How to bind a TextBlock to a resource containing formatted text?

Question

I have a TextBlock in my WPF window.

 
     Some formatted text.
 

When it is r

Answer 1

I've added hyperlink and image support to Vincents solution:

public static class FormattedTextBlock
{
    public static string GetFormattedText(DependencyObject obj)
    {
        return (string)obj.GetValue(FormattedTextProperty);
    }

    public static void SetFormattedText(DependencyObject obj, string value)
    {
        obj.SetValue(FormattedTextProperty, value);
    }

    public static readonly DependencyProperty FormattedTextProperty =
        DependencyProperty.RegisterAttached("FormattedText",
        typeof(string),
        typeof(FormattedTextBlock),
        new UIPropertyMetadata("", FormattedTextChanged));

    static Inline Traverse(string value)
    {
        // Get the sections/inlines
        string[] sections = SplitIntoSections(value);

        // Check for grouping
        if(sections.Length.Equals(1))
        {
            string section = sections[0];
            string token; // E.g 
            int tokenStart, tokenEnd; // Where the token/section starts and ends.

            // Check for token
            if(GetTokenInfo(section, out token, out tokenStart, out tokenEnd))
            {
                // Get the content to further examination
                string content = token.Length.Equals(tokenEnd - tokenStart) ?
                    null :
                    section.Substring(token.Length, section.Length - 1 - token.Length * 2);

                switch(token.ToUpper())
                {
                    case "":
                    case "":
                        /* Bold text */
                        return new Bold(Traverse(content));
                    case "":
                    case "":
                        /* Italic text */
                        return new Italic(Traverse(content));
                    case "":
                    case "":
                        /* Underlined text */
                        return new Underline(Traverse(content));
                    case "
":
                    case "
":
                    case "":
                        /* Line 1
line 2 */
                        return new LineBreak();
                    case "":
                    case "":
                        /* {http://www.google.de}Google */
                        var start = content.IndexOf("{");
                        var end = content.IndexOf("}");
                        var url = content.Substring(start + 1, end - 1);
                        var text = content.Substring(end + 1);
                        var link = new Hyperlink();
                        link.NavigateUri = new System.Uri(url);
                        link.RequestNavigate += Hyperlink_RequestNavigate;
                        link.Inlines.Add(text);
                        return link;
                    case "":
                    case "":
                        /* pack://application:,,,/ProjectName;component/directory1/directory2/image.png */
                        var image = new Image();
                        var bitmap = new BitmapImage(new Uri(content));
                        image.Source = bitmap;
                        image.Width = bitmap.Width;
                        image.Height = bitmap.Height;
                        var container = new InlineUIContainer();
                        container.Child = image;
                        return container;
                    default:
                        return new Run(section);
                }
            }
            else return new Run(section);
        }
        else // Group together
        {
            Span span = new Span();

            foreach(string section in sections)
                span.Inlines.Add(Traverse(section));

            return span;
        }
    }

    static void Hyperlink_RequestNavigate(object sender, RequestNavigateEventArgs e)
    {
        Process.Start(e.Uri.ToString());
    }

    /// 

    /// Examines the passed string and find the first token, where it begins and where it ends.
    /// 
    /// The string to examine.
    /// The found token.
    /// Where the token begins.
    /// Where the end-token ends.
    /// True if a token was found.
    static bool GetTokenInfo(string value, out string token, out int startIndex, out int endIndex)
    {
        token = null;
        endIndex = -1;

        startIndex = value.IndexOf("<");
        int startTokenEndIndex = value.IndexOf(">");

        // No token here
        if(startIndex < 0)
            return false;

        // No token here
        if(startTokenEndIndex < 0)
            return false;

        token = value.Substring(startIndex, startTokenEndIndex - startIndex + 1);

        // Check for closed token. E.g. 
        if(token.EndsWith("/>"))
        {
            endIndex = startIndex + token.Length;
            return true;
        }

        string endToken = token.Insert(1, "/");

        // Detect nesting;
        int nesting = 0;
        int temp_startTokenIndex = -1;
        int temp_endTokenIndex = -1;
        int pos = 0;
        do
        {
            temp_startTokenIndex = value.IndexOf(token, pos);
            temp_endTokenIndex = value.IndexOf(endToken, pos);

            if(temp_startTokenIndex >= 0 && temp_startTokenIndex < temp_endTokenIndex)
            {
                nesting++;
                pos = temp_startTokenIndex + token.Length;
            }
            else if(temp_endTokenIndex >= 0 && nesting > 0)
            {
                nesting--;
                pos = temp_endTokenIndex + endToken.Length;
            }
            else // Invalid tokenized string
                return false;

        } while(nesting > 0);

        endIndex = pos;

        return true;
    }

    /// 

    /// Splits the string into sections of tokens and regular text.
    /// 
    /// The string to split.
    /// An array with the sections.
    static string[] SplitIntoSections(string value)
    {
        List sections = new List();

        while(!string.IsNullOrEmpty(value))
        {
            string token;
            int tokenStartIndex, tokenEndIndex;

            // Check if this is a token section
            if(GetTokenInfo(value, out token, out tokenStartIndex, out tokenEndIndex))
            {
                // Add pretext if the token isn't from the start
                if(tokenStartIndex > 0)
                    sections.Add(value.Substring(0, tokenStartIndex));

                sections.Add(value.Substring(tokenStartIndex, tokenEndIndex - tokenStartIndex));
                value = value.Substring(tokenEndIndex); // Trim away
            }
            else
            { // No tokens, just add the text
                sections.Add(value);
                value = null;
            }
        }

        return sections.ToArray();
    }

    private static void FormattedTextChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        string value = e.NewValue as string;

        TextBlock textBlock = sender as TextBlock;

        if(textBlock != null)
            textBlock.Inlines.Add(Traverse(value));
    }
}

Thanks Vincent for the great template, it works like a charm!