A C# .NET developer is responsible for designing, implementing and maintaining software applications using Microsoft's .NET framework and C# language. They create server-side applications, web applications, and provide solutions for business needs. Their duties also include debugging, troubleshooting, and improving application performance. They often collaborate with other team members to integrate .NET applications with other systems. Additionally, they are involved in the entire software development life cycle, including requirement analysis, design, development, testing, deployment, and maintenance. Their work can greatly influence a business's operational efficiency and effectiveness.