Make Async function run completely in the background

Question

I have a timer that calls a method every 15 seconds and that method takes some time to finish. I have converted it to async as much as possible for me but it still freezes the UI for almost 1 sec when it runs and since it runs every 15 seconds, it becomes annoying.

Any idea how to make this async method run completely off the grid? This is the timer method:

public static DispatcherTimer UpdateList = new DispatcherTimer();
//GlobalVars.MainList = new List<string>();  //saved list from previous declaration

public MainFunction()
{
    this.InitializeComponent();
    UpdateList.Tick += UpdateList_Tick; ;
    UpdateList.Interval = new TimeSpan(0, 0, 0, 0, 15000);
    UpdateList.Start();
    //...
}
private async void UpdateList_Tick(object sender, object e)
    {   
    using (var client = new ImapClient())
            {
                using (var cancel = new CancellationTokenSource())
                {
                    await client.ConnectAsync("imap.gmail.com", 993, true, cancel.Token);
                    client.AuthenticationMechanisms.Remove("XOAUTH");

                    await client.AuthenticateAsync("email.com", "mail12345", cancel.Token);

                    var inbox = client.Inbox;
                    await inbox.OpenAsync(FolderAccess.ReadOnly, cancel.Token);

                    // let's try searching for some messages...
                    DateTime date = DateTime.Now;
                    DateTime mondayOfLastWeek = date.AddDays(-(int)date.DayOfWeek - 6);
                    var query = SearchQuery.DeliveredAfter(mondayOfLastWeek)
                        .And(SearchQuery.SubjectContains("search"))
                        .And(SearchQuery.All);
                    List<string> newList = new List<string>();
                    foreach (var uid in inbox.Search(query, cancel.Token))
                    {
                        var message = inbox.GetMessage(uid, cancel.Token);
                        string trimmedMSGEmtyLines = Regex.Replace(message.TextBody, @"^\s+$[\r\n]*", "", RegexOptions.Multiline);
                        newList.Add(message.Date.LocalDateTime + Environment.NewLine + trimmedMSGEmtyLines);
                    }
                    await client.DisconnectAsync(true, cancel.Token);
                    if (!GlobalVars.MainList.SequenceEqual(newList))
                    {
                        GlobalVars.MainList = newList;
                    }
                }
            }
}

Update: MailKit, Portable.Text.Encoding, MimeKit

Answer 1

async / await is useful when executing long running operations, which are decoupled from the UI, while running on a UI thread. This is exactly what you are doing - but it seems you have no business being on the UI thread in the first place. Really, you could await the entire contents of your event handler as it's wholly decoupled from the UI. So it's not the right tool for the job.

BackgroundWorker was suggested, which would be a great option. I chose to provide a solution using System.Threading.Timer because there is not much code to change vs. your existing code. If you add any code which updates the UI, you will need to invoke it back to the dispatcher thread.

And all those async methods could probably run synchronously. I would remove async / await then choose synchronous methods, for example client.ConnectAsync() >> client.Connect()

public System.Threading.Timer UpdateTimer;

public MainFunction()
{
    this.InitializeComponent();
    UpdateTimer = new System.Threading.Timer(UpdateList_Tick);
    UpdateTimer.Change(0, 15000);
    //...
}

private async void UpdateList_Tick(object state)
{   
    using (var client = new ImapClient())
    {
        using (var cancel = new CancellationTokenSource())
        {
            await client.ConnectAsync("imap.gmail.com", 993, true, cancel.Token);
            client.AuthenticationMechanisms.Remove("XOAUTH");

            await client.AuthenticateAsync("email.com", "mail12345", cancel.Token);

            var inbox = client.Inbox;
            await inbox.OpenAsync(FolderAccess.ReadOnly, cancel.Token);

            // let's try searching for some messages...
            DateTime date = DateTime.Now;
            DateTime mondayOfLastWeek = date.AddDays(-(int)date.DayOfWeek - 6);
            var query = SearchQuery.DeliveredAfter(mondayOfLastWeek)
                .And(SearchQuery.SubjectContains("search"))
                .And(SearchQuery.All);
            List<string> newList = new List<string>();
            foreach (var uid in inbox.Search(query, cancel.Token))
            {
                var message = inbox.GetMessage(uid, cancel.Token);
                string trimmedMSGEmtyLines = Regex.Replace(message.TextBody, @"^\s+$[\r\n]*", "", RegexOptions.Multiline);
                newList.Add(message.Date.LocalDateTime + Environment.NewLine + trimmedMSGEmtyLines);
            }
            await client.DisconnectAsync(true, cancel.Token);
            if (!GlobalVars.MainList.SequenceEqual(newList))
            {
                GlobalVars.MainList = newList;
            }
        }
    }
}