Question

I am using SignalR. The function on the Hub often return a Task. I now have a function that will add a connection to a bunch of groups. I would like to return a Task that represents all of these Tasks.

I found a perfect function for that: Task.WhenAll. However this is a new function in .NET 4.5 and I am still stuck on .NET 4.

Hence I decided to write my own version of it until we can move to .NET 4.5. Because there are often some caveats when it comes to multithreading (e.g. thread pool stuff), I am not sure if my implementation is correct:

public static Task WhenAll(IEnumerable<Task> tasks)
{
    return Task.Factory.StartNew(() => Task.WaitAll(tasks.ToArray()));
}

Functionally, it works I think, but don’t I get an extra blocked thread for the new Task? Or is this unavoidable?

Edit:
Here is how I would use it with SignalR:

public static Task Add(this IGroupManager groupManager, string connectionId,
                                                   IEnumerable<string> groups)
{
   return WhenAll(groups.Select(group => groupManager.Add(connectionId, group)));
}

Answer 1

Your solution will work fine, but you’re right that it would block a thread the whole time.

I think the simplest way to efficiently implement WhenAll() on .Net 4.0 is to use ContinueWhenAll(). It performs an action when all Tasks from a collection are finished and returns a Task representing that action. Since we want just that Task, we don’t need the action, passing an empty lambda will work:

public static Task WhenAll(IEnumerable<Task> tasks)
{
    return Task.Factory.ContinueWhenAll(tasks.ToArray(), _ => {});
}