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copied!<p>I've decided on a custom solution here... The locks are messy and undesired but at the moment, this does the job I want.</p> <pre><code>public interface ITaskExecutionQueue { Task<TResult> QueueTask<TResult>(Func<Task<TResult>> taskGenerator); Task<TResult> QueueTask<TResult>(Task<Task<TResult>> taskGenerator); int OutstandingTaskCount { get; } event EventHandler OutstandingTaskCountChanged; } /// This class ensures that only a single Task is executed at any one time. They are executed sequentially in order being queued. /// The advantages of this class over OrderedTaskScheduler is that you can use any type of Task such as FromAsync (I/O Completion ports) /// which are not able to be scheduled using a traditional TaskScheduler. /// Ensure that the `outer` tasks you queue are unstarted. E.g. <![CDATA[ /// _taskExeQueue.QueueTask(new Task<Task<TResult>>(() => StartMyRealTask())); /// ]]> class OrderedTaskExecutionQueue : ITaskExecutionQueue { private readonly Queue<Task> _queuedTasks = new Queue<Task>(); private Task _currentTask; private readonly object _lockSync = new object(); /// <summary> /// Queues a task for execution /// </summary> /// <typeparam name="TResult"></typeparam> /// <param name="taskGenerator">An unstarted Task that creates your started real-work task</param> /// <returns></returns> public Task<TResult> QueueTask<TResult>(Func<Task<TResult>> taskGenerator) { return QueueTask(new Task<Task<TResult>>(taskGenerator)); } public Task<TResult> QueueTask<TResult>(Task<Task<TResult>> taskGenerator) { Task<TResult> unwrapped = taskGenerator.Unwrap(); unwrapped.ContinueWith(_ => { EndTask(); StartNextTaskIfQueued(); }, TaskContinuationOptions.ExecuteSynchronously); lock (_lockSync) { _queuedTasks.Enqueue(taskGenerator); if (_currentTask == null) { StartNextTaskIfQueued(); } } TaskCompletionSource<TResult> tcs = new TaskCompletionSource<TResult>(); tcs.TrySetFromTaskIncomplete(unwrapped); OutstandingTaskCountChanged.Raise(this); return tcs.Task; } private void EndTask() { lock (_lockSync) { _currentTask = null; _queuedTasks.Dequeue(); } OutstandingTaskCountChanged.Raise(this); } private void StartNextTaskIfQueued() { lock (_lockSync) { if (_queuedTasks.Count > 0) { _currentTask = _queuedTasks.Peek(); _currentTask.RunSynchronously(); } } } /// <summary> /// Includes the currently executing task. /// </summary> public int OutstandingTaskCount { get { lock (_lockSync) { return _queuedTasks.Count; } } } public event EventHandler OutstandingTaskCountChanged; } </code></pre> <p>Takes in an unstarted <code>Task<Task<TResult>></code> - this allows the queue to decide when to execute it and begin the <code>FromAsync</code> call (which is the inner task). Usage:</p> <pre><code>Task<Task<TResult>> queueTask = new Task<Task<TResult>>(() => Task.Factory.FromAsync(beginAction, endAction)); Task<TResult> asyncCallTask = _taskExecutionQueue.QueueTask(queueTask); </code></pre>

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