How to make any class awaitable

Asynchronous programming in C# is a powerful feature that enables developers to build responsive and efficient applications. The async and await keywords make it easy to work with asynchronous operations using the Task-based Asynchronous Pattern (TAP). But what if you need to integrate with legacy systems or third-party libraries that use non-standard asynchronous patterns, such as callback-based APIs? In this post, we’ll explore how to create custom async/await mechanisms to bridge the gap between these different asynchronous paradigms.

Understanding the Basics of Async/Await

Before diving into custom implementations, let’s quickly recap how async and await work in C#:

• Async Methods: Methods marked with the async keyword can use the await keyword to asynchronously wait for tasks to complete.
• Awaitable Objects: For an object to be awaitable, it needs to implement the GetAwaiter method, which returns an awaiter. The awaiter must implement the INotifyCompletion interface.

The Scenario: Integrating with a Callback-Based API

Imagine you’re working with a legacy message queue library that provides an asynchronous SendMessage method using callbacks. Your goal is to integrate this library into your modern C# application using the async/await pattern for improved readability and maintainability.

Step-by-Step Implementation

Step 1: Define the Legacy API

Here’s a simplified version of a legacy message queue API:

public class LegacyMessageQueue
{
    public void SendMessage(string message, Action<bool> callback)
    {
        // Simulate async operation with a delay
        Timer timer = new Timer(_ =>
        {
            bool success = true; // Simulate success
            callback(success);
        }, null, 1000, Timeout.Infinite);
    }
}

This API uses a callback to notify when the message sending operation is complete.

Step 2: Create the Custom Task Type

To create a custom awaitable type, start by defining a MessageTask class that wraps the callback-based API:

public class MessageTask
{
    private readonly Action<Action<bool>> _continuationAction;

    public MessageTask(Action<Action<bool>> continuationAction)
    {
        _continuationAction = continuationAction;
    }

    public MessageAwaiter GetAwaiter()
    {
        return new MessageAwaiter(_continuationAction);
    }
}

Step 3: Implement the Awaiter

Next, implement the MessageAwaiter class to handle the continuation logic:

public class MessageAwaiter : INotifyCompletion
{
    private readonly Action<Action<bool>> _continuationAction;
    private Action _continuation;
    private bool _result;

    public MessageAwaiter(Action<Action<bool>> continuationAction)
    {
        _continuationAction = continuationAction;
    }

    public bool IsCompleted { get; private set; }

    public void OnCompleted(Action continuation)
    {
        _continuation = continuation;
        _continuationAction(result =>
        {
            _result = result;
            IsCompleted = true;
            _continuation?.Invoke();
        });
    }

    public bool GetResult()
    {
        return _result;
    }
}

Step 4: Create an Extension Method

To make it easier to use the custom task type, create an extension method for the LegacyMessageQueue:

public static class LegacyMessageQueueExtensions
{
    public static MessageTask SendMessageAsync(this LegacyMessageQueue queue, string message)
    {
        return new MessageTask(continuation => queue.SendMessage(message, continuation));
    }
}

Step 5: Use the Custom Async/Await

With everything set up, you can now use the custom async/await pattern in your code:

public class Program
{
    public static async Task Main(string[] args)
    {
        var queue = new LegacyMessageQueue();
        bool result = await queue.SendMessageAsync("Hello, World!");
        Console.WriteLine(result ? "Message sent successfully!" : "Failed to send message.");
    }
}

Benefits of Custom Async/Await

• Seamless Integration: Integrates legacy or third-party callback-based APIs with modern async/await syntax.
• Readability: Enhances code readability and maintainability by using the familiar async/await pattern.
• Flexibility: Customizable to fit specific needs that the built-in Task types may not cover.

Conclusion

Creating custom async/await mechanisms in C# is a powerful technique for integrating with non-standard asynchronous patterns. By defining your own task and awaiter types, you can seamlessly bridge the gap between legacy callback-based APIs and modern async/await syntax, making your code cleaner and more maintainable. This approach not only improves readability but also leverages the full power of asynchronous programming in C#.