TaskCompletionSource by Example

In this article, we’ll learn how to use TaskCompletionSource. It’s one of those tools which you will rarely need to use, but when you do, you’ll be glad that you knew about it. Let’s dive right into it.

Basic Usage

The source code for this section is in the TaskCompletionSource1 folder at the Gigi Labs BitBucket Repository.

Let’s create a new console application, and in Main(), we’ll have my usual workaround for running asynchronous code in a console application:

        static void Main(string[] args)
        {
            Run();
            Console.ReadLine();
        }

In the Run() method, we have a simple example showing how TaskCompletionSource works:

        static async void Run()
        {
            var tcs = new TaskCompletionSource<bool>();

            var fireAndForgetTask = Task.Delay(5000)
                                        .ContinueWith(task => tcs.SetResult(true));

            await tcs.Task;
        }

TaskCompletionSource is just a wrapper for a Task, giving you control over its completion. Thus, a TaskCompletionSource<bool> will contain a Task<bool>, and you can set the bool result based on your own logic.

Here, we are using TaskCompletionSource as a synchronization mechanism. Our main thread spawns off an operation and waits for its result, using the Task in the TaskCompletionSource. Even if the operation is not Task-based, it can set the result of the Task in the TaskCompletionSource, allowing the main thread to resume its execution.

Let’s add some diagnostic code so that we can understand what’s going on from the output:

        static async void Run()
        {
            var stopwatch = Stopwatch.StartNew();

            var tcs = new TaskCompletionSource<bool>();

            Console.WriteLine($"Starting... (after {stopwatch.ElapsedMilliseconds}ms)");

            var fireAndForgetTask = Task.Delay(5000)
                                        .ContinueWith(task => tcs.SetResult(true));

            Console.WriteLine($"Waiting...  (after {stopwatch.ElapsedMilliseconds}ms)");

            await tcs.Task;

            Console.WriteLine($"Done.       (after {stopwatch.ElapsedMilliseconds}ms)");

            stopwatch.Stop();
        }

And here is the output:

Starting... (after 0ms)
Waiting...  (after 41ms)
Done.       (after 5072ms)

As you can see, the main thread waited until tcs.SetResult(true) was called; this triggered completion of the TaskCompletionSource’s underlying task (which the main thread was awaiting), and allowed the main thread to resume.

Aside from SetResult(), TaskCompletionSource offers methods to cancel a task or fault it with an exception. There are also safe Try...() equivalents:

SDK Example

The source code for this section is in the TaskCompletionSource2 folder at the Gigi Labs BitBucket Repository.

One scenario where I found TaskCompletionSource to be extremely well-suited is when you are given a third-party SDK which exposes events. Imagine this: you submit an order via an SDK method, and it gives you an ID for that order, but not the result. The SDK goes off and does what it has to do to perhaps talk to an external service and have the order processed. When this eventually happens, the SDK fires an event to notify the calling application on whether the order was placed successfully.

We’ll use this as an example of the SDK code:

    public class MockSdk
    {
        public event EventHandler<OrderOutcome> OnOrderCompleted;

        public Guid SubmitOrder(decimal price)
        {
            var orderId = Guid.NewGuid();

            // do a REST call over the network or something
            Task.Delay(3000).ContinueWith(task => OnOrderCompleted(this,
                new OrderOutcome(orderId, true)));

            return orderId;
        }
    }

The OrderOutcome class is just a simple DTO:

    public class OrderOutcome
    {
        public Guid OrderId { get; set; }
        public bool Success { get; set; }

        public OrderOutcome(Guid orderId, bool success)
        {
            this.OrderId = orderId;
            this.Success = success;
        }
    }

Notice how MockSdk‘s SubmitOrder does not return any form of Task, and we can’t await it. This doesn’t necessarily mean that it’s blocking; it might be using another form of asynchrony such as the Asynchronous Programming Model or a messaging framework with a request-response fashion (such as RPC over RabbitMQ).

At the end of the day, this is still asynchrony, and we can use TaskCompletionSource to build a Task-based Asynchronous Pattern abstraction over it (allowing the application to simply await the call).

First, we start building a simple proxy class that wraps the SDK:

    public class SdkProxy
    {
        private MockSdk sdk;

        public SdkProxy()
        {
            this.sdk = new MockSdk();
            this.sdk.OnOrderCompleted += Sdk_OnOrderCompleted;
        }

        private void Sdk_OnOrderCompleted(object sender, OrderOutcome e)
        {
            // TODO
        }
    }

We then add a dictionary, which allows us to relate each OrderId to its corresponding TaskCompletionSource. Using a ConcurrentDictionary instead of a normal Dictionary helps deal with multithreading scenarios without needing to lock:

        private ConcurrentDictionary<Guid,
            TaskCompletionSource<bool>> pendingOrders;
        private MockSdk sdk;

        public SdkProxy()
        {
            this.pendingOrders = new ConcurrentDictionary<Guid,
                TaskCompletionSource<bool>>();

            this.sdk = new MockSdk();
            this.sdk.OnOrderCompleted += Sdk_OnOrderCompleted;
        }

The proxy class exposes a SubmitOrderAsync() method:

        public Task SubmitOrderAsync(decimal price)
        {
            var orderId = sdk.SubmitOrder(price);

            Console.WriteLine($"OrderId {orderId} submitted with price {price}");

            var tcs = new TaskCompletionSource<bool>();
            this.pendingOrders.TryAdd(orderId, tcs);

            return tcs.Task;
        }

This method calls the underlying SubmitOrder() in the SDK, and uses the returned OrderId to add a new TaskCompletionSource in the dictionary. The TaskCompletionSource’s underlying Task is returned, so that the application can await it.

        private void Sdk_OnOrderCompleted(object sender, OrderOutcome e)
        {
            string successStr = e.Success ? "was successful" : "failed";
            Console.WriteLine($"OrderId {e.OrderId} {successStr}");

            this.pendingOrders.TryRemove(e.OrderId, out var tcs);
            tcs.SetResult(e.Success);
        }

When the SDK fires a completion event, the proxy will remove the TaskCompletionSource from the pending order and set its result. The application awaiting the underlying task will resume and take a decision depending on the logic.

We can test this with the following program code in a console application:

        static void Main(string[] args)
        {
            Run();
            Console.ReadLine();
        }

        static async void Run()
        {
            var sdkProxy = new SdkProxy();

            await sdkProxy.SubmitOrderAsync(10);
            await sdkProxy.SubmitOrderAsync(20);
            await sdkProxy.SubmitOrderAsync(5);
            await sdkProxy.SubmitOrderAsync(15);
            await sdkProxy.SubmitOrderAsync(4);
        }

The output shows that the program did indeed wait for each order to complete before starting the next one:

OrderId 3e2d4577-8bbb-46b7-a5df-2efec23bae6b submitted with price 10
OrderId 3e2d4577-8bbb-46b7-a5df-2efec23bae6b was successful
OrderId e22425b9-3aa3-48db-a40f-8b8cfbdcd3af submitted with price 20
OrderId e22425b9-3aa3-48db-a40f-8b8cfbdcd3af was successful
OrderId 3b5a2602-a5d2-4225-bbdb-10642a63f7bc submitted with price 5
OrderId 3b5a2602-a5d2-4225-bbdb-10642a63f7bc was successful
OrderId ffd61cea-343e-4a9c-a76f-889598a45993 submitted with price 15
OrderId ffd61cea-343e-4a9c-a76f-889598a45993 was successful
OrderId b443462c-f949-49b9-a6f0-08bbbb82fe7e submitted with price 4
OrderId b443462c-f949-49b9-a6f0-08bbbb82fe7e was successful

Summary

Use TaskCompletionSource to adapt an arbitrary form of asynchrony to use Tasks, and enable elegant async/await usage.

Do not use it simply expose an asynchronous wrapper for a synchronous method. You should either not do that at all, or use Task.FromResult() instead.

If you’re concerned that the the asynchronous call might never resume, consider adding a timeout.

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