Execute implementation

gistfile1.cs

public IEnumerable<Row> Execute(IEnumerable<Row> rows)
{
    var blockingCollection = new BlockingCollection<Row>();
    var count = _operations.Count;
    if (count == 0)
    {
        yield break;
    }
    var tasks = _operations.Select(currentOp =>
        Task.Factory.StartNew(() =>
            {
                try
                {
                    foreach (var row in currentOp.Execute(null))
                    {
                        blockingCollection.Add(row);
                    }
                }
                finally
                {
                    if (Interlocked.Decrement(ref count) == 0)
                    {
                        blockingCollection.CompleteAdding();
                    }
                }
            }
        )
    ).ToArray();
    foreach (var row in blockingCollection.GetConsumingEnumerable())
    {
        yield return row;
    }
    Task.WaitAll(tasks); // raise any exception that were raised during execution
}

Works great. Thanks