Custom Task-like Types (Extending async in C#)

Task is the default, but it’s not the only way to be awaitable. This post explains how C#‘s “awaitable pattern” lets you build custom task-like types for performance, control, and domain-specific async behavior.

What you’ll learn

• Why you’d want a custom awaitable: avoiding heap allocations, controlling scheduling/thread context, and integrating with other platforms
• The exact awaitable pattern — GetAwaiter(), INotifyCompletion, IsCompleted, GetResult(), and OnCompleted(Action) — with no inheritance required
• Building a minimal MyAwaitable/awaiter step by step, and how IsCompleted, OnCompleted, and GetResult interact
• How ValueTask<T> and struct-based awaitables reduce GC pressure in high-throughput code
• Real-world use cases (game engines, high-performance servers, embedded/IoT, testing) and the caveats around debugging and complexity

Worth a read if you want to level up your understanding of how async/await really works under the hood.