C# and Tasks - UI Thread Hang - Pre-Async/Await keywords

Question

I'm trying to understand what the correct code to grab a set of data asynchronously when I do not have access to the client lib I am using to retrieve the data. I specify an endpoint and a date range and I'm supposed to retrieve a list of playlists. What I have now never comes back after the Start() call. Note: this is running in a WinForm. I am trying to better understand Tasks and don't just want to jump to awaits or a BackgroundWorker. I know I'm getting lost somewhere.

    private void GoButtonClick(object sender, EventArgs e)
    {
        string baseUrl = "http://someserver/api";
        var startDateTime = this._startDateTimePicker.Value;
        var endDateTime = this._endDateTimePicker.Value;
        _getPlaylistsFunc = delegate()
            {
                var client = new PlaylistExportClient(baseUrl);
                return client.GetPlaylistsByDateRange(startDateTime, endDateTime).ToList();
            };
        var task = new Task<List<Playlist>>(_getPlaylistsFunc);
        task.ContinueWith((t) => DisplayPlaylists(t.Result));
        task.Start();
    }

    private void DisplayPlaylists(List<Playlist> playlists)
    {
        _queueDataGridView.DataSource = playlists;
    }

UPDATE I made these changes but now the application seems to hang the UI thread.

    private void GoButtonClick(object sender, EventArgs e)
    {
        string baseUrl = "http://someserver/api";
        var startDateTime = this._startDateTimePicker.Value;
        var endDateTime = this._endDateTimePicker.Value;
        var token = Task.Factory.CancellationToken;

        var context = TaskScheduler.FromCurrentSynchronizationContext();
        Task.Factory.StartNew(() =>
            {
                var client = new PlaylistExportClient(baseUrl);
                _queueDataGridView.DataSource = client.GetPlaylistsByDateRange(startDateTime, endDateTime).ToList();

            },token,TaskCreationOptions.None,context);

    }

Answer 1

I recommend you use the Task-based Asynchronous Pattern. It's quite straightforward:

private async void GoButtonClick(object sender, EventArgs e)
{
    string baseUrl = "http://someserver/api";
    var startDateTime = this._startDateTimePicker.Value;
    var endDateTime = this._endDateTimePicker.Value;
    var playlists = await Task.Run(() =>
    {
        var client = new PlaylistExportClient(baseUrl);
        return client.GetPlaylistsByDateRange(startDateTime, endDateTime).ToList();
    });
    _queueDataGridView.DataSource = playlists;
}

Note that this will block a threadpool thread; if you can modify the library to have a GetPlaylistsByDateRangeAsync method, you can make this more efficient.

Edit: If you're stuck on .NET 4.0, you can install Microsoft.Bcl.Async to get full async/await capabilities. If - for some inexplicable reason - you still can't use async/await, then you can do it like this:

private void GoButtonClick(object sender, EventArgs e)
{
    string baseUrl = "http://someserver/api";
    var startDateTime = this._startDateTimePicker.Value;
    var endDateTime = this._endDateTimePicker.Value;
    var context = TaskScheduler.FromCurrentSynchronizationContext();
    Task.Run(() =>
    {
        var client = new PlaylistExportClient(baseUrl);
        return client.GetPlaylistsByDateRange(startDateTime, endDateTime).ToList();
    }).ContinueWith(t =>
    {
        _queueDataGridView.DataSource = t.Result;
    }, context);
}

However, note that your error handling is more complex with this approach.