C# Async/Await SmtpClient.SendMailAsync

Question

I have written a simple class for me to send reports which works well, however I now need to incorporate a log to check whether the send was successful or not.

A quick brief on my app: It has a main form that you can manually run the reports via buttons and if the app is run from the command line then the form is hidden.

When the form is shown the SmtpClient.SendComplete event fires, but when the form is hidden it does not and also doesn't send the email. I have tried every which way to get it to work.

public static async Task SendAsync(string sendTo, string subject, string body = "", string attachment = "")
{
#if (DEBUG)
    // If in DEBUG mode then send reports to lee.
    sendTo = "developer";
#endif
    try
    {
        SmtpClient client = new SmtpClient("smtp.server", 587)
        { Credentials = new NetworkCredential("username", "password") };
        MailMessage message = new MailMessage
        { From = new MailAddress("mailFrom") };
        message.To.Add(sendTo);
        message.Subject = subject;
        message.Body = body;
        if (!string.IsNullOrEmpty(attachment))
            message.Attachments.Add(new Attachment(attachment));
        client.SendCompleted += (s,e) =>
            {
                if (e.Error != null)
                {
                    Logger.FileLog(message.Subject, e.Error.ToString());
                }
                else
                {
                    Logger.FileLog(message.Subject, "Message Sent");
                }
                client.Dispose();
                message.Dispose();
            };
        Logger.FileLog(_subject, "Running Async");
        await client.SendMailAsync(message);

        Logger.FileLog(_subject, "Finished");
    }
    catch (Exception e)
    {
        Logger.FileLog(subject, $"Error {e.HResult}, {e.Message}, {e.InnerException.ToString()}");
    }
}

I have tried async Task, async void, SendAsync, SendMailAsync, loops to wait for a callback.

Some of the logging is there just so I can see how far it got when running from the command line and it only ever gets as far as "Running Async" which is before it tries to send the email.

The code for detecting whether to show or hide the form as requested, the original project was written in VB, the above code is in a class library which was working until I tried to run Async.

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    If CommandLineArgs.Count > 0 Then
        Visible = False
        RunSilent = True
        ParseCommandLine()
        Close()
    Else
        'Update to add from db, XML or Excel
        Dim t As Task = Task.Run(
            Sub()
                LoadReports()
            End Sub)
        t.Wait()
    End If
End Sub

Any help or advice will be greatly appreciated.

It might help if you show us the logic around detecting if it is running from the console and hiding the form. — Anders Carstensen
– Anders Carstensen, Commented Jul 21, 2018 at 10:28
The described behavior may be due to a deadlock but without a minimal reproducible example, I can't say for sure — Nkosi
– Nkosi, Commented Jul 21, 2018 at 11:11
Investigate the parse command method and see if you are mixing async and blocking calls — Nkosi
– Nkosi, Commented Jul 21, 2018 at 11:14
thanks, you've all given me something to look at, I will report back what I find. — Lee Morgan
– Lee Morgan, Commented Jul 21, 2018 at 11:30

Nkosi · Accepted Answer · 2018-07-21 12:16:28Z

1

You close right after processing command line, so chances are that you are not awaiting the method and Close is being invoked before the email can be sent.

assuming ParseCommandLine and LoadReports are async functions then you should await them in the event handler

Private Async Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
    If CommandLineArgs.Count > 0 Then
        Visible = False
        RunSilent = True
        Await ParseCommandLine() 'wait for task to complete then close
        Close()
    Else
        'Update to add from db, XML or Excel
        Await LoadReports()
    End If
End Sub

Note the removal of .Wait() blocking call that could cause deadlock when mixed with async calls.

Reference Async/Await - Best Practices in Asynchronous Programming

edited Jul 21, 2018 at 12:16

answered Jul 21, 2018 at 11:59

Nkosi

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5 Comments

Lee Morgan Over a year ago

Ok, I will look at all of the methods and make sure they all use async and await, didn't realise it would be this difficult when I started it:(

Nkosi Over a year ago

@LeeMorgan When choosing to use async/await, it tends to spread to the point where you tend to go async all the way. Once you get the hang of it, things tend to get easier.

Lee Morgan Over a year ago

Just so I am clear on it all, any method that calls an async method also needs to be async/await?

Nkosi Over a year ago

@LeeMorgan Yes, if you want to wait on said task. If you do not want to wait you can still call it from a non async method, but it will be running on a separate thread, which can lead to the behavior you are seeing with the close happening before it is desired.

Lee Morgan Over a year ago

Mega, not only have you answered my question but you have also helped me understand it some more. I really appreciate your time and explanation.

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