Async SmtpClient Extensions

//--------------------------------------------------------------------------
//
//  Copyright (c) Microsoft Corporation.  All rights reserved.
//
//  File: SmtpClientExtensions.cs
//
//--------------------------------------------------------------------------

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

namespace System.Net.NetworkInformation
{
    internal class EAPCommon
    {
        internal static void HandleCompletion<T>(
            TaskCompletionSource<T> tcs,
            ComponentModel.AsyncCompletedEventArgs e,
            Func<T> getResult, Action unregisterHandler)
        {
            // Transfers the results from the AsyncCompletedEventArgs and getResult() to the
            // TaskCompletionSource, but only AsyncCompletedEventArg's UserState matches the TCS
            // (this check is important if the same WebClient is used for multiple, asynchronous
            // operations concurrently). Also unregisters the handler to avoid a leak.
            if (e.UserState != tcs) return;
            if (e.Cancelled) tcs.TrySetCanceled();
            else if (e.Error != null) tcs.TrySetException(e.Error);
            else tcs.TrySetResult(getResult());
            unregisterHandler();
        }
    }

    /// <summary>Extension methods for working with SmtpClient asynchronously.</summary>
    public static class SmtpClientExtensions
    {
        /// <summary>Sends an e-mail message asynchronously.</summary>
        /// <param name="smtpClient">The client.</param>
        /// <param name="message">A MailMessage that contains the message to send.</param>
        /// <param name="userToken">A user-defined object stored in the resulting Task.</param>
        /// <returns>A Task that represents the asynchronous send.</returns>
        public static Task SendTask(this SmtpClient smtpClient, MailMessage message, object userToken)
        {
            return SendTaskCore(smtpClient, userToken, tcs => smtpClient.SendAsync(message, tcs));
        }

        /// <summary>Sends an e-mail message asynchronously.</summary>
        /// <param name="smtpClient">The client.</param>
        /// <param name="message">A MailMessage that contains the message to send.</param>
        /// <param name="from">A String that contains the address information of the message sender.</param>
        /// <param name="recipients">A String that contains the address that the message is sent to.</param>
        /// <param name="subject">A String that contains the subject line for the message.</param>
        /// <param name="body">A String that contains the message body.</param>
        /// <param name="userToken">A user-defined object stored in the resulting Task.</param>
        /// <returns>A Task that represents the asynchronous send.</returns>
        public static Task SendTask(this SmtpClient smtpClient, string from, string recipients, string subject, string body, object userToken)
        {
            return SendTaskCore(smtpClient, userToken, tcs => smtpClient.SendAsync(from, recipients, subject, body, tcs));
        }

        /// <summary>The core implementation of SendTask.</summary>
        /// <param name="smtpClient">The client.</param>
        /// <param name="userToken">The user-supplied state.</param>
        /// <param name="sendAsync">
        /// A delegate that initiates the asynchronous send.
        /// The provided TaskCompletionSource must be passed as the user-supplied state to the actual SmtpClient.SendAsync method.
        /// </param>
        /// <returns></returns>
        private static Task SendTaskCore(SmtpClient smtpClient, object userToken, Action<TaskCompletionSource<object>> sendAsync)
        {
            // Validate we're being used with a real smtpClient.  The rest of the arg validation
            // will happen in the call to sendAsync.
            if (smtpClient == null) throw new ArgumentNullException(nameof(smtpClient));

            // Create a TaskCompletionSource to represent the operation
            var tcs = new TaskCompletionSource<object>(userToken);

            // Register a handler that will transfer completion results to the TCS Task
            SendCompletedEventHandler handler = null;
            handler = (sender, e) => EAPCommon.HandleCompletion(tcs, e, () => null, () => smtpClient.SendCompleted -= handler);
            smtpClient.SendCompleted += handler;

            // Try to start the async operation.  If starting it fails (due to parameter validation)
            // unregister the handler before allowing the exception to propagate.
            try
            {
                sendAsync(tcs);
            }
            catch (Exception exc)
            {
                smtpClient.SendCompleted -= handler;
                tcs.TrySetException(exc);
            }

            // Return the task to represent the asynchronous operation
            return tcs.Task;
        }
    }
}