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Posted 2 Aug 2017


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SmtpClient.SendMailAsync: CancellationToken Support

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2 Aug 2017CPOL1 min read
Adding cancellation support to the SendMailAsync method


Since .NET 4.5, the SmtpClient class has provided the TPL-friendly SendMailAsync method to make it easier to send email asynchronously from an async method. Unfortunately, none of the overloads support passing a CancellationToken, which makes it tricky to cancel the operation.

I ran into this shortcoming recently with an asynchronous ASP.NET MVC action configured to cancel when the user disconnected. If that happened whilst an email was being sent, the application would log an exception with the message: "An asynchronous module or handler completed while an asynchronous operation was still pending". This was because the action was being cancelled, but the task to send the email was not.

Using the Code

The following extension method adds support for passing a CancellationToken. When canceled, it will call the SendAsyncCancel method to cancel the operation.

using System;
using System.ComponentModel;
using System.Net.Mail;
using System.Threading;
using System.Threading.Tasks;

namespace System.Net.Mail
    /// <summary>
    /// Extension methods for the <see cref="SmtpClient"/> class.
    /// </summary>
    public static class SmtpClientExtensions
        /// <summary>
        /// Sends the specified message to an SMTP server for delivery as an asynchronous operation.
        /// </summary>
        /// <param name="client">
        /// The <see cref="SmtpClient"/> instance.
        /// </param>
        /// <param name="message">
        /// The <see cref="MailMessage"/> to send.
        /// </param>
        /// <param name="cancellationToken">
        /// The <see cref="CancellationToken"/> to monitor for cancellation requests.
        /// </para>
        /// <returns>
        /// A <see cref="Task"/> representing the asynchronous operation.
        /// </returns>
        /// <exception cref="ArgumentNullException">
        /// <para><paramref name="client"/> is <see langword="null"/>.</para>
        /// <para>-or-</para>
        /// <para><paramref name="message"/> is <see langword="null"/>.</para>
        /// </exception>
        public static Task SendMailAsync(
            this SmtpClient client, 
            MailMessage message, 
            CancellationToken cancellationToken)
            if (client == null) throw new ArgumentNullException(nameof(client));
            if (message == null) throw new ArgumentNullException(nameof(message));
            if (!cancellationToken.CanBeCanceled) return client.SendMailAsync(message);

            var tcs = new TaskCompletionSource<object>();
            var registration = default(CancellationTokenRegistration);
            SendCompletedEventHandler handler = null;
            handler = (sender, e) =>
                if (e.UserState == tcs)
                        if (handler != null)
                            client.SendCompleted -= handler;
                            handler = null;

                        if (e.Error != null)
                        else if (e.Cancelled)

            client.SendCompleted += handler;

                client.SendAsync(message, tcs);
                registration = cancellationToken.Register(client.SendAsyncCancel);
                client.SendCompleted -= handler;

            return tcs.Task;

Using it is fairly simple:

using (var message = new MailMessage())
    var cts = new CancellationTokenSource();
    var smtp = new SmtpClient();
    await smtp.SendMailAsync(message, cts.Token).ConfigureAwait(false);

Points of Interest

There are various simpler versions of this method around - for example, this one posted in 2016 by Matt Benic. However, I wanted to keep the code as close as possible to the source of the existing method.

As pointed out by Todd Aspeotis in the comments to Matt's version, it's necessary to call SendAsync before registering the callback on the CancellationToken; otherwise, if the token is already cancelled, the SendAsyncCancel method will be called too soon.


  • 2017-08-02: Initial version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Richard Deeming
Software Developer CodeProject
United Kingdom United Kingdom
I started writing code when I was 8, with my trusty ZX Spectrum and a subscription to "Input" magazine. Spent many a happy hour in the school's computer labs with the BBC Micros and our two DOS PCs.

After a brief detour into the world of Maths, I found my way back into programming during my degree via free copies of Delphi and Visual C++ given away with computing magazines.

I went straight from my degree into my first programming job, at Trinet Ltd. Eleven years later, the company merged to become ArcomIT. Three years after that, our project manager left to set up Nevalee Business Solutions, and took me with him. Since then, we've taken on four more members of staff, and more work than you can shake a stick at. Smile | :)

Between writing custom code to integrate with Visma Business, developing web portals to streamline operations for a large multi-national customer, and maintaining RedAtlas, our general aviation airport management system, there's certainly never a dull day in the office!

Outside of work, I enjoy real ale and decent books, and when I get the chance I "tinkle the ivories" on my Technics organ.

Comments and Discussions

SuggestionConfigureAwait Pin
hedidin3-Aug-17 8:41
professionalhedidin3-Aug-17 8:41 
PraiseRe: ConfigureAwait Pin
Richard Deeming3-Aug-17 9:56
mveRichard Deeming3-Aug-17 9:56 

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