C# Wait Till All Threads Complete Execution

Question

I've tried this...

public ArrayList GetAllObjectAttributes()
    {
        List<Task> taskList = new List<Task>();
        ArrayList allObjectAttributes = new ArrayList();
        taskList.Add(Task.Factory.StartNew(() => { allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Folder));}));
        taskList.Add(Task.Factory.StartNew(() => { allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.XMLFile)); }));
        taskList.Add(Task.Factory.StartNew(() => { allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.TextFile)); }));
        taskList.Add(Task.Factory.StartNew(() => { allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Parent)); }));
        Task.WaitAll(taskList.ToArray());
        return allObjectAttributes;
    }

and this...

public ArrayList GetAllObjectAttributes()
    {
        Thread[] threads = new Thread[4];
        ArrayList allObjectAttributes = new ArrayList();
        threads[0] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Folder)));
        threads[1] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.XMLFile)));
        threads[2] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.TextFile)));
        threads[3] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Parent)));

        foreach(Thread thread in threads)
        {
            thread.Start();
            thread.Join();
        }
        return allObjectAttributes;
    }

and this too...

public ArrayList GetAllObjectAttributes()
    {
        Thread[] threads = new Thread[4];
        ArrayList allObjectAttributes = new ArrayList();
        threads[0] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Folder)));
        threads[1] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.XMLFile)));
        threads[2] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.TextFile)));
        threads[3] = new Thread(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Parent)));

        foreach(Thread thread in threads)
        {
            thread.Start();
        }
        while(threads[0].IsAlive || threads[1].IsAlive || threads[2].IsAlive || threads[3].IsAlive)
        {
            Thread.Sleep(500);
        }
        return allObjectAttributes;
    }

I also tried Spawn Multiple Threads for work then wait until all finished

I still get a null in one of the arraylist items in allObjectAttributes.

However, when I do

public ArrayList GetAllObjectAttributes()
    {
        ArrayList allObjectAttributes = new ArrayList();
        allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Folder)));
        allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.XMLFile)));
        allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.TextFile)));
        allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Parent)));
        return allObjectAttributes;
    }

I never get a null item in the arraylist items.

1. What am i doing wrong to wait until all threads complete?
2. Any other advise, so that the arraylist is returned only after all the 4 threads complete execution.

private List GetObjectAttributes(TreeViewAttrs tv)
{
    List objectAttributes = new List();
    string command = "COMMAND_TO_EXECUTE";
    if (command != "")
    {
        List results = RunCommand(command);
        if (results == null) { return null; }
        if (results.Count > 0)
        {
            foreach (string result in results)
            {
                if (!result.Contains("" + tv + ""))
                {
                    string[] res = reformatResponseString(result); //reformat the strings as per custom structure
                    if (res != null) { objectAttributes.Add(res); }
                }
            }
            return objectAttributes;
        }
    }
    return null;
}

Answer 1

Slightly improved by using a thread-safe collection (.NET 4.0 compatible):

List<Task> taskList = new List<Task>();
ConcurrentBag<object> allObjectAttributes = new ConcurrentBag<object>();

taskList.Add(Task.Factory.StartNew(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Folder))));
taskList.Add(Task.Factory.StartNew(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.XMLFile))));
taskList.Add(Task.Factory.StartNew(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.TextFile))));
taskList.Add(Task.Factory.StartNew(() => allObjectAttributes.Add(GetObjectAttributes(TreeViewAttrs.Parent))));

Task.WaitAll(taskList.ToArray());

return allObjectAttributes;

Alternative approach: use Task.Result once all tasks have completed (thread-safe collection no longer required as only one thread modifies ArrayList):

Task<object>[] taskList = {
    Task.Factory.StartNew(() => (object)GetObjectAttributes(TreeViewAttrs.Folder)),
    Task.Factory.StartNew(() => (object)GetObjectAttributes(TreeViewAttrs.XMLFile)),
    Task.Factory.StartNew(() => (object)GetObjectAttributes(TreeViewAttrs.TextFile)),
    Task.Factory.StartNew(() => (object)GetObjectAttributes(TreeViewAttrs.Parent))
};

Task.WaitAll(taskList);

ArrayList allObjectAttributes = new ArrayList();

foreach (Task<object> task in taskList) {
    allObjectAttributes.Add(task.Result);
}

return allObjectAttributes;

Significantly improved by using Task.WhenAll (.NET 4.5 only):

object[] allObjectAttributes = await Task.WhenAll(
    Task.Run(() => GetObjectAttributes(TreeViewAttrs.Folder)),
    Task.Run(() => GetObjectAttributes(TreeViewAttrs.XMLFile)),
    Task.Run(() => GetObjectAttributes(TreeViewAttrs.TextFile)),
    Task.Run(() => GetObjectAttributes(TreeViewAttrs.Parent))
);

return allObjectAttributes;

*Note: I used object as the generic parameter as you left the return type of GetObjectAttributes unspecified.