How to write an “awaitable” method?

Question

I\'m finally looking into the async & await keywords, which I kind of "get", but all the examples I\'ve seen call async methods in the .Net framework, e.g. thi

Answer 1

If you don't want to use a Task, you may write a completely customized awaitable object. Such object is one implementing a method GetAwaiter () returning an object implementing INotifyCompletion, which can be the object itself.

More: INotifyCompletion

The awaiter implements:

• IsCompleted is get the state
• GetResult () to get the result
• OnCompleted (Action continuation) to set the continuation delegate.

The awaitable object contains some method for actual payload (e.g. below, the method is Run).

class Program {
    // Need to change the declaration of Main() in order to use 'await'
    static async Task Main () {
        // Create a custom awaitable object
        MyAwaitable awaitable = new MyAwaitable ();

        // Run awaitable payload, ignore returned Task
        _ = awaitable.Run ();

        // Do some other tasks while awaitable is running
        Console.WriteLine ("Waiting for completion...");

        // Wait for completion
        await awaitable;

        Console.WriteLine ("The long operation is now complete. " + awaitable.GetResult());
    }
}

public class MyAwaitable : INotifyCompletion {
    // Fields
    private Action continuation = null;
    private string result = string.Empty;

    // Make this class awaitable
    public MyAwaitable GetAwaiter () { return this; }

    // Implementation of INotifyCompletion for the self-awaiter
    public bool IsCompleted { get; set; }
    public string GetResult () { return result; }
    public void OnCompleted (Action continuation) {
        // Store continuation delegate
        this.continuation = continuation;
        Console.WriteLine ("Continuation set");
    }

    // Payload to run
    public async Task Run () {
        Console.WriteLine ("Computing result...");

        // Wait 2 seconds
        await Task.Delay (2000);
        result = "The result is 10";

        // Set completed
        IsCompleted = true;

        Console.WriteLine ("Result available");

        // Continue with the continuation provided
        continuation?.Invoke ();
    }
}