How would you implement a custom awaitable object?

Question

Anubhav Kumar · Answer

To implement a custom awaitable object in C#, you need to create a type that can be used with the
await keyword. This requires understanding how the C# compiler translates
await into a series of method calls.
High-Level Summary
To be awaitable, your custom type must:
Have a method called GetAwaiter().That method returns an awaiter object which must:
Implement INotifyCompletion or ICriticalNotifyCompletionHave a bool IsCompleted { get; } propertyHave a void OnCompleted(Action) methodHave a T GetResult() method (returns result after awaiting)
Components of a Custom Awaitable
1. Awaitable Object
This is the object you call await on.
2. Awaiter Object
Returned by GetAwaiter(); it contains logic for managing the async wait.
Example: Simple Custom Awaitable
Let’s create a custom awaitable that just delays and returns a message.
Custom Awaitable
public class MyCustomAwaitable
{
    private readonly int _delay;
    public MyCustomAwaitable(int delay)
    {
        _delay = delay;
    }
    public MyCustomAwaiter GetAwaiter()
    {
        return new MyCustomAwaiter(_delay);
    }
}
Custom Awaiter
using System;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
public class MyCustomAwaiter : INotifyCompletion
{
    private readonly int _delay;
    private Task _task;
    public MyCustomAwaiter(int delay)
    {
        _delay = delay;
        _task = Task.Delay(delay); // simulate async operation
    }
    public bool IsCompleted =andgt; _task.IsCompleted;
    public void OnCompleted(Action continuation)
    {
        _task.ContinueWith(t =andgt; continuation());
    }
    public string GetResult()
    {
        // Can also throw exceptions or return results
        return $"Completed after {_delay}ms!";
    }
}
Usage
public async Task UseCustomAwaitable()
{
    var result = await new MyCustomAwaitable(1000);
    Console.WriteLine(result);  // Output: Completed after 1000ms!
}
Key Concepts
Element
Purpose
GetAwaiter()
Returns the awaiter
IsCompleted
Indicates if the task is already done
OnCompleted()
Registers the continuation (what to do after await finishes)
GetResult()
Returns the result after await is done
Real-World Use Cases
Custom throttling or retry logicAsync wrapper around I/OAwaiting external signals (file system, messages, etc.)Simulation/test awaitables
Let me know if you want:
A generic version (returning T)Integration with CancellationTokenA non-Task-based awaiter (e.g., for events or custom I/O)

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How would you implement a custom awaitable object?

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1 Answers

High-Level Summary

Components of a Custom Awaitable

1. Awaitable Object

2. Awaiter Object

Usage

Key Concepts

Real-World Use Cases

Liked By

Element	Purpose
`GetAwaiter()`	Returns the awaiter
`IsCompleted`	Indicates if the task is already done
`OnCompleted()`	Registers the continuation (what to do after await finishes)
`GetResult()`	Returns the result after await is done