Run a bunch async tasks when a certain execution order needs to be preserved

Question

I am trying to asynchronize my original code

for(var item in items) { dbAccess.Save(item); }

which works fine:

 var tasks = new List<Task>(); for(var item in items) { tasks.Add(dbAccess.SaveAsync(item)); } await Task.WhenAll(tasks);

However, I need to add an additional clean-up call before I save and item to my DB:

 var tasks = new List<Task>(); for(var item in items) { tasks.Add(dbAccess.DeleteAsync(item.id)); tasks.Add(dbAccess.SaveAsync(item)); } await Task.WhenAll(tasks);

This code above is incorrect since SaveAsync should not be executed until the corresponding DeleteAsync is finished. In other words Deletion must always go before Saving for each item but the order for the items does not matter.

Is there a perfect way to do this ?

How about issuing all the deletes first, waiting for them, then issuing all the saves and then waiting for those? Otherwise you should package up each delete+save into a single task. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Feb 25, 2016 at 16:31
Good point Lasse, I simplified my code for the purpose of this question. I cannot implement your suggestion because there is a bit more I am not showing. Actually DeleteAsync returns something else that needs to be passed to SaveAsync. — AstroSharp
– AstroSharp, Commented Feb 25, 2016 at 16:36
So? Package them up into a task that first calls deleteasync, then calls saveasync afterwards? — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Feb 25, 2016 at 16:37
I can't. I have to pass another parameter to SaveAsync that can only be obtained from DeleteAsync. I gave an overly simplified example for demonstration purposes only. It is more involving here. All I want is just call Method1Async strictly before Method2Async while both methods can be called in a random order on each element of a collection. — AstroSharp
– AstroSharp, Commented Feb 25, 2016 at 18:10
Yes, I understand that, but if you on a single task first call DeleteAsync, and obtain the value from it, then pass that to SaveAsync, you should be able to do it. And frankly, we can only provide answers related to the code you posted. If the solutions suggested fits the code you posted but not the real code, that's really not our problem. That's yours. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Feb 25, 2016 at 18:13

Servy · Accepted Answer · 2016-02-25 16:35:50Z

Create an async method that performs the delete, then the save, for a single item, and perform all of those composite operations on each item in parallel:

var tasks = items.Select(async item => { await dbAccess.DeleteAsync(item.id); await dbAccess.SaveAsync(item); }); await Task.WhenAll(tasks);

Yacoub Massad · Accepted Answer · 2016-02-25 16:47:28Z

You can use a Func<Task> to encapsulate the two async calls into a single Task like this:

for(var item in items) { //We store the value of item in a local variable //because it is not recommended to close over a loop variable var my_item = item; Func<Task> task_fact = async () => { await dbAccess.DeleteAsync(my_item.id); await dbAccess.SaveAsync(my_item); }; tasks.Add(task_fact()); }

This will create a Task that calls the delete method, asynchronously wait for it, and then calls the save method and asynchronously wait for it.

You can do the same thing using a method.

Sean · Accepted Answer · 2016-02-25 16:38:13Z

How about using ContinueWith:

var tasks = new List<Task>(); for(var item in items) { var task = dbAccess.DeleteAsync(item.id) .ContinueWith(antecedent => dbAccess.SaveAsync(item)) .Unwrap(); tasks.Add(task); } await Task.WhenAll(tasks);

You need to unwrap the result, as currently you're not waiting for the save to complete. It's also a lot messier than using await, and likely doesn't have the error handling semantics you'd want.

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