The only setup absolutely required is to reference dlr.js from any HTML file you wish write Ruby code in:

<script src=""

If you want to develop without a network connection, or deploy IronRuby yourself, simply extract the re-distributable package and reference it:

<script type="text/javascript">
    window.DLR = {path: 'path/to/gestalt.latest'}
<script src="path/to/gestalt.latest/dlr.js" type="text/javascript">


using the version is preferred, especially in production. However, please pick a specific version (like dlr-20100305.js, rather than dlr-latest.js), as this will ensure your application keeps working between releases; dlr-latest.js will always point to the most recent version.

By default just doing this sets up your HTML page for executing Ruby code. See the dlr.js API documentation for more advanced uses.

Writing Ruby code

Ruby code is executed by using HTML script-tags; either inline or as an external file. This implementation aims to be compliant with the HTML4 specification for scripting in HTML pages.

Inline scripts

The following attributes are used with a HTML script-tag to embed Ruby:

  • type: defines the mime-type the script code should map to; the following prefixes are allowed: application/, text/, and application/x-. The actual language name will be passed to the DLR hosting API, so the above example could have used application/ironruby and it would still work:

    <script type="application/ruby">
      window.alert "Ruby inline script-tag"
    <script type="text/ruby">
      window.alert "Also a Ruby inline script-tag"
  • (Optional) defer: if this attribute is not present, its value is false. Otherwise, the value is true (even if it's explicitly set to defer="false"; this is how all modern browsers behave). If set to true the code is not run; but it can be used to evaluate later:

    <script type="application/ruby" defer="true" id="for_later">
      puts 2 + 2
    <script type="application/ruby">
      eval document.for_later.innerHTML

External scripts

The following attributes are used with a HTML script-tag to reference Ruby:

  • src: specify the path to a Ruby file. If it's a relative URI, it will be considered relative to the HTML file. The src URI is downloaded and cached in memory, building a virtual file-system of external script code. Then this file is executed in its own DLR ScriptScope, which properly isolates execution between scripts, and most closely matches what Ruby's import statement does.

    # foo.rb
    window.alert "Hello from a ruby file"
    <!-- foo.html -->
    <script type="application/ruby" src="foo.rb"></script>
  • type: specifies the mime-type of the script-tag, which is used to figure out the language; see Inline Scripts type attribute.

    Technically this is not required, as the extension of the file will be used to detect the language if type is omitted, but most browsers will then attempt to run the code with it's built-in JavaScript engine, and most likely throw a JavaScript syntax exception. So, it's recommended to always using the type attribute.

  • (Optional) defer: See Inline Scripts defer attribute for symantics. If this is true, the src URI is just downloaded and cached, but is not run. This allows full control over when the script gets run, as another script can get the first shot at importing it:

    <script type="application/ruby" src="foo.rb" defer="true"></script>
    <script type="application/ruby">import foo</script>

Execution order

Script-tags will be executed in the order they are defined, but before the start script is executed (if one is provided). All inline code is to be executed in the same scope, basically as if they all one Ruby file. This allow methods defined in one script-tag to be called from another:

<script type="application/ruby">
  window.alert "in first script-tag"
  def foo
    "In Foo"
<script type="application/ruby">
  window.alert "in second script-tag"
  window.alert foo

File-system operations

Silverlight runs in a sand-box, not allowing programs access to the machine's file system, as well as forbidding native user-code from being loaded. However, IronRuby's implementation abstracts file-system operations, allowing it to provide different behavior when running in Silverlight. External script tags are used to define the file system entries.

Ruby script files

Each time an external script-tag is downloaded, it is also cached in-memory so the same file isn't re-downloaded. This download cache is actually presented to Ruby as a read-only file system, which is how things like require still work; they are actually asking if the file exists, except all file-system operations in Silverlight are redirected to the download cache.

Zip files

The external script tag's src attribute can be a *.zip file; this is useful for larger libraries where it may be cumbersome to list all the script files out as script-tags.

The following attributes are used with a HTML script-tag to reference zip files:

  • src: URI to a *.zip file.

    The value of the src attribute will be placed on the language's path, and basically treated as a folder. When a script file is requested from any other script, the language will try to find it by using its path and checking for the existence of the file. If the path contains a known zip file name, then it will continue to look inside the zip file:

    <script type="application/x-zip-compressed" src=""></script>
    <script type="application/ruby">
      $LOAD_PATH << 'lib'
      require 'erb'
  • type: must be set to application/x-zip-compressed

  • Not implemented yet (Optional) defer: toggles whether the zip file is placed on the path. Defaults to false which adds it to the path, while true will not add it to the path. When defer="true" you can always programmatically add it to the path using Ruby's sys module:

    <script type="application/x-zip-compressed" src="" defer="true">
    <script type="application/x-ruby">
      $LOAD_PATH << "ruby-stdlib"

Note: "added the zip file to the path" is not implemented at the moment, so it will always behave as defer="true".

Since zip files are treated just like a folder, you can put anything inside the ZIP file; DLLs, XAML files, text files, images, etc, and use them just like you would if they were part of the file-system:

<script type="application/x-zip-compressed" src=""></script>
<script type="application/ruby">
  require ""
  txt = "my-archive/foo.txt"

When accessing files inside a zip file, just use the zip filename as if it were a folder name.

Note: Today only the zip file's filename (without the .zip extension) is required to access it (example: open('my-archive/foo.txt')), though that's a bug in the implementation, not the spec.


Silverlight not only provides an execution model for Ruby scripts, but it also allows for rendering vector graphics in the browser, for animations or rich user-interfaces. This can be accomplished by using eXtensible Application Markup Language (XAML), or directly from Ruby.


XAML markup can be embedded into a script-tag, either inline or as an external file:

<!-- inline XAML file -->
<script type="application/xaml+xml" id="inlineXAML" width="200" height="75">
  <Canvas Background="Wheat">
    <TextBlock Canvas.Left="20" FontSize="24" />

<!-- external XAML file -->
<script type="application/xaml+xml" id="externalXAML" src="foo.xaml">

The following attributes are used with a HTML script-tag to embed XAML content:

  • width: the width of Silverlight control surface.

  • height: the height of Silverlight control surface.

  • type: should be set to application/xaml+xml, though application/xml+xaml also works.

  • src: URI to a XAML file. It behaves like external scripts src attribute with regard to downloading and caching. If it is not set, the XAML content is expected to be provided in the script-tag's innerText.

  • id: DOM ID the generated Silverlight control will have; this is needed to tell Ruby code to run against a specific Silverlight control.

  • (Optional) defer: By default either the external or inline XAML causes dlr.js to inject a Silverlight control, and set the RootVisual of that Silverlight instance to the XAML provided by the script-tag. However, if this is true, the Silverlight control is still injected into the DOM, but the XAML content is not set as the RootVisual of that control. If the XAML content was provided by the src attribute, then the file is still downloaded and cached. Setting the RootVisual can be done manually, however:

    <script type="application/xaml+xml" id="xamlContent" defer="true">
      <Canvas Background="Wheat">
        <TextBlock Canvas.Left="20" FontSize="24" />
    <script type="application/ruby" class="xamlContent">
      include System::Windows
      Application.current.load_root_visual_from_string document.xamlContent.innerHTML

    If you do not want to even have the control added, then you'll have to disable dlr.js's auto-adding:

    <script type="text/javascript">
      window.DLR = {autoAdd: false}
    <script type="text/javascript" src="dlr.js"></script>
    <script type="application/xaml+xml" id="xamlContent" defer="true">
      <Canvas Background="Wheat">
        <TextBlock Canvas.Left="20" FontSize="24" />

    Then you can add a control at any time:

    <script type="text/javascript">
      DLR.createObject({width: 200, height: 200});

This is similar to the way that Silverlight 1.0 allowed XAML to be embedded.

From Ruby

XAML is simply a markup language for creating objects, so the same thing can be done directly from Ruby. Given this XAML:

<script type="application/xaml+xml" id="xamlContent">
  <Canvas Background="Wheat">
    <TextBlock Canvas.Left="20" FontSize="24" />

The equivalent in Ruby would be:

import System::Windows
import System::Windows::Media
import System::Windows::Controls
c =
c.background = Colors.wheat
t =
t.font_size = 24
c.children.add t
Canvas.set_left t, 20
Application.current.root_visual = c

Unclassified documentation

This is just random documentation, which has yet to be incorporated into a place that makes sense.

Multiple controls

Browsers allow for multiple object-controls to be on a single page, so you could have multiple Silverlight controls on the same page. This introduces an unexpected side-effect to having Silverlight run code inside script-tags; every Silverlight would run run every script-tag. Consider the following:

<div id="message"></div>
<script src="dlr.js"></script>
<script type="text/javascript">
  DLR.createObject({width: '100', height: '100'})
<script type="application/ruby">
  include System::Windows
  Application.current.root_visual =

Both Silverlight controls will get their root_visual set, since the Ruby script-tag is executed twice, once for each Silverlight control. To avoid this, script-tags must be scoped to a specific Silverlight control. dlr.js instructs dlr.xap to only run "un-scoped" script-tags on the first control added to a page, and only run "scoped" script-tags with subsequent added controls. To "scope" a script-tag, the class attribute contains the same value as its corresponding Silverlight control's xamlid initParam:

<script type="text/javascript">
  DLR.createObject({xamlid: 'control1'})
<script type="application/ruby" class="control1">
  # will only run in the "control1" object

An un-scoped script-tag is simply a script-tag without a class attribute. These will run in a Silverlight control that does not have the "xamlid" initParam set; dlr.js does this for only the first control it injects.

If you intend to not use Silverlight graphics through script-tags, or only use them in one control, then you don't need to worry about scoping; scoping only comes into play when you have multiple controls. If you want to use Silverlight graphics, you can use this same strategy on script-tags containing XAML to make sure the proper RootVisual is set.

A script-tag having a "*" class attribute will cause it to run in every script-tag, so the first-example's behavior is still possible.

Changes to existing behavior

Though there are no major breaking changes to any existing behavior of existing applications, there needs to be some changes to existing features to make this new activation-model work properly.

Previously, the "start" initParam (entry-point/start-script to the DLR Silverlight app) is required if there is no app.* file in the XAP file. If the "start" initParam is omitted in this condition, an error would have been raised, complaining about not finding an app.* file.

This requirement is now completely relaxed; neither an app.* file or a "start" initParam is required. If no "start" script or defer=false script-tags exist on the page; then nothing runs and no error is raised. This is relaxed because a Silverlight application can be only inline XAML.

<script type="application/ruby">
<object ...>
  <params name="source" value="app.xap" />
  <params name="initParams" value="" /> <!-- no initParams value needed -->

Though these changes are being introduced to remove the need for Chiron, it is still a useful tool for generating XAP files on the fly. Chiron now serves files out of the "externalUrlPrefix" path if it is a relative path, so extensions can be developed locally and Chiron instantly picks them up. Also, Chiron's XAP building features will build an appropriate XAP file depending on whether you're using slvx files or not.

Interacting with markup

To make accessing the HTML and XAML easier and more like how JavaScript works, variables pointing to them are added to the scope in which script-tags are executed in.

HTML accessors

document maps to System.Windows.Browser.HtmlPage.Document, which is of type HtmlDocument, and window maps to System.Windows.Browser.HtmlPage.Window, which is of type HtmlWindow.

When a method is called on an HtmlDocument that does not exist, it calls GetElementById(methodName). The following examples are in Ruby:

# same as ...

document.doesnotexist # None

When a method is called on an HtmlElement that does not exist, it should call GetProperty(methodName). When calling the non-existent method as a setter, call SetProperty(methodName, value):

# same as ...

document.a_div_id.innerHTML = "Hi"
# same as ...
document.a_div_id.SetProperty("innerHTML", "Hi")

When an indexer is used on an HtmlElement, it should call GetAttribute(methodName). When setting the indexer, call SetAttribute(methodName, value):

# same as ...

document.link_id['href'] = ''
# same as ...
document.a_div_id.SetAttribute('href', '')

XAML accessors

me and xaml both map to System.Windows.Application.Current.RootVisual, having a base-type of FrameworkElement. When a method is called that does not exist on me or xaml, then FindName(methodName) is called. This allows access to any XAML elements with an x:Name value to be accessed by the x:Name value as a method call:

xaml.Message.Text = "New Message"

To load a XAML file into the root visual, use the LoadRootVisual methods:


Event handling from code

From code, events on both HTML and XAML elements can be hooked via the language's specific .NET event hookup syntax. Given the following HTML:

<a id="cm">Click Me</a>

You can hook the onclick event from Ruby:

<script type="application/ruby"> do |s, e|
    s.innerHTML = "Clicked!"

Hooking XAML events also works:

<script type="application/xml+xaml">
  <TextBox x:Name="xcm" Text="Click Me" />

<script type="application/ruby">
  include System::Windows
  root_visual = Application.current.root_visual
  root_visual.xcm.MouseLeftButtonDown do |s, e|
    s.text = "Clicked!"

Event handling from HTML or XAML markup is not supported!


Visual Studio Debugger

When you have debug mode turned on, it will just work as it used to. Attach the debugger to the browser, open the script file in Visual Studio, place a breakpoint, etc. Having the script files in the XAP does not make a difference for debugging; it's all about the debug-able code being generated and having the file open in VS.

Implementation details

Silverlight XAP file structure

With both user scripts and larger libraries outside the main XAP file, the main XAP only serves as a container for the AppManifest.xaml and any dynamic language assemblies required by the application. Silverlight 3 introduced "Transparent Silverlight Extensions", a way to package your own libraries into a .slvx (Silverlight versioned extension) file (really just zip file) which applications can depend on by referencing it from their AppManifest.xaml. Using this feature all the assemblies can be removed from the XAP file, put in a slvx file, and hosted on an internet location so other applications can depend on it. Instead of IronRuby and IronRuby releases containing the assemblies built for Silverlight, they will just contain a dlr.xap file. This xap file will be shared between all applications; only advanced scenarios will need to modify the xap file. It will only containing just two files:


The AppManifest.xaml file just references the Microsoft.Scripting.slvx file, and points the Silverlight application at the static entry point in Microsoft.Scripting.Silverlight.dll (included in Microsoft.Scripting.slvx):

   <ExtensionPart Source=""/>


The languages.config file lists the configuration information for DLR languages that can be used in Silverlight. This file can be present in a DLR-based xap today for defining configuration information for languages other than Ruby and Ruby, but now this file must be present if an application depends on the Microsoft.Scripting.slvx file. Included in this information is the URL for each language's slvx file:

    <Language names="IronRuby;Ruby;rb"
              external="" />

The language node can have the following attributes:

  • names: ;-separated list of names the language can use
  • extensions: ;-separated list of file extensions the language can use
  • languageContext: language's type that inherits from LanguageContext
  • assemblies: URIs to assemblies which make up the language
    • Optional: but if external is missing, then this list of assemblies is assumed to be in the XAP
  • external: SLVX file for all language assemblies


Microsoft.Scripting.slvx will contain the following DLLs: - Microsoft.Scripting.dll - Microsoft.Dynamic.dll - Microsoft.Scripting.Core.dll - Microsoft.Scripting.ExtensionAttribute.dll - Microsoft.Scripting.Silverlight.dll

When an application starts up, Silverlight downloads the Microsoft.Scripting.slvx file, loads all the assemblies inside it, and then kicks off the static entry point, Microsoft.Scripting.Silverlight.DynamicApplication. During its startup logic, it tries to load language configuration from the languages.config file; if that fails it looks to already loaded assemblies referenced in the AppManifest.xaml and loads the configuration info off the assemblies directly. Because of this, XAP files must have a languages.config to download languages on-demand. After the language configuration is loaded, the script-tags on the HTML page are processed; for each language used, the existence of all the language's assemblies in the XAP file is checked, and if they are not all found the language's external-package is downloaded, assemblies inside loaded, and a ScriptEngine created for the language. Both the list of assemblies and external-package URI are provided by languages.config.

If an application cannot depend on the slvx files hosted on the internet, they can be hosted on any machine. Just change the AppManifest.xaml and languages.config to point to the new location. If Chiron is still being used to generate the XAP file, then the externalUrlPrefix in Chiron.exe.config is the only setting that needs to be changed.


These are clearly non-goals for IronRuby, though some persuasion might move them up into ideas.

jQuery-influenced selectors

Though the idea of having a jQuery-like selector API for DLR languages is attractive, it is less feasible since each language will want a different way to specify the syntax. Also, libraries in those languages may exist (eg. Ruby's Hpricot), so it'd be best to use those directly. This might be addressed in a future change, or another library, but is out of scope for this change.


The "start" script referenced in the Inline Scripts section ... what is it?

The "start" script is another term for the entry-point script. By default it's app.*, and * is used to figure out the correct language to instantiate. However, the user can specify the specific start-script in the initParams:

<param name="initParams" value="start=myapp.rb" />

See the original dynamic languages in Silverlight specification for more information TODO add link.

Can I write offline Silverlight applications with this?

Not with Silverlight 3. Offline Silverlight applications do not allow using the browser DOM APIs, since they just run the Silverlight control outside the browser. Therefore, offline Silverlight applications cannot use <script> tag code. If you'd like to write a Silverlight application that runs both in the browser and on the desktop, you'll need to keep everything in the XAP file and use the "start" script as the application's entry-point. Silverlight 4 supports HTML hosted in an OOB app, so it's possible to directly support this in the future.


Public APIs

  • dlr.js
  • Microsoft.Scripting.Silverlight.dll
  • DLR Hosting API