Backend Developer C# / .NET – ASP.NET, SQL Server (m/f/d)

Who We Are

Rockstardevelopers, founded in 2015 in Mannheim, with further offices in Stuttgart and Munich. We are a software development company with experience from hundreds of individual projects for enterprise and medium-sized clients.

Short communication lines, honest feedback, no political games. No hierarchy levels between you and the people making decisions.

Your Responsibilities

You will work on client projects at large companies, embedded in their teams. Your focus will be on the backend with C# and .NET: business logic, databases, and interfaces to other systems.

• Backend development with C# and .NET, web applications with ASP.NET
• Design data models and implement them with Microsoft SQL Server and T-SQL, including performance optimization
• Develop interfaces and integrate systems
• Further develop existing applications, for example, billing and financial systems or identity management solutions
• Generate, review, and refactor code with Claude Code, Copilot, or Cursor
• Coordinate with product owners, business departments, and operations

Assignments are predominantly remote. Client appointments are usually in Frankfurt, with occasional on-site days. Those who wish can also come to one of our offices. This is not mandatory.

Where You Will Land

Depending on the project, you will work with large corporations, in the public sector, or with enterprise and medium-sized clients from various industries. Our clients almost exclusively fill these projects with permanent employees. That's why we are looking for you in permanent employment.

What You Bring

At least three years of professional experience as a developer, counted after vocational training or university studies. For some projects, we are looking for senior developers with more than five years of experience.

We expect:

• C# and .NET, at least three years productively used in the last five years
• Microsoft SQL Server and T-SQL, including data modeling
• Web applications with ASP.NET
• Git, automated tests, and clean handover to operations and team

Advantageous (not a must):

• One Identity Manager (backend, process designer) or other Identity and Access Management systems
• SAP user and role management or SAP connectors
• Financial, billing, or controlling systems
• Experience in the environment of railway or transport companies
• LLM integration into applications (RAG, Tool Calling, Agents), even from a side project
• React or another frontend framework

You don't have to know everything. What's important is that you are proficient in .NET backend and SQL Server and can quickly learn new domain knowledge.

Project Language is German

You will work in the client's team, in German, in meetings and in documentation. We expect German at C1 level or higher.

AI Tools Are Mandatory, Not a Bonus

Claude Code, Copilot, Cursor: You must be able to work productively with them. Our clients expect it, and that's how we work. Internally, we invest in your AI skills.

What We Offer

• Predominantly remote, plus office if you like
• MacBook (unless the client provides their own hardware)
• Various clients, various projects
• The founders are your direct contact persons
• Internal upskilling in AI development

Application

Curriculum Vitae is sufficient. No cover letter needed. After a short video question round (2x90 seconds), we will get back to you within a few days.

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