using System;
using System.IO;
using System.Net.Mail;
using System.Reflection;

namespace Swan {
  /// <summary>
  /// Extension methods.
  /// </summary>
  public static class SmtpExtensions {
    private static readonly BindingFlags PrivateInstanceFlags = BindingFlags.Instance | BindingFlags.NonPublic;

    /// <summary>
    /// The raw contents of this MailMessage as a MemoryStream.
    /// </summary>
    /// <param name="this">The caller.</param>
    /// <returns>A MemoryStream with the raw contents of this MailMessage.</returns>
    public static MemoryStream ToMimeMessage(this MailMessage @this) {
      if(@this == null) {
        throw new ArgumentNullException(nameof(@this));
      }

      MemoryStream result = new MemoryStream();
      Object mailWriter = MimeMessageConstants.MailWriterConstructor.Invoke(new Object[] { result });
      _ = MimeMessageConstants.SendMethod.Invoke(@this, PrivateInstanceFlags, null, MimeMessageConstants.IsRunningInDotNetFourPointFive ? new[] { mailWriter, true, true } : new[] { mailWriter, true }, null);

      result = new MemoryStream(result.ToArray());
      _ = MimeMessageConstants.CloseMethod.Invoke(mailWriter, PrivateInstanceFlags, null, Array.Empty<Object>(), null);
      result.Position = 0;
      return result;
    }

    internal static class MimeMessageConstants {
#pragma warning disable DE0005 // API is deprecated
      public static readonly Type MailWriter = typeof(SmtpClient).Assembly.GetType("System.Net.Mail.MailWriter");
#pragma warning restore DE0005 // API is deprecated
      public static readonly ConstructorInfo MailWriterConstructor = MailWriter.GetConstructor(PrivateInstanceFlags, null, new[] { typeof(Stream) }, null);
      public static readonly MethodInfo CloseMethod = MailWriter.GetMethod("Close", PrivateInstanceFlags);
      public static readonly MethodInfo SendMethod = typeof(MailMessage).GetMethod("Send", PrivateInstanceFlags);
      public static readonly Boolean IsRunningInDotNetFourPointFive = SendMethod.GetParameters().Length == 3;
    }
  }
}