While using the Task[] Object reference not set to an instance of an object

Question

I am using Tasks to do some processes. The tasks will be in a loop, I am not sure sure how many tasks/iterations will be there. It will vary time to time. How to use the Tasks? Below is my code.

void func1(string loc) 
{
    var CurrentDirectoryInfo = new DirectoryInfo(loc);
    Task[] tasks; // null
    int index = 0;

    foreach (DirectoryInfo D1 in CurrentDirectoryInfo.GetDirectories)
    {
        tasks[index] = Task.Factory.StartNew(() =>func1(d1.FullName));
        index++;
    }

If I use null for the Task[] tasks, I am getting Object reference not set to an instance of an object error.

If I leave it unassigned, I am getting Use of unassigned variable error.

This is very very bad code. You should re-visit your knowledge of StartNew and also decide whether you need an individual task created per directory/subdirectory. Chances are... you don't (how many cores does your PC have?). — Simon Whitehead
– Simon Whitehead, Commented Nov 28, 2013 at 4:26
How would you go about solving the problem if tasks were any other data type, and you were trying to access an instance method/property? — Factor Mystic
– Factor Mystic, Commented Nov 28, 2013 at 4:29
Also the root of this question has nothing to do with tasks, so it should probably be retitled and retagged. — Factor Mystic
– Factor Mystic, Commented Nov 28, 2013 at 4:31
What might be the best solution if I want to have per sub-directory, Each task takes only less than five seconds to complete. — user3044315
– user3044315, Commented Nov 28, 2013 at 4:34

Adil · Accepted Answer · 2013-11-28 04:59:00Z

1

You can use list of tasks, if you do not know how many task object you need at compile time.

List<Task> tasks = new List<Task>();

Add newly created takss in list.

foreach(DirectoryInfo D1 in CurrentDirectoryInfo.GetDirectories)
{
    tasks.Add(Task.Factory.StartNew(() =>func1(d1.FullName)));
}

The code for creating task object for directory object does not make sense. You should consider alternative approach, you might achieve this using single task in more efficient way.

edited Nov 28, 2013 at 4:59

answered Nov 28, 2013 at 4:27

Adil

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5 Comments

user3044315 Over a year ago

I need to wait for the tasks to finish. How Can I use the WaitAll in this case?

Adil Over a year ago

Yes WaitAll could be used.

user3044315 Over a year ago

How can this be done in a single thread in more effective way? I want to compute something on each task which will consume some time.

Adil Over a year ago

It depends on what you do in thread if processing in task is small and the task overhead is big then using single or less threads could be better approach. You can compare both after making them.

user3044315 Over a year ago

Great. So I will use the thread for the top level folders only then. Thanks :)

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