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How would you implement a custom awaitable object?

ICSM Computer 569 16 Jun 2025

How would you implement a custom awaitable object?

c# c#
ICSM Computer
Ask by ICSM Computer
Updated 18 Jun 2025

1 Answers

Anubhav Sharma
Anubhav Sharma 18 Jun 2025
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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 ICriticalNotifyCompletion
    • Have a bool IsCompleted { get; } property
    • Have a void OnCompleted(Action) method
    • Have 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 => _task.IsCompleted;

    public void OnCompleted(Action continuation)
    {
        _task.ContinueWith(t => 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 logic
  • Async wrapper around I/O
  • Awaiting external signals (file system, messages, etc.)
  • Simulation/test awaitables

Let me know if you want:

  • A generic version (returning T)
  • Integration with CancellationToken
  • A non-Task-based awaiter (e.g., for events or custom I/O)
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