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)));
}

```