foreach loop with async calls, last one to call onsuccess

return Coroutine.StartParallel(InitializeCore)
                .AsOperationResult()
                .ContinueWith(
                    or => Coroutine.Start(InitializeApplicationStorage, op => op.OnComplete(onSuccess, onFail))
                              .AsOperationResult()
                              .Execute()
                );

the private IEnumerable<INotifyCompleted> InitializeCore() executes a few 
yield return EntityManager.insertentitytypehere.Where(q => predicatehere).ExecuteAsync();
When the above routine finishes, the InitializeApplicationStorage routine is executed and here lies my problem; I have to do something like this:
	foreach (var questionObject in qoWithImages)
            // ReSharper restore LoopCanBeConvertedToQuery
            {
                var o = questionObject;
 
                yield return EntityManager.InvokeServerMethodAsync(
                    typeName, methodName,
                    callBack =>
                    {
                        //omitted for brevity => writing the received byte[] to isolated storage
                    },
                    null,
                    new object[] { questionObject.image_name, questionObject.test_id }
                    );
            }
I want the main LoadAsync onSuccess to be called when the last callback has finished, with other words, to continue when all the async functions have completed.
 
How can I do that?
 
Thanks again!