您的解决方案类似于我在MimeKit 的 MessageReader 示例中使用的逻辑，但现在 MimeKit 提供了更好的解决方案：
/// <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 ();
}
string text = entity.Text;
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);
}
}
然后要使用这个自定义HtmlPreviewVisitor类，你会有一个类似这样的方法：
void Render (WebBrowser browser, MimeMessage message)
{
var tmpDir = Path.Combine (Path.GetTempPath (), message.MessageId);
var visitor = new HtmlPreviewVisitor (tmpDir);
Directory.CreateDirectory (tmpDir);
message.Accept (visitor);
browser.DocumentText = visitor.HtmlBody;
}
我知道这看起来像是很多代码，但它涵盖的不仅仅是简单的案例。您会注意到，如果 HTML 不可用，它还会处理渲染text/plain和正文。text/plain; format=flowed它还正确地仅使用属于封装multipart/related树的图像。
您可以修改此代码的一种方法是将图像嵌入到img标签中，而不是使用临时目录。为此，您需要将SaveImage方法修改为如下所示（请注意，下一段代码未经测试）：
string SaveImage (MimePart image, string url)
{
using (var output = new MemoryStream ()) {
image.ContentObject.DecodeTo (output);
var buffer = output.GetBuffer ();
int length = (int) output.Length;
return string.Format ("data:{0};base64,{1}", image.ContentType.MimeType, Convert.ToBase64String (buffer, 0, length));
}
}