When you mark a method async, the Roslyn compiler transforms the method into an IAsyncStateMachine struct behind the scenes.

public async Task<string> FetchDataAsync()
{
    var result = await _httpClient.GetStringAsync("https://api.example.com");
    return result.ToUpper();
}

Internal Lowering:

1. Roslyn generates a hidden struct state machine with an int _state field (-1 = running, 0 = waiting on awaiter, -2 = completed).
2. When execution hits await, it calls awaiter.IsCompleted. If false, it hooks a continuation delegate and returns the uncompleted Task to the caller.
3. When the I/O completion port fires, the runtime calls MoveNext() on the state machine, restoring execution at the exact state without blocking an OS thread!

Understanding that async methods compile to struct state machines explains why async methods with zero awaits still incur state machine initialization overhead.

Also explains why SynchronizationContext.Current captures execution context in UI frameworks (WPF/WinForms) unless ConfigureAwait(false) is used.