Question

I have a long running Task that uses callbacks to feed data incrementally (rather than a single ContinueWith() callback at the end).

I want to be able to pass the Task object back in this callback for task identification purposes (using Task.CurrentId)

However, I cannot work out how to pass the Task object into the task delegate. There appears to be no overload to do this, and I can’t use the closure to do it since the task object is not defined at that point.

eg.

public Task StartDoingSomeStuff(CallbackDelegate callback)
{
    Task task = Task.Factory.StartNew(() =>
    {
         while(whatever)
         {
             var results = DoSomeStuff();
             callback(results, task); //CS0165. How do I get hold of the task?
         }
    });

    return task;
}

gives:

error CS0165: Use of unassigned local variable ‘task’

Answer 1

Split declaring the variable and assigning the task to it into two statements. Make sure that you don’t use the variable before the task has been assigned:

public Task StartDoingSomeStuff(CallbackDelegate callback)
{
    var gate = new object();
    lock (gate)
    {
        Task task = null;
        task = Task.Factory.StartNew(() =>
        {
            lock (gate)
            {
                while (whatever)
                {
                    var results = DoSomeStuff();
                    callback(results, task);
                }
            }
        });
        return task;
    }
}