/*DataflowBlock.cs(70,9,70,16): error CS0117: 'System.Threading.Tasks.Task' does not contain a definition for 'WhenAll'*/
public static Task<int> Choose<T1, T2> (
            ISourceBlock<T1> source1, Action<T1> action1,
            ISourceBlock<T2> source2, Action<T2> action2,
            DataflowBlockOptions dataflowBlockOptions)
        {
            if (source1 == null)
                throw new ArgumentNullException ("source1");
            if (source2 == null)
                throw new ArgumentNullException ("source2");
            if (action1 == null)
                throw new ArgumentNullException ("action1");
            if (action2 == null)
                throw new ArgumentNullException ("action2");
            if (dataflowBlockOptions == null)
                throw new ArgumentNullException ("dataflowBlockOptions");

            var chooser = new ChooserBlock<T1, T2, object> (action1, action2, null, dataflowBlockOptions);
            source1.LinkTo (chooser.Target1);
            source2.LinkTo (chooser.Target2);

            Task.WhenAll (source1.Completion, source2.Completion)
                .ContinueWith (_ => chooser.AllSourcesCompleted ());

            return chooser.Completion;
        }

/*DataflowBlock.cs(300,17,300,27): error CS0117: 'System.Threading.Tasks.Task' does not contain a definition for 'FromResult' */
        public static Task<bool> OutputAvailableAsync<TOutput> (
            this ISourceBlock<TOutput> source, CancellationToken cancellationToken)
        {
            if (source == null)
                throw new ArgumentNullException ("source");

            cancellationToken.ThrowIfCancellationRequested ();

            if (source.Completion.IsCompleted || source.Completion.IsCanceled
                || source.Completion.IsFaulted)
                return Task.FromResult (false);

            var block = new OutputAvailableBlock<TOutput> ();
            var bridge = source.LinkTo (block,
                new DataflowLinkOptions { PropagateCompletion = true });
            return block.AsyncGet (bridge, cancellationToken);
        }