Passing data into Task Continuation

Question

I have the following method :

private void GetHistoricalRawDataFromTextFiles(MessageHeader header, HistoricalRawDataFromTextFileSubscriptionDto dto)
{
    var computeInstance = GetComputeInstance(dto.SubscriberId, dto.ComputeInstanceId);
    var task = computeInstance.GetHistoricalRawDataFromTextFiles(dto, progress => SendProgress(header.Sender, progress));

    task.ContinueWith(myTask =>
    {
        dto.TimeSeries = myTask.Result;
        Messenger.SendTo(SubscriberId, header.Sender, MessageType.Reply, MessageTopic.HistoricalRawDataFromTextFiles, dto);
    });
}

Method computeInstance.GetHistoricalRawDataFromTextFiles returns a Task<List<string>> and my question is

whether this is the correct way to pass header and dto into the lambda expression and task continuations. It is important that the header and dto instance values are captured within the lambda expression and task continuation at the time the outer method is called. The same method may be called again before the task from the previous call completes.

Yuval Itzchakov · Accepted Answer · 2015-10-04 10:02:26Z

2

It is important that the header and dto instance values are captured within the lambda expression and task continuation at the time the outer method is called.

When using lambda expressions, what gets closed over is the variable, and not the value of that variable.

As long as header and dto aren't global variables which you modify each time before making the method call, you should be fine. If they are global variables, then you'll need to find a way to create a local copy for each of them. If they're reference types, you'll need to clone them, and if they're value types, you'll need to copy them to a local variable inside the lambda.

answered Oct 4, 2015 at 10:02

Yuval Itzchakov

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7 Comments

Matt Over a year ago

they are not global variables in that they are globally accessible but, yes, header and dto change on each outer message call. So, the only way would be to clone them and then pass the clone into the lambda and task?

Yuval Itzchakov Over a year ago

No, if they are not globally modifiable and you pass a new instance each time, you are fine with the code as is.

Matt Over a year ago

Thanks, that makes it clearer, so, it just comes down to the fact that I pass in a new references to values of header and dto, correct?

Yuval Itzchakov Over a year ago

Yes. Don't be worried about the captured variables from previous methods, you should be fine as is.

Royi Namir Over a year ago

Just to be careful : Captured variables can be modified on different threads.

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Royi Namir · Accepted Answer · 2015-10-04 10:29:38Z

1

I think that your question boils down to : "Is my method thread safe?"

I don't think it's related to your captured variable here.

Your method doesn't seem to access a shared/global source ( static/global variables or fields).(otherwise you'd need synchronization)

Even if this method was called by a multiple threads simultaneously then it still be thread safe and each call to GetHistoricalRawDataFromTextFiles would deal with a separate "realm" - This is because each thread has its own stack.

So whether or not you use captured variable (refers to the same memory location) - you still get unique dto and header for each iteration realm.

I don't see here any same shared memory location issue since each invocation (even if threaded) - gets its own space.

Again. this is assuming you're not using any global state.

answered Oct 4, 2015 at 10:29

Royi Namir

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2 Comments

Matt Over a year ago

what do you define exactly as global variable? The outer method private void GetHistoricalRawDataFromTextFiles(MessageHeader header, HistoricalRawDataFromTextFileSubscriptionDto dto) can be invoked even on the same thread multiple times in short succession (each with different header and dto instances) and the idea is to spin up tasks and run their continuations that "capture" the header and dto that is passed into the method at the time. I guess I am worried that a second method call will impact the task and its continuation that was spun up during the first method call.

Royi Namir Over a year ago

@MattWolf By global variable I mean a variable that is accessible from outside the function. it doesn't have to be static though

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