Extension method to have SmtpClient's SendMailAsync respond to a CancellationToken

SmtpClientExtensions.cs

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

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 = SendCompleted;

        client.SendCompleted += handler;

        try
        {
            client.SendAsync(message, tcs);
            registration = cancellationToken.Register(client.SendAsyncCancel);
        }
        catch
        {
            client.SendCompleted -= handler;
            registration.Dispose();
            throw;
        }

        return tcs.Task;

        void SendCompleted(object sender, AsyncCompletedEventArgs e)
        {
            if (e.UserState == tcs)
            {
                try
                {
                    if (handler != null)
                    {
                        client.SendCompleted -= handler;
                        handler = null;
                    }
                }
                finally
                {
                    registration.Dispose();

                    if (e.Error != null)
                    {
                        tcs.TrySetException(e.Error);
                    }
                    else if (e.Cancelled)
                    {
                        tcs.TrySetCanceled();
                    }
                    else
                    {
                        tcs.TrySetResult(null);
                    }
                }
            }
        }
    }
}

This should really be baked in to the BCL.

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