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MimeKit

HtmlToHtml Class

An HTML to HTML converter.
Inheritance Hierarchy
SystemObject
  MimeKit.TextTextConverter
    MimeKit.TextHtmlToHtml

Namespace: MimeKit.Text
Assembly: MimeKit (in MimeKit.dll) Version: 4.3.0
Syntax
C#
public class HtmlToHtml : TextConverter

The HtmlToHtml type exposes the following members.

Constructors
 NameDescription
Public methodHtmlToHtml Initialize a new instance of the HtmlToHtml class.
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Properties
 NameDescription
Public propertyDetectEncodingFromByteOrderMark Get or set whether the encoding of the input is detected from the byte order mark or determined by the InputEncoding property.
(Inherited from TextConverter)
Public propertyFilterComments Get or set whether or not the converter should remove HTML comments from the output.
Public propertyFilterHtml Get or set whether or not executable scripts should be stripped from the output.
Public propertyFooter Get or set the text that will be appended to the end of the output.
(Inherited from TextConverter)
Public propertyFooterFormat Get or set the footer format.
Public propertyHeader Get or set text that will be prepended to the beginning of the output.
(Inherited from TextConverter)
Public propertyHeaderFormat Get or set the header format.
Public propertyCode exampleHtmlTagCallback Get or set the HtmlTagCallback method to use for custom filtering of HTML tags and content.
Public propertyInputEncoding Get or set the input encoding.
(Inherited from TextConverter)
Public propertyInputFormat Get the input format.
(Overrides TextConverterInputFormat)
Public propertyInputStreamBufferSize Get or set the size of the input stream buffer.
(Inherited from TextConverter)
Public propertyOutputEncoding Get or set the output encoding.
(Inherited from TextConverter)
Public propertyOutputFormat Get the output format.
(Overrides TextConverterOutputFormat)
Public propertyOutputStreamBufferSize Get or set the size of the output stream buffer.
(Inherited from TextConverter)
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Methods
 NameDescription
Public methodCode exampleConvert(String) Convert text from the InputFormat to the OutputFormat.
(Inherited from TextConverter)
Public methodConvert(Stream, Stream) Convert the contents of source from the InputFormat to the OutputFormat and writes the resulting text to destination.
(Inherited from TextConverter)
Public methodConvert(Stream, TextWriter) Convert the contents of source from the InputFormat to the OutputFormat and uses the writer to write the resulting text.
(Inherited from TextConverter)
Public methodConvert(TextReader, TextWriter) Convert the contents of reader from the InputFormat to the OutputFormat and uses the writer to write the resulting text.
(Overrides TextConverterConvert(TextReader, TextWriter))
Public methodConvert(TextReader, Stream) Convert the contents of reader from the InputFormat to the OutputFormat and writes the resulting text to destination.
(Inherited from TextConverter)
Public methodEquals
(Inherited from Object)
Protected methodFinalize
(Inherited from Object)
Public methodGetHashCode
(Inherited from Object)
Public methodGetType
(Inherited from Object)
Protected methodMemberwiseClone
(Inherited from Object)
Public methodToString
(Inherited from Object)
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Remarks
Used to convert HTML into HTML.
Example
C#
/// <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> ();
    string body;

    /// <summary>
    /// Creates a new HtmlPreviewVisitor.
    /// </summary>
    public HtmlPreviewVisitor ()
    {
    }

    /// <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;
    }

    /// <summary>
    /// Get a data: URI for the image attachment.
    /// </summary>
    /// <remarks>
    /// Encodes the image attachment into a string suitable for setting as a src= attribute value in
    /// an img tag.
    /// </remarks>
    /// <returns>The data: URI.</returns>
    /// <param name="image">The image attachment.</param>
    string GetDataUri (MimePart image)
    {
        using (var memory = new MemoryStream ()) {
            image.Content.DecodeTo (memory);
            var buffer = memory.GetBuffer ();
            var length = (int) memory.Length;
            var base64 = Convert.ToBase64String (buffer, 0, length);

            return string.Format ("data:{0};base64,{1}", image.ContentType.MimeType, base64);
        }
    }

    // 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.Meta && !ctx.IsEndTag) {
            bool isContentType = false;

            ctx.WriteTag (htmlWriter, false);

            // replace charsets with "utf-8" since our output will be in utf-8 (and not whatever the original charset was)
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Id == HtmlAttributeId.Charset) {
                    htmlWriter.WriteAttributeName (attribute.Name);
                    htmlWriter.WriteAttributeValue ("utf-8");
                } else if (isContentType && attribute.Id == HtmlAttributeId.Content) {
                    htmlWriter.WriteAttributeName (attribute.Name);
                    htmlWriter.WriteAttributeValue ("text/html; charset=utf-8");
                } else {
                    if (attribute.Id == HtmlAttributeId.HttpEquiv && attribute.Value != null
                        && attribute.Value.Equals ("Content-Type", StringComparison.OrdinalIgnoreCase))
                        isContentType = true;

                    htmlWriter.WriteAttribute (attribute);
                }
            }
        } else if (ctx.TagId == HtmlTagId.Image && !ctx.IsEndTag && stack.Count > 0) {
            ctx.WriteTag (htmlWriter, false);

            // replace the src attribute with a "data:" URL
            foreach (var attribute in ctx.Attributes) {
                if (attribute.Id == HtmlAttributeId.Src) {
                    if (!TryGetImage (attribute.Value, out var image)) {
                        htmlWriter.WriteAttribute (attribute);
                        continue;
                    }

                    var dataUri = GetDataUri (image);

                    htmlWriter.WriteAttributeName (attribute.Name);
                    htmlWriter.WriteAttributeValue (dataUri);
                } 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.Equals ("oncontextmenu", StringComparison.OrdinalIgnoreCase))
                    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.Equals ("yes", StringComparison.OrdinalIgnoreCase);

            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);
    }
}
See Also