When should TaskCompletionSource<T> be used?

I mostly use it when only an event based API is available (for example Windows Phone 8 sockets):

public Task<Args> SomeApiWrapper()
{
    TaskCompletionSource<Args> tcs = new TaskCompletionSource<Args>(); 

    var obj = new SomeApi();

    // will get raised, when the work is done
    obj.Done += (args) => 
    {
        // this will notify the caller 
        // of the SomeApiWrapper that 
        // the task just completed
        tcs.SetResult(args);
    }

    // start the work
    obj.Do();

    return tcs.Task;
}

So it's especially useful when used together with the C#5 async keyword.

In my experiences, TaskCompletionSource is great for wrapping old asynchronous patterns to the modern async/await pattern.

The most beneficial example I can think of is when working with Socket. It has the old APM and EAP patterns, but not the awaitable Task methods that TcpListener and TcpClient have.

I personally have several issues with the NetworkStream class and prefer the raw Socket. Being that I also love the async/await pattern, I made an extension class SocketExtender which creates several extension methods for Socket.

All of these methods make use of TaskCompletionSource<T> to wrap the asynchronous calls like so:

    public static Task<Socket> AcceptAsync(this Socket socket)
    {
        if (socket == null)
            throw new ArgumentNullException("socket");

        var tcs = new TaskCompletionSource<Socket>();

        socket.BeginAccept(asyncResult =>
        {
            try
            {
                var s = asyncResult.AsyncState as Socket;
                var client = s.EndAccept(asyncResult);

                tcs.SetResult(client);
            }
            catch (Exception ex)
            {
                tcs.SetException(ex);
            }

        }, socket);

        return tcs.Task;
    }

I pass the socket into the BeginAccept methods so that I get a slight performance boost out of the compiler not having to hoist the local parameter.

Then the beauty of it all:

 var listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
 listener.Bind(new IPEndPoint(IPAddress.Loopback, 2610));
 listener.Listen(10);

 var client = await listener.AcceptAsync();

To me, a classic scenario for using TaskCompletionSource is when it's possible that my method won't necessarily have to do a time consuming operation. What it allows us to do is to choose the specific cases where we'd like to use a new thread.

A good example for this is when you use a cache. You can have a GetResourceAsync method, which looks in the cache for the requested resource and returns at once (without using a new thread, by using TaskCompletionSource) if the resource was found. Only if the resource wasn't found, we'd like to use a new thread and retrieve it using Task.Run().

A code example can be seen here: How to conditionally run a code asynchonously using tasks