c# – 如何使用MimeKit获取电子邮件的WYSIWYG正文

using (var client = new ImapClient())
{
    client.Connect(emailServer,993,true);
    client.AuthenticationMechanisms.Remove("XOAUTH2");
    client.Authenticate(username,password);
    var inbox = client.Inbox;
    inbox.Open(FolderAccess.ReadOnly);
    SearchQuery query;
    if (checkBox.IsChecked == false)
    {
        query = SearchQuery.DeliveredBefore((DateTime)dateEnd).And(
            SearchQuery.DeliveredAfter((DateTime)dateStart)).And(
            SearchQuery.SubjectContains("Subject to find"));
    }
    else
    {
        query = SearchQuery.SubjectContains("Subject to find");
    }
    foreach (var uid in inbox.Search(query))
    {
        var message = inbox.GetMessage(uid);
        formEmails.Add(message.TextBody);
        messageDate.Add(message.Date.LocalDateTime);
    }
    client.Disconnect(true);
}

对电子邮件的一个常见误解是,有一个明确定义的邮件正文,然后是一个附件列表.实际情况并非如此.实际情况是MIME是内容的树结构,很像文件系统.

幸运的是,MIME确实定义了一组关于邮件客户端应该如何解释MIME部分的树结构的一般规则. Content-Disposition标头用于向接收客户端提供关于哪些部分要作为消息正文的一部分显示以及哪些部分应被解释为附件的提示.

Content-Disposition标头通常具有以下两个值之一：内联或附件.

这些值的含义应该相当明显.如果值是附件,则所述MIME部分的内容意味着呈现为与核心消息分开的文件附件.但是,如果值是内联的,则该MIME部分的内容应在邮件客户端呈现核心邮件正文中内联显示.如果Content-Disposition标头不存在,则应将其视为值为内联.

从技术上讲,缺少Content-Disposition标头或标记为内联的每个部分都是核心消息体的一部分.

不过,还有更多的东西.

现代MIME消息通常包含一个多部分/备用MIME容器,它通常包含发送者写入的文本的text / plain和text / html版本. text / html版本的格式通常更接近发件人在他或她所见即所得编辑器中看到的文本/纯文本版本.

以两种格式发送消息文本的原因是并非所有邮件客户端都能够显示HTML.

接收客户端应仅显示multipart / alternative容器中包含的备用视图之一.由于备选视图按照最忠实于最忠实的顺序列出发件人在其所见即所得编辑器中看到的内容,因此接收客户端应该从最后开始的备用视图列表中进行操作并向后工作,直到找到它的一部分为止能够显示.

例：

multipart/alternative
  text/plain
  text/html

如上例所示,text / html部分列在最后,因为它最忠实于发送者在编写消息时在其所见即所得编辑器中看到的内容.

为了使问题更加复杂,有时现代邮件客户端将使用多部分/相关MIME容器而不是简单的text / html部分,以便在HTML中嵌入图像和其他多媒体内容.

例：

multipart/alternative
  text/plain
  multipart/related
    text/html
    image/jpeg
    video/mp4
    image/png

在上面的示例中,备用视图之一是多部分/相关容器,其中包含引用兄弟视频和图像的消息主体的HTML版本.

现在您已经大致了解了消息的结构以及如何解释各种MIME实体,我们可以开始弄清楚如何按预期实际呈现消息.

使用MimeVisitor(呈现消息的最准确方式)

MimeKit包含一个MimeVisitor类,用于访问MIME树结构中的每个节点.例如,以下MimeVisitor子类可用于生成由浏览器控件(如WebBrowser)呈现的HTML：

/// <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.ContentObject.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);
    }
}

您使用此访问者的方式可能如下所示：

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TextBody和HtmlBody属性(最简单的方法)

为了简化获取消息文本的常见任务,MimeMessage包含两个属性,可以帮助您获取消息正文的text / plain或text / html版本.这些分别是TextBody和HtmlBody.

但请记住,至少在HtmlBody属性中,HTML部分可能是多部分/相关的子部分,允许它引用也包含在该部分/相关内的图像和其他类型的媒体.实体.这个属性实际上只是一个方便的属性,并不是自己遍历MIME结构的真正好的替代品,因此您可以正确地解释相关内容.