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Posted 2 May 2008


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Silverlight 2 Beta 2: The first native rich text editor

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4.88/5 (18 votes)
8 Jun 2008MIT9 min read
In contrast to existing ones, my RTE is fully written in C#, and doesn't use any HTML rendering or JavaScript. Most common features are supported!


1. Introduction

Silverlight 2 Beta 2 is now supported!

Please visit for discussions, bug reports, latest bug-fixes, and releases. A first stable version named "RichTextEdit beta 2.0" is now available. I also have updated the files here on CodeProject!

A live demo is available here.

The whole article is also available as PDF in the documentation download.

Many people are waiting for a way to type rich text in Silverlight. Even if I think that Microsoft will bring out their own solution, nobody knows how long to wait for and what features would be included. My RTE ships with various common features and an extensive documentation. Please note that the whole thing is still in BETA state, and there are some weird bugs on XP and some others on Vista. It seems as if they are not caused by my component at all, because Visual Studio would notify me in that case. So, wait for the final version of Silverlight 2, before you use this component in any production environment! It is, of course, possible that some things are still undocumented, or that the documented ones will not behave as expected. Don’t hesitate to report such incoherencies.

An incomplete feature list:

  • Copy/paste formatted text between rich text boxes and copy/paste from/to clipboard of unformatted but macro-enabled text. This means, in the Windows clipboard, even things like emoticons will be kept.
  • You may insert line breaks, unordered lists and blockquotes.
  • You may use various keyboard selection features like End/Home/PageUp/PageDown/Left/Up/Right/Down, Ctrl+A/End/Home, Ctrl+Shift+End/Home/Left/Right, "Shift"+"End/Home/PageUp/PageDown/Left/Up/Right/Down" and so forth...
  • Supports direct Unicode character input using "Ctrl"+[NumPad].
  • All Silverlight font formatting is supported, and even some more like SUP/SUB formatting.
  • You may define macros and a proper object class that should replace matching text, like emoticons...
  • In contrast to many other rich text editors, this one is fully real-time. That means, no preview is required because the editor allows editing all things directly.
  • If you only use macros and IRichTextObject to extend the control, you will automatically get support for secure content serialization of all control elements. Content serialization also supports reloading content and editing it again.
  • Secure content serialization gets rid of any potential security leak when storing user typed formatted text on a server and presenting it to visitors, because it is fully verifiable.
  • You may restrict font formatting to a well defined custom subset. This allows you to ensure that all user-typed input matches your needs or website design (this feature is currently not implemented, but only prototyped).
  • Snapshots allow convenient access to formatted content, and also Find&Replace with regular expressions, for example...

Because there is a complete API documentation available in the documentation download, this section concentrates on introducing the component. If you really want to understand it, you will have to read the documentation and work with the demo.

1.1. A first look

The above screenshot shows some formatting and the use of macros (as emoticons are). It also uses a custom designed border, and you can see that the inner content automatically gets realigned to fit into the outer border. You may also customize the selection back- and foreground color as well as the cursor color.

There obviously are two uncommon features. The first thing is that you may insert any object derived from FrameworkElement into the text flow. The second thing is that such objects, as macros, are capable of assigning the formatting information, which will allow emoticons, for example, to also be underlined and background colored as the surrounding text is.

As you can see, there is a little problem with Silverlight’s interpolation feature. It leads to small separators which are visible after each char when the font background is unequal to the control background. For obscurity, this seems to be an issue of Vista, because under XP, it is working as expected!

If someone has an idea of how to workaround this issue, please tell me!

The following is a compressed screenshot of the demo application:


1.2. Clipboard support

But it goes further. If you select custom objects, for example, the button, you will be able to copy/paste it with Ctrl+c/v or internal Clipboard methods, including all text formatting. If you select and copy macros to the Clipboard, they will be translated back to their text representation, allowing to copy/paste macros between application boundaries, where in contract formatted text operations are restricted to the same application only. Clipboard operations are also supported, if the browser does not support it or the user has denied access. But then, of course, all operations are restricted to the current application.

This way, you are able to thread custom objects as letters, and also remove/overwrite them as usual text with backspace, for example.

1.3. Future features

Of course, this component is not perfect. The following features will be implemented within the next few months:

  • An XML scripting language to initialize rich text content in Expression Blend, for example. This also eases the management of multiple languages, because content and design are separated.
  • A way to wrap text around controls, like in Microsoft Office. Currently, they are only inserted like a letter, which limits the capabilities for inserting images or any other kind of rich non-text content. But, this feature is really far away from being simple, so don’t wait for it.
  • Non-editable XAML output which requires the XamlWriter that is currently not available.
  • Features that other developers may submit.

2. A tour through the demo

To understand how to use this component, I really recommend stepping through the demo file. It utilizes nearly all the features, and shows some tricks to realize hyperlinks or Find&Replace with regular expressions using the abstract RTE interface. The following sections refer to “RichText\Demo\Page.xaml.cs”.

2.1. Initialization

As RichtextEdit is derived from UserControl, you may use and initialize it as any other framework element.

RichEdit = new RichTextEdit();
RichEdit.AutoFocus = true; 
RichEdit.OnSelectionChanged += new NotificationHandler(RichEdit_OnSelectionChanged);
RichEdit.OnContentChanged += new NotificationHandler(RichEdit_OnContentChanged);


RichEdit.RegisterObject(new SerializableButton(null));

The above code initializes the content with a test string, and makes the control ready to receive two special events. This allows your code to get notified if either the selection or content has changed. Also, the MSNEmoticons extension is applied, which will replace common emoticons like “:-)” with a matching MSN icon. The last line is something special, and will be covered later. All lines except the object creation are optional!

2.2. Updating the ToolBar

void RichEdit_OnSelectionChanged(object  sender)
    IsUpdating = true;

This event will update the whole demo GUI according to the formatting at the current selection. This method is very important for any useful ToolBar-Editor pair, and you should try to understand what is going on there… The most important thing to mention is that all formatting properties like RichTextEdit.FontAttributes are set according to the current selection. That means, you just have to read them out within the OnSelectionChanged handler and update your ToolBar properly.

2.3. Find&Replace

To find text, we need to utilize the RichTextEdit.Snapshot. This allows us to directly operate on string content, which is not common when dealing with rich text:

private MatchCollection  REGEX_Matches;
private Int32 REGEX_Index = 0;
private RichTextEdit.Snapshot REGEX_Snapshot;

private void BTN_Find_Click(object sender, RoutedEventArgs e)
    REGEX_Snapshot = RichEdit.QueryText();
    Regex Exp = new Regex(EDIT_Find.Text, RegexOptions.IgnoreCase |
    RegexOptions.Multiline | RegexOptions.ECMAScript);

    REGEX_Matches = Exp.Matches(REGEX_Snapshot.Text);
    REGEX_Index = 0;

    BTN_Replace.IsEnabled = true;
    BTN_FindNext.IsEnabled = true;
    BTN_FindNext_Click(null, null);

To realize the “Find Next” method, we just loop through all the matches…

private void BTN_FindNext_Click(object sender, RoutedEventArgs  e)
    if ((REGEX_Matches == null) || (REGEX_Matches.Count == 0))
        BTN_FindNext.IsEnabled = false;


    if (REGEX_Matches.Count <= REGEX_Index)
        REGEX_Index = 0;

    // select match
    Match m = REGEX_Matches[REGEX_Index++];
    REGEX_Snapshot.Select(CursorPosition.End, m.Index, m.Length);

As you can see, a snapshot also allows us to select rich text based on string offsets.

If you also want to replace rich text, this will get a little bit more complicated. First, we need to remove the text referred to by the current match entry. Then, we insert the replacement and select it.

private void BTN_Replace_Click(object sender, RoutedEventArgs  e)
    if ((REGEX_Matches == null) || (REGEX_Matches.Count == 0))

    if (REGEX_Matches.Count <= REGEX_Index)
        REGEX_Index = 1;

    // replace selection
    Match m = REGEX_Matches[REGEX_Index - 1];
    Int32 iStart = m.Index;
    Int32 iLen = m.Length;

    REGEX_Snapshot.Remove(ref iStart, ref iLen);
    REGEX_Snapshot.Select(CursorPosition.Start, iStart, 0);
    REGEX_Snapshot.SelectionStart - EDIT_Replace.Text.Length, 

Even if this might look strange, it is a very consistent way to realize Find&Replace. Imagine there are custom objects between the letters that you wouldn’t know about. Such circumstances are handled within a snapshot, and you don’t have to worry about it.

3. Secure content serialization

After a user has typed and formatted his text, you need to save it. My control provides a secure way to do this:

private void BTN_Serialize_Click(object sender, RoutedEventArgs  e)
    MemoryStream Buffer = new MemoryStream();
    RichTextEdit.Snapshot Snapshot = RichEdit.QuerySelectionText();

    Snapshot.Serialize(false, Buffer);

    // convert to base64
    LABEL_Binary.Text = Convert.ToBase64String(Buffer.GetBuffer(), 0, (int)Buffer.Length);
    BTN_Deserialize.IsEnabled = true;

As you can see, again the snapshot is involved; you need a snapshot of what you want to serialize. If your web server does not support binary serialization, just encode it to Base64 as shown above. All formatting, all macros, and all custom rich text objects will be included in such a serialization stream.

Deserialization works similarly:

private void BTN_Deserialize_Click(object sender, RoutedEventArgs e)
    MemoryStream Buffer = new
    RichEdit.InsertDeserialization(false, Buffer);

3.1 Why is this secure?

It is secure because it is 100% verifiable. Secure does not mean that it is encrypted; you still have to use SSL for encrypted content transmission. Verifiability prevents you from a whole range of common attacks because it is simply not possible to store harmful serialization content on your server or invoke harmful operations when visiting your site.

In future versions, serialization will be improved to be compressed, which will heavily reduce the final stream size and reduce your storage costs.

4. Custom rich text objects

Even if you may insert normal framework elements (FE), I don’t recommend it. A normal FE is not included in any kind of serialization and also not in Clipboard operations. To allow FEs to be serialized, you have to create a class which implements the “IRichTextObject” interface. The following shows a rich text object wrapper around a simple button:

class SerializableButton : UserControl, IRichTextObject
    private String m_Caption;
    private Button m_Instance;
    private SerializableButton() : base() { }

    public SerializableButton(String InCaption)
        m_Caption = InCaption;

    public Int16 GetTypeID() { return 0x100; }

    public Boolean IsFocusable { get { return true; } }

    public void Serialize(FrameworkElement InElement, BinaryWriter InTarget)
        SerializableButton Button = (SerializableButton)InElement;


    public FrameworkElement Deserialize(
        Boolean InIgnoreWarnings,
        BinaryReader InSource)
        return CreateInstance(InSource.ReadString());

    public FrameworkElement CreateInstance()
        return CreateInstance(m_Caption);

    private FrameworkElement CreateInstance(String InCaption)
        if (InCaption == null)
            throw new InvalidOperationException();

        SerializableButton Result = new SerializableButton();

        Result.m_Instance = new Button();
        Result.m_Instance.Content = InCaption;
        Result.Content = Result.m_Instance;
        Result.Width = 100;
        Result.Height = 25;

        return Result;

The above code combines a framework element and the interface. In general, this is the easiest way, because during serialization and deserialization, you will only get a reference to the FE, and it’s your duty to find the proper rich text object interface.

The class implementing the interface should have a constructor that takes all the parameters that are required to instantiate it. In our case, we only need a caption for the underlying button. Those are also the parameters that should be serialized, because this way, you only have to deserialize and pass them to the constructor to deserialize the whole rich text object.

The instance creator is something special. The rich text object (RTO) can be threaded as a wrapper around a normal framework element. It provides a way to keep all required data through serialization, to later recreate a technically equal instance. An RTO instance should always provide technically equal framework elements through “CreateInstance”, but never return any object twice. It should just ensure that all returned objects referring to the same RTO constructor parameters will behave and look the same.

Before all that, you have to register the RTO with your editor, as shown in the initialization:

RichEdit = new RichTextEdit();
RichEdit.RegisterObject(new SerializableButton(null));

You don’t need to pass any parameters, because this RTO instance should just provide the “Serialize”, “Deserialize”, and “GetTypeID” methods which can be threaded as static, but static methods are not supported with interfaces…

This technology is something hard to explain, but it is very powerful if understood. Please look at the demo and try to find out how it works.

Have fun!


This article, along with any associated source code and files, is licensed under The MIT License


About the Author

Christoph Husse
Software Developer SecurityRevolutions
Germany Germany
No Biography provided

Comments and Discussions

QuestionAny Lightswitch support Pin
adudley25626-Sep-11 23:23
Memberadudley25626-Sep-11 23:23 
Questioncan you help me Pin
mheidari3-Aug-09 4:32
Membermheidari3-Aug-09 4:32 
Generalgrate job,but other language(2byte) can't use [modified] Pin
Jery123413-Feb-09 21:48
MemberJery123413-Feb-09 21:48 
QuestionIs there any possible way of making MoveCursor to actually move the cursor to the end of the text? Pin
User 309585914-Dec-08 17:33
MemberUser 309585914-Dec-08 17:33 
GeneralMy vote of 1 Pin
User 309585914-Dec-08 17:09
MemberUser 309585914-Dec-08 17:09 
GeneralGenerating/Loading HTML Pin
AttaShahzad15-Aug-08 0:36
MemberAttaShahzad15-Aug-08 0:36 
GeneralRe: Generating/Loading HTML Pin
Christoph Husse15-Aug-08 1:51
MemberChristoph Husse15-Aug-08 1:51 
GeneralThanks man! Pin
Aamer Alduais28-Jul-08 1:30
MemberAamer Alduais28-Jul-08 1:30 
GeneralThanks Pin
chris_ashford9-Jun-08 1:00
Memberchris_ashford9-Jun-08 1:00 
GeneralRe: Thanks Pin
Christoph Husse9-Jun-08 4:32
MemberChristoph Husse9-Jun-08 4:32 
GeneralGreat! Pin
Hugo de Vreugd8-Jun-08 20:41
MemberHugo de Vreugd8-Jun-08 20:41 
GeneralRe: Great! Pin
Christoph Husse8-Jun-08 22:28
MemberChristoph Husse8-Jun-08 22:28 
QuestionWhat? Pin
Heywood8-Jun-08 6:04
MemberHeywood8-Jun-08 6:04 
AnswerRe: What? [modified] Pin
Christoph Husse8-Jun-08 6:11
MemberChristoph Husse8-Jun-08 6:11 
GeneralRe: What? Pin
Heywood8-Jun-08 6:25
MemberHeywood8-Jun-08 6:25 
GeneralRe: What? Pin
Christoph Husse8-Jun-08 6:31
MemberChristoph Husse8-Jun-08 6:31 
GeneralRe: What? Pin
Michael Sync9-Jun-08 0:56
MemberMichael Sync9-Jun-08 0:56 
GeneralRe: What? Pin
Simon Capewell10-Jun-08 4:29
MemberSimon Capewell10-Jun-08 4:29 
GeneralRe: What? Pin
Christoph Husse10-Jun-08 5:05
MemberChristoph Husse10-Jun-08 5:05 
AnswerRe: What? Pin
David Roh27-Jul-08 5:52
MemberDavid Roh27-Jul-08 5:52 
Thank you very much Christopher - your work is deeply appreciated.

David Roh

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