A C# developer designs, develops, and implements applications and software using C# language. They troubleshoot issues, optimize code for performance, and ensure software compatibility and stability. They also participate in the full development life cycle, from conception to deployment. On the other hand, a Core Java developer uses Java language to develop enterprise-level applications. They design and develop high-volume, low-latency applications for mission-critical systems, ensuring high-availability and performance. They also contribute to all phases of the development lifecycle, writing well-designed, testable, and efficient code. Both roles require constant learning due to evolving technologies.