Sure, are you looking for a tool to teach C# programming or a tool to help you create C# programming assignments?

I recommend Grok. It's an online platform that specializes in teaching programming and computer science. It offers a variety of courses and resources, including problem-based learning, interactive notes, and auto-marking features that provide instant feedback to learners.

Key features for teachers include:

1. Courses and Competitions: Helps teachers confidently bring coding into classrooms with varying skill levels.
2. Problem-Based Learning: Matches each new concept with a problem so students learn by doing.
3. Automarker: Breaks each problem down into tests and gives students useful feedback.
4. Teacher Verification: Gives teachers access to sign up students and manage student accounts at their school. Verified teachers also receive free access to all of Grok's materials.
5. Professional Development Opportunities: Provides step-by-step walkthroughs for each problem, making it a valuable resource for teachers who are not too confident with coding.

This tool can be used for collaborative coding projects, cross-curricular integration, and organizing coding competitions and events.