C# is missing discriminated unions, and it has a flawed async programming model. Namely, there are multiple models (for historical reasons / backwards compatibility), and the more current one (task-based) throws a wrench in your ability to effectively design interfaces, functions, delegates etc. that can be shared between synchronous and asynchronous code. Green threads would have fixed this, at the cost of some other potential issues, but it looks we’re stuck with tasks for now.