C#中使用IMAP协议读取一些附件的问题

Question

我想请教您以下问题：

我有两个代码可以阅读带有各自附件的电子邮件：

第一个，我用S22.IMAP dll:

using (ImapClient Client = new ImapClient(imap, 993, usuario, psw, AuthMethod.Login, true))
{
    IEnumerable<uint> uids = Client.Search(SearchCondition.Unseen());//Correos no leídos

    IEnumerable<MailMessage> messages = Client.GetMessages(uids, FetchOptions.Normal); 

    conexion.stringconeccion = stringconeccion;
    conexion.conectar();

    String ts = "start transaction";

    MySqlCommand datos_ts = new MySqlCommand(ts, conexion.con);
    datos_ts.ExecuteScalar();

    DataTable dt_existeXML = new DataTable();

    int insercion = 0;

    foreach (MailMessage msg in messages)
    {
        foreach (Attachment atc in msg.Attachments)
        {
            if (System.IO.Path.GetExtension(msg.Attachments[0].Name) == ".xml")
            {
                String archivoXML_texto = "";
                byte[] allBytes = new byte[msg.Attachments[0].ContentStream.Length];
                int bytesRead = msg.Attachments[0].ContentStream.Read(allBytes, 0, (int)msg.Attachments[0].ContentStream.Length);

                using (MemoryStream memory = new MemoryStream(allBytes))
                {
                    StreamReader archivoXML = new StreamReader(memory);

                    archivoXML_texto = archivoXML.ReadToEnd();
                    archivoXML.Close();

                    memory.Dispose();
                }
            }
        }
    }

第二个代码，使用 MailKit DLL：

using (var client = new ImapClient ()) {
    client.Connect ("imap.gmail.com", 993, SecureSocketOptions.SslOnConnect);
    client.Authenticate ("correo@gmail.com", "clave");
    client.Inbox.Open (FolderAccess.ReadOnly);

    var uids = client.Inbox.Search(SearchQuery.NotSeen);

    foreach (var uid in uids)
    {
        var message = client.Inbox.GetMessage(uid);
        foreach (var attachment in message.Attachments.OfType<MimePart>())
        {   
                byte[] allBytes = new byte[attachment.Content.Stream.Length];
                int bytesRead = attachment.Content.Stream.Read(allBytes, 0, (int)attachment.Content.Stream.Length);
                string texto_definitivo = "";
                String archivoXML_textoBase64 = "";
                using (MemoryStream memory = new MemoryStream(allBytes))
                {
                    StreamReader archivoXML = new StreamReader(memory);
                    archivoXML_textoBase64 = archivoXML.ReadToEnd();
                    byte[] temp_backToBytes = Convert.FromBase64String(archivoXML_textoBase64);
                    texto_definitivo = Encoding.ASCII.GetString(temp_backToBytes);
                    archivoXML.Close();
                    memory.Dispose();
                }   
        }
    }
    client.Disconnect (true);
}

但我意识到有些附件没有阅读，而且，我注意到一些未阅读的附件的共同点，没有附件图标。但是当我打开邮件时，我看到它们有附件：

enter image description here

例如标红的图片，右侧的附件图标不显示。但是当我打开它时，我确认它有一个附加文件。只是那些文件，两个代码中的一个，不看附件。

我的问题是：

我怎么也看那些附件？我应该配置或启用邮件中的任何选项吗？什么以及如何？发邮件有误吗？怎么解决的？

更新

希望我已经理解了。以下代码尝试在可能的情况下获取附件的文本/内容。如有不妥请指正

    public static void DownloadBodyParts ()
    {
        using (var client = new ImapClient ()) {
            client.Connect ("imap.gmail.com", 993, SecureSocketOptions.SslOnConnect);             
            client.Authenticate("correo@gmail.com", "clave");
            client.Inbox.Open(FolderAccess.ReadWrite);
            var uids = client.Inbox.Search(SearchQuery.NotSeen);          

            foreach (var uid in uids)
            {
                var message = client.Inbox.GetMessage(uid);                    
                var attachments = message.BodyParts.OfType<MimePart>().Where(part => !string.IsNullOrEmpty(part.FileName));

                foreach (MimePart atch in attachments)
                {
                    using (var memory = new MemoryStream())
                    {
                        atch.Content.DecodeTo(memory);
                        var buffer = memory.ToArray();
                        var text = Encoding.UTF8.GetString(buffer);
                    }
                }
             }
        client.Disconnect (true);
         }

Answer 1

来自 MailKit FAQ:

问：如何判断邮件是否有附件？

在大多数情况下，body 的 MIME-type 或 multipart/mixed 包含多个部分的邮件可能有附件。如上所示，multipart/mixed 的第一部分通常是消息的文本 body，但并不总是那么简单。

一般来说，MIME 附件会有一个 Content-Disposition header 的值是 attachment。要获取符合此条件的 body 部分的列表，您可以使用 MimeMessage.Attachments 属性.

遗憾的是，并非所有邮件客户端都遵循此约定，因此您可能需要编写自己的自定义逻辑。例如，您可能希望处理设置了 name 或 filename 参数的所有 body 部分：

var attachments = message.BodyParts.OfType<MimePart> ().Where (part => !string.IsNullOrEmpty (part.FileName));

一种更复杂的方法是将邮件的主要文本 body 部分未引用的 body 部分视为附件。换句话说，将任何未用于呈现消息的 body 部分视为附件。有关如何执行此操作的示例，请考虑以下代码片段：

/// <summary>
/// Visits a MimeMessage and generates HTML suitable to be rendered by a browser control.
/// </summary>
class HtmlPreviewVisitor : MimeVisitor
{
    List<MultipartRelated> stack = new List<MultipartRelated> ();
    List<MimeEntity> attachments = new List<MimeEntity> ();
    readonly string tempDir;
    string body;

    /// <summary>
    /// Creates a new HtmlPreviewVisitor.
    /// </summary>
    /// <param name="tempDirectory">A temporary directory used for storing image files.</param>
    public HtmlPreviewVisitor (string tempDirectory)
    {
        tempDir = tempDirectory;
    }

    /// <summary>
    /// The list of attachments that were in the MimeMessage.
    /// </summary>
    public IList<MimeEntity> Attachments {
        get { return attachments; }
    }

    /// <summary>
    /// The HTML string that can be set on the BrowserControl.
    /// </summary>
    public string HtmlBody {
        get { return body ?? string.Empty; }
    }

    protected override void VisitMultipartAlternative (MultipartAlternative alternative)
    {
        // walk the multipart/alternative children backwards from greatest level of faithfulness to the least faithful
        for (int i = alternative.Count - 1; i >= 0 && body == null; i--)
            alternative[i].Accept (this);
    }

    protected override void VisitMultipartRelated (MultipartRelated related)
    {
        var root = related.Root;

        // push this multipart/related onto our stack
        stack.Add (related);

        // visit the root document
        root.Accept (this);

        // pop this multipart/related off our stack
        stack.RemoveAt (stack.Count - 1);
    }

    // look up the image based on the img src url within our multipart/related stack
    bool TryGetImage (string url, out MimePart image)
    {
        UriKind kind;
        int index;
        Uri uri;

        if (Uri.IsWellFormedUriString (url, UriKind.Absolute))
            kind = UriKind.Absolute;
        else if (Uri.IsWellFormedUriString (url, UriKind.Relative))
            kind = UriKind.Relative;
        else
            kind = UriKind.RelativeOrAbsolute;

        try {
            uri = new Uri (url, kind);
        } catch {
            image = null;
            return false;
        }

        for (int i = stack.Count - 1; i >= 0; i--) {
            if ((index = stack[i].IndexOf (uri)) == -1)
                continue;

            image = stack[i][index] as MimePart;
            return image != null;
        }

        image = null;

        return false;
    }

    // Save the image to our temp directory and return a "file://" url suitable for
    // the browser control to load.
    // Note: if you'd rather embed the image data into the HTML, you can construct a
    // "data:" url instead.
    string SaveImage (MimePart image, string url)
    {
        string fileName = url.Replace (':', '_').Replace ('\', '_').Replace ('/', '_');

        string path = Path.Combine (tempDir, fileName);

        if (!File.Exists (path)) {
            using (var output = File.Create (path))
                image.Content.DecodeTo (output);
        }

        return "file://" + path.Replace ('\', '/');
    }

    // Replaces <img src=...> urls that refer to images embedded within the message with
    // "file://" urls that the browser control will actually be able to load.
    void HtmlTagCallback (HtmlTagContext ctx, HtmlWriter htmlWriter)
    {
        if (ctx.TagId == HtmlTagId.Image && !ctx.IsEndTag && stack.Count > 0) {
            ctx.WriteTag (htmlWriter, false);

            // replace the src attribute with a file:// URL
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Id == HtmlAttributeId.Src) {
                    MimePart image;
                    string url;

                    if (!TryGetImage (attribute.Value, out image)) {
                        htmlWriter.WriteAttribute (attribute);
                        continue;
                    }

                    url = SaveImage (image, attribute.Value);

                    htmlWriter.WriteAttributeName (attribute.Name);
                    htmlWriter.WriteAttributeValue (url);
                } else {
                    htmlWriter.WriteAttribute (attribute);
                }
            }
        } else if (ctx.TagId == HtmlTagId.Body && !ctx.IsEndTag) {
            ctx.WriteTag (htmlWriter, false);

            // add and/or replace oncontextmenu="return false;"
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Name.ToLowerInvariant () == "oncontextmenu")
                    continue;

                htmlWriter.WriteAttribute (attribute);
            }

            htmlWriter.WriteAttribute ("oncontextmenu", "return false;");
        } else {
            // pass the tag through to the output
            ctx.WriteTag (htmlWriter, true);
        }
    }

    protected override void VisitTextPart (TextPart entity)
    {
        TextConverter converter;

        if (body != null) {
            // since we've already found the body, treat this as an attachment
            attachments.Add (entity);
            return;
        }

        if (entity.IsHtml) {
            converter = new HtmlToHtml {
                HtmlTagCallback = HtmlTagCallback
            };
        } else if (entity.IsFlowed) {
            var flowed = new FlowedToHtml ();
            string delsp;

            if (entity.ContentType.Parameters.TryGetValue ("delsp", out delsp))
                flowed.DeleteSpace = delsp.ToLowerInvariant () == "yes";

            converter = flowed;
        } else {
            converter = new TextToHtml ();
        }

        body = converter.Convert (entity.Text);
    }

    protected override void VisitTnefPart (TnefPart entity)
    {
        // extract any attachments in the MS-TNEF part
        attachments.AddRange (entity.ExtractAttachments ());
    }

    protected override void VisitMessagePart (MessagePart entity)
    {
        // treat message/rfc822 parts as attachments
        attachments.Add (entity);
    }

    protected override void VisitMimePart (MimePart entity)
    {
        // realistically, if we've gotten this far, then we can treat this as an attachment
        // even if the IsAttachment property is false.
        attachments.Add (entity);
    }
}
And the way you'd use this visitor might look something like this:

void Render (MimeMessage message)
{
    var tmpDir = Path.Combine (Path.GetTempPath (), message.MessageId);
    var visitor = new HtmlPreviewVisitor (tmpDir);

    Directory.CreateDirectory (tmpDir);

    message.Accept (visitor);

    DisplayHtml (visitor.HtmlBody);
    DisplayAttachments (visitor.Attachments);
}

使用上述技术呈现邮件后，您将获得未使用的附件列表，即使它们不符合 MimeMessage.Attachments [=60 使用的简单标准=].

更新：

下面是如何获取 MimePart 的文本内容（假设 MimePart 不能转换为 TextPart，这使得这个过程非常简单）。

using (var memory = new MemoryStream ()) {
    mimePart.Content.DecodeTo (memory);

    var buffer = memory.ToArray ();
    var text = Encoding.UTF8.GetString (buffer);
}

当然，如果MimePart可以转换成TextPart，那就更简单了：

var textPart = (TextPart) mimePart;
var text = textPart.Text;

C#中使用IMAP协议读取一些附件的问题

Problems with reading some attachments using IMAP protocol in C #

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gmail

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attachment

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