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.