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

I have a TextBlock in my WPF window.

 <TextBlock>
     Some <Bold>formatted</Bold> text.
 </TextBlock>

When it is rendered it looks like this,

Some formatted text.

My question is, can I bind this inline "content" to a resource in my application?

I got as far as:

Making an application resource string,

myText="Some <Bold>formatted</Bold> text."

and the following xaml (Some code omitted for brevity)

 <Window xmlns:props="clr-namespace:MyApp.Properties">
     <Window.Resources>
         <props:Resources x:Key="Resources"/>
     </Window.Resources>
      <TextBlock x:Name="Try1" 
          Text="{Binding Source={StaticResource Resources} Path=myText}"/>
     <TextBlock x:Name="Try2">
          <Binding Source="{StaticResourc开发者_Go百科e Resources}" Path="myText" />
     </TextBlock>
 </Window>

Try1 renders with the tags in place and not effecting formatting.

Some <Bold>formatted<Bold> text.

Try2 will not compile or render because the resource "myText" is not of type Inline but a string.

Is this seemingly simple task possible and if so how?

---

Here is my modified code for recursively format text. It handles Bold, Italic, Underline and LineBreak but can easily be extended to support more (modify the switch statement).

public static class MyBehavior
{
    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(MyBehavior),
        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 <Bold>
            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)
                {
                    case "<Bold>":
                        return new Bold(Traverse(content));
                    case "<Italic>":
                        return new Italic(Traverse(content));
                    case "<Underline>":
                        return new Underline(Traverse(content));
                    case "<LineBreak/>":
                        return new LineBreak();
                    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;
        }
    }

    /// <summary>
    /// Examines the passed string and find the first token, where it begins and where it ends.
    /// </summary>
    /// <param name="value">The string to examine.</param>
    /// <param name="token">The found token.</param>
    /// <param name="startIndex">Where the token begins.</param>
    /// <param name="endIndex">Where the end-token ends.</param>
    /// <returns>True if a token was found.</returns>
    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. <LineBreak/>
        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;
    }

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

        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));
    }
}

---

Edit: (proposed by Spook)

A shorter version, but requires the text to be XML-valid:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Xml;

// (...)

public static class TextBlockHelper
{
    #region FormattedText Attached dependency property

    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(TextBlockHelper),
        new UIPropertyMetadata("", FormattedTextChanged));

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

        TextBlock textBlock = sender as TextBlock;

        if (textBlock != null)
        {
            textBlock.Inlines.Clear();
            textBlock.Inlines.Add(Process(value));
        }
    }

    #endregion

    static Inline Process(string value)
    {
        XmlDocument doc = new XmlDocument();
        doc.LoadXml(value);

        Span span = new Span();
        InternalProcess(span, doc.ChildNodes[0]);

        return span;
    }

    private static void InternalProcess(Span span, XmlNode xmlNode)
    {
        foreach (XmlNode child in xmlNode)
        {
            if (child is XmlText)
            {
                span.Inlines.Add(new Run(child.InnerText));
            }
            else if (child is XmlElement)
            {
                Span spanItem = new Span();
                InternalProcess(spanItem, child);
                switch (child.Name.ToUpper())
                {
                    case "B":
                    case "BOLD":
                        Bold bold = new Bold(spanItem);
                        span.Inlines.Add(bold);
                        break;
                    case "I":
                    case "ITALIC":
                        Italic italic = new Italic(spanItem);
                        span.Inlines.Add(italic);
                        break;
                    case "U":
                    case "UNDERLINE":
                        Underline underline = new Underline(spanItem);
                        span.Inlines.Add(underline);
                        break;
                }
            }
        }
    }
}

And an example of usage:

<RootItem xmlns:u="clr-namespace:MyApp.Helpers">
    <TextBlock u:TextBlockHelper.FormattedText="{Binding SomeProperty}" />
</RootItem>

---

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 <Bold>
            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 "<B>":
                    case "<BOLD>":
                        /* <b>Bold text</b> */
                        return new Bold(Traverse(content));
                    case "<I>":
                    case "<ITALIC>":
                        /* <i>Italic text</i> */
                        return new Italic(Traverse(content));
                    case "<U>":
                    case "<UNDERLINE>":
                        /* <u>Underlined text</u> */
                        return new Underline(Traverse(content));
                    case "<BR>":
                    case "<BR/>":
                    case "<LINEBREAK/>":
                        /* Line 1<br/>line 2 */
                        return new LineBreak();
                    case "<A>":
                    case "<LINK>":
                        /* <a>{http://www.google.de}Google</a> */
                        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 "<IMG>":
                    case "<IMAGE>":
                        /* <image>pack://application:,,,/ProjectName;component/directory1/directory2/image.png</image> */
                        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());
    }

    /// <summary>
    /// Examines the passed string and find the first token, where it begins and where it ends.
    /// </summary>
    /// <param name="value">The string to examine.</param>
    /// <param name="token">The found token.</param>
    /// <param name="startIndex">Where the token begins.</param>
    /// <param name="endIndex">Where the end-token ends.</param>
    /// <returns>True if a token was found.</returns>
    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. <LineBreak/>
        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;
    }

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

        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!

---

How about using attached behavior? Below code only handles bold tags. Each word which should be bold needs to be wrapped in bold tags. You probably want to make the class accept other formats as well. Also spaces needs to be handled better, the class strips out consecutive spaces and add one extra to the end. So consider below class as demo code only which will need further work to be useful but it should get you started.

XAML:

<Window x:Class="FormatTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:FormatTest="clr-namespace:FormatTest"
    Title="Window1" Height="300" Width="300">

    <TextBlock FormatTest:FormattedTextBehavior.FormattedText="{Binding Path=Text}" />

</Window>

Code behind:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;

namespace FormatTest
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            DataContext = this;
        }

        public string Text { get { return "Some <Bold>formatted</Bold> text."; } }
    }

    public static class FormattedTextBehavior
    {
        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(FormattedTextBehavior),
                                                new UIPropertyMetadata("", FormattedTextChanged));

        private static void FormattedTextChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            TextBlock textBlock = sender as TextBlock;
            string value = e.NewValue as string;
            string[] tokens = value.Split(' ');
            foreach (string token in tokens)
            {
                if (token.StartsWith("<Bold>") && token.EndsWith("</Bold>"))
                {
                    textBlock.Inlines.Add(new Bold(new Run(token.Replace("<Bold>", "").Replace("</Bold>", "") + " ")));
                }
                else
                {
                    textBlock.Inlines.Add(new Run(token + " "));
                }
            }
        }
    }
}

---

EDIT:

This line,

<props:Resources x:Key="Resources"/>

is a bad approach to accesing the Project.Properties.Resources namespace. It causes awkward glitches when recompiling.

Much better to use x:Static to do somthing like this,

Text="{x:Static props:Resources.SomeText}"

in your binding. Thx to Ben

---

Okay, this is how I did it. It's not perfect but it works.

Remember, there is a project resource called FormattedText.

cs:

// TextBlock with a bindable InlineCollection property.

// Type is List(Inline) not InlineCollection becuase
// InlineCollection makes the IDE xaml parser complain
// presumably this is caused by an inherited attribute.

public class BindableTextBlock : TextBlock
{
    public static readonly DependencyProperty InlineCollectionProperty =
        DependencyProperty.Register(
            "InlineCollection",
            typeof(List<Inline>),
            typeof(BindableTextBlock),
            new UIPropertyMetadata(OnInlineCollectionChanged));

    private static void OnInlineCollectionChanged(DependencyObject sender,
        DependencyPropertyChangedEventArgs e)
    {
        BinableTextBlock instance = sender as BindableTextBlock;

        if (instance != null)
        {
            List<Inline> newText = e.NewValue as List<Inline>;
            if (newText != null)
            {
                // Clear the underlying Inlines property
                instance.Inlines.Clear();
                // Add the passed List<Inline> to the real Inlines
                instance.Inlines.AddRange(newText.ToList());
            }
        }
    }

    public List<Inline> InlineCollection
    {
        get
        {
            return (List<Inline>)GetValue(InlineCollectionProperty);
        }
        set
        {
            SetValue(InlineCollectionProperty, value);
        }
    }
}

// Convertor between a string of xaml with implied run elements
// and a generic list of inlines

[ValueConversion(typeof(string), typeof(List<Inline>))]
public class StringInlineCollectionConvertor : IValueConverter
{
    public object Convert(object value, 
        Type targetType, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        string text = value as String;

        // a surrogate TextBlock to host an InlineCollection
        TextBlock results = new TextBlock();

        if (!String.IsNullOrEmpty(text))
        {
            //Arbritary literal acting as a replace token, 
            //must not exist in the empty xaml definition.
            const string Replace = "xxx";

            // add a dummy run element and replace it with the text
            results.Inlines.Add(new Run(Replace));
            string resultsXaml = XamlWriter.Save(results);
            string resultsXamlWithText = resultsXaml.Replace(Replace, text);

            // deserialise the xaml back into our TextBlock
            results = XamlReader.Parse(resultsXamlWithText) as TextBlock;
        }
        return results.Inlines.ToList<Inline>();
    }

    // Not clear when this will be called but included for completeness

    public object ConvertBack(
        object value, 
        Type targetType, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        String results = String.Empty;

        InlineCollection inlines = value as InlineCollection;
        if (inlines != null)
        {
            //read the xaml as xml and return the "content"
            var reader = 
                XElement.Parse(XamlWriter.Save(inlines)).CreateReader();
            reader.MoveToContent();
            results = reader.ReadInnerXml();
        }
        return results;
    }
}

xaml:

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:props="clr-namespace:Project.Properties"
    xmlns:local="clr-namespace:Project">
    <Window.Resources>
        <props:Resources x:Key="Resources"/>
        <local:StringInlineCollectionConvertor x:Key="InlineConvert"/>
    </Window.Resources>
    <local:BindableTextBlock InlineCollection="
        {Binding Source={StaticResource Resources}, 
        Path=FormattedText, 
        Converter={StaticResource InlineConvert}}"/>
</Window>

I made 2 classes. A sub-classed TextBlock with a "bindable" InlineCollection and an IValueConverter to convert the collection from and to a String.

Using InlineCollection directly as the type of the property made VS2010 complain, although the code still ran fine. I changed to a generic list of Inlines. I assume that there is an inherited attribute telling VS that the InlineCollection has no constructor.

I tryed making the InlineCollection property the BindableTextBlock's ContentProperty but ran into issues and out of time. Please feel free to take the next step and tell me about it.

I apologise for any errata but this code had to be transcribed and sanitised.

If there is a better way of doing this, surely there must be, please tell me that too. Wouldn't it be nice if this functionality was built in or, have I missed something?

---

I ended up needing to do this in my application and had to support many of the markup possible normally in TextBlock inlines, so I took Wallstreet Programmer's answer above (which works beautifully and is much less complicated than most other answers I found on this topic) and expanded on it. I figure someone else might find this useful.

I haven't thoroughly tested this with ALL the tags yet, but every one I've tested has worked like a charm. I also suspect it's not the fastest code in the world, but my own testing with several thousand formatted messages in a ListView seemed surprisingly zippy. YMMV. Code is below:

XAML:

<Window x:Class="FormatTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:FormatTest="clr-namespace:FormatTest"
    Title="Window1" Height="300" Width="300">

    <TextBlock FormatTest:FormattedTextBehavior.FormattedText="{Binding Path=Text}" />

</Window>

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Media;

namespace FormatTest
{

public static class FormattedTextBehavior
{
    public class TextPart
    {
        public String mType = String.Empty;
        public Inline mInline = null;
        public InlineCollection mChildren = null;

        public TextPart() {}
        public TextPart(String t, Inline inline, InlineCollection col)
        {
            mType = t;
            mInline = inline;
            mChildren = col;
        }
    }

    private static Regex mRegex = new Regex(@"<(?<Span>/?[^>]*)>", RegexOptions.Compiled | RegexOptions.IgnoreCase);
    private static Regex mSpanRegex = new Regex("(?<Key>[^\\s=]+)=\"(?<Val>[^\\s\"]*)\"", RegexOptions.Compiled | RegexOptions.IgnoreCase);

    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(FormattedTextBehavior),
                                            new UIPropertyMetadata("", FormattedTextChanged));

    private static void FormattedTextChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        TextBlock textBlock = sender as TextBlock;
        FormatText(e.NewValue as string, new TextPart("TextBlock", null, textBlock.Inlines));
    }

    public static void FormatText(String s, TextPart root)
    {
        int len = s.Length;
        int lastIdx = 0;
        List<TextPart> parts = new List<TextPart>();
        parts.Add(root);
        Match m = mRegex.Match(s);
        while (m.Success)
        {
            String tag = m.Result("${Span}");
            if (tag.StartsWith("/"))
            {
                String prevStr = s.Substring(lastIdx, m.Index - lastIdx);
                TextPart part = parts.Last();
                if (!String.IsNullOrEmpty(prevStr))
                {
                    if (part.mChildren != null)
                    {
                        part.mChildren.Add(new Run(prevStr));
                    }
                    else if (part.mInline is Run)
                    {
                        (part.mInline as Run).Text = prevStr;
                    }
                }
                if (!tag.Substring(1).Equals(part.mType, StringComparison.InvariantCultureIgnoreCase))
                {
                    Logger.LogD("Mismatched End Tag '" + tag.Substring(1) + "' (expected </" + part.mType + ">) at position " + m.Index.ToString() + " in String '" + s + "'");
                }
                if (parts.Count > 1)
                {
                    parts.RemoveAt(parts.Count - 1);
                    TextPart parentPart = parts.Last();
                    if (parentPart.mChildren != null)
                    {
                        parentPart.mChildren.Add(part.mInline);
                    }
                }
            }
            else
            {
                TextPart prevPart = parts.Last();
                String prevStr = s.Substring(lastIdx, m.Index - lastIdx);
                if (!String.IsNullOrEmpty(prevStr))
                {
                    if (prevPart.mChildren != null)
                    {
                        prevPart.mChildren.Add(new Run(prevStr));
                    }
                    else if (prevPart.mInline is Run)
                    {
                        (prevPart.mInline as Run).Text = prevStr;
                    }
                }

                bool hasAttributes = false;
                TextPart part = new TextPart();
                if (tag.StartsWith("bold", StringComparison.InvariantCultureIgnoreCase))
                {
                    part.mType = "BOLD";
                    part.mInline = new Bold();
                    part.mChildren = (part.mInline as Bold).Inlines;
                }
                else if (tag.StartsWith("underline", StringComparison.InvariantCultureIgnoreCase))
                {
                    part.mType = "UNDERLINE";
                    part.mInline = new Underline();
                    part.mChildren = (part.mInline as Underline).Inlines;
                }
                else if (tag.StartsWith("italic", StringComparison.InvariantCultureIgnoreCase))
                {
                    part.mType = "ITALIC";
                    part.mInline = new Italic();
                    part.mChildren = (part.mInline as Italic).Inlines;
                }
                else if (tag.StartsWith("linebreak", StringComparison.InvariantCultureIgnoreCase))
                {
                    part.mType = "LINEBREAK";
                    part.mInline = new LineBreak();
                }
                else if (tag.StartsWith("span", StringComparison.InvariantCultureIgnoreCase))
                {
                    hasAttributes = true;
                    part.mType = "SPAN";
                    part.mInline = new Span();
                    part.mChildren = (part.mInline as Span).Inlines;
                }
                else if (tag.StartsWith("run", StringComparison.InvariantCultureIgnoreCase))
                {
                    hasAttributes = true;
                    part.mType = "RUN";
                    part.mInline = new Run();
                }
                else if (tag.StartsWith("hyperlink", StringComparison.InvariantCultureIgnoreCase))
                {
                    hasAttributes = true;
                    part.mType = "HYPERLINK";
                    part.mInline = new Hyperlink();
                    part.mChildren = (part.mInline as Hyperlink).Inlines;
                }

                if (hasAttributes && part.mInline != null)
                {
                    Match m2 = mSpanRegex.Match(tag);
                    while (m2.Success)
                    {
                        String key = m2.Result("${Key}");
                        String val = m2.Result("${Val}");
                        if (key.Equals("FontWeight", StringComparison.InvariantCultureIgnoreCase))
                        {
                            FontWeight fw = FontWeights.Normal;
                            try
                            {
                                fw = (FontWeight)new FontWeightConverter().ConvertFromString(val);
                            }
                            catch (Exception)
                            {
                                fw = FontWeights.Normal;
                            }
                            part.mInline.FontWeight = fw;
                        }
                        else if (key.Equals("FontSize", StringComparison.InvariantCultureIgnoreCase))
                        {
                            double fs = part.mInline.FontSize;
                            if (Double.TryParse(val, out fs))
                            {
                                part.mInline.FontSize = fs;
                            }
                        }
                        else if (key.Equals("FontStretch", StringComparison.InvariantCultureIgnoreCase))
                        {
                            FontStretch fs = FontStretches.Normal;
                            try
                            {
                                fs = (FontStretch)new FontStretchConverter().ConvertFromString(val);
                            }
                            catch (Exception)
                            {
                                fs = FontStretches.Normal;
                            }
                            part.mInline.FontStretch = fs;
                        }
                        else if (key.Equals("FontStyle", StringComparison.InvariantCultureIgnoreCase))
                        {
                            FontStyle fs = FontStyles.Normal;
                            try
                            {
                                fs = (FontStyle)new FontStyleConverter().ConvertFromString(val);
                            }
                            catch (Exception)
                            {
                                fs = FontStyles.Normal;
                            }
                            part.mInline.FontStyle = fs;
                        }
                        else if (key.Equals("FontFamily", StringComparison.InvariantCultureIgnoreCase))
                        {
                            if (!String.IsNullOrEmpty(val))
                            {
                                FontFamily ff = new FontFamily(val);
                                if (Fonts.SystemFontFamilies.Contains(ff))
                                {
                                    part.mInline.FontFamily = ff;
                                }
                            }
                        }
                        else if (key.Equals("Background", StringComparison.InvariantCultureIgnoreCase))
                        {
                            Brush b = part.mInline.Background;
                            try
                            {
                                b = (Brush)new BrushConverter().ConvertFromString(val);
                            }
                            catch (Exception)
                            {
                                b = part.mInline.Background;
                            }
                            part.mInline.Background = b;
                        }
                        else if (key.Equals("Foreground", StringComparison.InvariantCultureIgnoreCase))
                        {
                            Brush b = part.mInline.Foreground;
                            try
                            {
                                b = (Brush)new BrushConverter().ConvertFromString(val);
                            }
                            catch (Exception)
                            {
                                b = part.mInline.Foreground;
                            }
                            part.mInline.Foreground = b;
                        }
                        else if (key.Equals("ToolTip", StringComparison.InvariantCultureIgnoreCase))
                        {
                            part.mInline.ToolTip = val;
                        }
                        else if (key.Equals("Text", StringComparison.InvariantCultureIgnoreCase) && part.mInline is Run)
                        {
                            (part.mInline as Run).Text = val;
                        }
                        else if (key.Equals("NavigateUri", StringComparison.InvariantCultureIgnoreCase) && part.mInline is Hyperlink)
                        {
                            (part.mInline as Hyperlink).NavigateUri = new Uri(val);
                        }
                        m2 = m2.NextMatch();
                    }
                }

                if (part.mInline != null)
                {
                    if (tag.TrimEnd().EndsWith("/"))
                    {
                        if (prevPart.mChildren != null)
                        {
                            prevPart.mChildren.Add(part.mInline);
                        }
                    }
                    else
                    {
                        parts.Add(part);
                    }
                }
            }
            lastIdx = m.Index + m.Length;
            m = m.NextMatch();
        }
        if (lastIdx < (len - 1))
        {
            root.mChildren.Add(new Run(s.Substring(lastIdx)));
        }
    }
}

}

---

The same I have implemented using Behavior. Code given below:

public class FormatTextBlock : Behavior<System.Windows.Controls.TextBlock>
{
    public static readonly DependencyProperty FormattedTextProperty = 
        DependencyProperty.Register(
            "FormattedText", 
            typeof(string),
            typeof(FormatTextBlock),
            new PropertyMetadata(string.Empty, OnFormattedTextChanged));

    public string FormattedText
    {
        get { return (string)AssociatedObject.GetValue(FormattedTextProperty); }
        set { AssociatedObject.SetValue(FormattedTextProperty, value); }
    }

    private static void OnFormattedTextChanged(DependencyObject textBlock, DependencyPropertyChangedEventArgs eventArgs)
    {
        System.Windows.Controls.TextBlock currentTxtBlock = (textBlock as FormatTextBlock).AssociatedObject;

        string text = eventArgs.NewValue as string;

        if (currentTxtBlock != null)
        {
            currentTxtBlock.Inlines.Clear();

            string[] strs = text.Split(new string[] { "<Bold>", "</Bold>" }, StringSplitOptions.None);

            for (int i = 0; i < strs.Length; i++)
            {
                currentTxtBlock.Inlines.Add(new Run { Text = strs[i], FontWeight = i % 2 == 1 ? FontWeights.Bold : FontWeights.Normal });
            }
        }
    }
}

XAML - import namespace

<UserControl x:Class="MyClass"
         xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
         xmlns:behav="clr-namespace:myAssembly.myNameSapce;assembly=myAssembly"
>

Then to use the behavior as:

    <TextBlock TextWrapping="Wrap">
        <i:Interaction.Behaviors>
            <behav:FormatTextBlock FormattedText="{Binding Path=UIMessage}" />
        </i:Interaction.Behaviors>
    </TextBlock>

---

This work for me:

XAML:

<phone:PhoneApplicationPage x:Class="MyAPP.Views.Class"
                        xmlns:utils="clr-namespace:MyAPP.Utils">

and your TextBlock XAML:

<TextBlock utils:TextBlockHelper.FormattedText="{Binding Text}" />

CODE:

public static class TextBlockHelper
{
    public static string GetFormattedText(DependencyObject textBlock)
    { 
        return (string)textBlock.GetValue(FormattedTextProperty); 
    }

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

    public static readonly DependencyProperty FormattedTextProperty =
        DependencyProperty.RegisterAttached("FormattedText", typeof(string), typeof(TextBlock),
        new PropertyMetadata(string.Empty, (sender, e) =>
        {
            string text = e.NewValue as string;
            var textB1 = sender as TextBlock;
            if (textB1 != null)
            {
                textB1.Inlines.Clear();
                var str = text.Split(new string[] { "<b>", "</b>" }, StringSplitOptions.None);
                for (int i = 0; i < str.Length; i++)
                    textB1.Inlines.Add(new Run { Text = str[i], FontWeight = i % 2 == 1 ? FontWeights.Bold : FontWeights.Normal });

            }
        }));
}

USE in your string binding:

String Text = Text <b>Bold</b>;

---

So, combining a behavior to get the attached property and Jodrells use of XamlReader, here's a version that can deal with most things you would expect to be able to have as inlines in a TextBlock. Only the default and x: namespaces supperted, but you could extend that.

public static readonly DependencyProperty FormattedTextProperty = DependencyProperty.RegisterAttached(
  "FormattedText",
  typeof(string),
  typeof(TextBlockBehaviour),
  new PropertyMetadata(default(string), FormattedTextChanged_));

public static bool GetFormattedText(TextBlock textBlock)
{
  return (bool)textBlock.GetValue(FormattedTextProperty);
}

public static void SetFormattedText(TextBlock textBlock, bool value)
{
  textBlock.SetValue(FormattedTextProperty, value);
}

private static void FormattedTextChanged_(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  TextBlock textBlock = d as TextBlock;
  if (textBlock == null)
    return;

  textBlock.Inlines.Clear();

  string value = e.NewValue as string;
  if (string.IsNullOrEmpty(value))
  {
    textBlock.Text = null;
    return;
  }

  using (var stringReader = new StringReader($"<TextBlock xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\" xmlns:x=\"http://schemas.microsoft.com/winfx/2006/xaml\">{value}</TextBlock>"))
  {
    using (var xmlReader = XmlReader.Create(stringReader))
    {
      TextBlock newTextBlock = (TextBlock)XamlReader.Load(xmlReader);
      if (newTextBlock.Inlines.Count == 0)
      {
        textBlock.Text = newTextBlock.Text;
      }
      else
      {
        foreach (var inline in newTextBlock.Inlines.ToArray())
        {
          textBlock.Inlines.Add(inline);
        }
      }
    }
  }
}