIs, and OpenAPI/Swagger • Experience implementing real-time communications using SignalR or similar technologies • Strong unit and integration testing experience using xUnit, Moq, or similar frameworks • Experience developing cross-platform applications for Windows and Linux • Strong understanding of modern software engineering practices, code quality, and CI/CD workflows Desirable Skills • Experience with data visualisation or plotting libraries • Understanding of networking and communication protocols • Experience modernising or migrating legacy desktop applications • Experience porting applications from Java/Swing, C/C++, or other legacy technology stacks • Experience working across both desktop and backend software projects Technology Stack • C# / .NET • ASP.NET Core • Avalonia UI • PostgreSQL • Dapper • SignalR • Swagger / OpenAPI • xUnit, Moq & Coverlet • Git & CI/CD Pipelines Package of Work Context This engagement is structured around defined software development work packages. The successful contractor will be responsible for taking agreed features and enhancements through design, implementation, testing, and delivery across desktop applications, backend services, APIs, and supporting tooling. Indicative Work Packages • Deliver defined software enhancements across desktop, backend, tooling, and API components • Build new application functionality within an established .NET architecture • Develop and extend ASP.NET Core services and REST APIs • Implement database-driven functionality using PostgreSQL and Dapper • Develop and maintain desktop UI features using Avalonia and MVVM principles • Add or improve automated unit and integration tests for new functionality • Contribute technical input to solution design, implementation, and delivery planning • Produce clean, maintainable code together with supporting documentation and handover information for completed deliverables Contractor Profile A hands-on Senior Software Engineer with strong modern .NET experience, capable of taking ownership of defined technical deliverables across desktop, backend, API, and tooling projects. The successful contractor will be comfortable working across both modern and legacy technologies, delivering high-quality, maintainable software with minimal supervision.