Background Tasks in Async Methods

In the next release of Halfwit RT (what, you hadn't heard of Halfwit RT?) you'll be able to start typing a username and have it auto-complete from a known list of users.

To populate this list initially, I wanted to get a list of all the people you follow as soon as you're authenticated. However, the act of fetching the users you follow takes some time, so I didn't want to "await" the call and prevent the authentication method from finishing until it was done.

The normal way to start a "fire and forget" background operation is to use Task.Factory.StartNew(), so at first I did something like this:

Task.Factory.StartNew(async () => /* fetch user names */);

This worked, but it raised a compiler warning:

Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call.

This makes perfect sense - the compiler doesn't think you want to start a background task and not wait for it to finish inside an async method.

So how did I get around it? I got tricky. Maybe too tricky, as I'm sure there's a different (and possibly better) way to do this.

await Task.Factory.StartNew(() =>
{
    Task.Factory.StartNew(async () => { /* fetch user names */ });
});

So I'm awaiting a task that effectively finishes immediately, because all it does is start a background operation in another task.

It got a little trickier than that, though, because when the act of fetching the user names is complete, I need to add them to a List<String>, and I need that to happen on the primary thread to avoid issues. So I had to pass in the current TaskScheduler and make sure the continuation happened there.

Here's the code in its entirety, as called from my authentication method:

Task LoadFriendUserNamesAsync()
{
    return Task.Factory.StartNew(scheduler =>
    {
        Task.Factory.StartNew(async () =>
        {
            var friendsResponse = await Client.GetFriendIdsAsync();
            if (friendsResponse.StatusCode != System.Net.HttpStatusCode.OK) return new string[0];

            var usersResponse = await Client.LookupUsersAsync(friendsResponse.Result);
            if (usersResponse.StatusCode != System.Net.HttpStatusCode.OK) return new string[0];

            return usersResponse.Result.Select(u => u.ScreenName).ToArray();
        }).ContinueWith(t =>
        {
            if (t.IsFaulted || t.IsCanceled) return;

            var task = t.Result;
            if (task.IsFaulted || task.IsCanceled) return;

            foreach (var user in task.Result) UserNames.Add(user);

        }, (TaskScheduler)scheduler);
    }, TaskScheduler.Current);
}