These are some notes on Silverlight Hosting APIs and how the plugin works / should work. They are likely incorrect

Table of contents

Moonlight plugin

How it works

First step: create an <object> element of type application/ag-plugin.

This is not mandatory unless you run silverlight apps written in Microsoft ways:

  • The web browser loads HTML, which contains "SilverlightControlHost" element.
  • SilverlightControlHost invokes "createSilverlight()" which is defined in the HTML's bound Javascript.
  • createSilverlight() invokes createObjectEx() which is defined in silverlight.js.
  • createObjectEx() either creates an <object> element of type application/ag-plugin, or download/support links.
  • <object> element of type application/ag-plugin is recognized by the moonlight plugin.

We could just directly include such an <object> element in the HTML instead.

Second step: bootstrapping Silverlight control

It is done by moonlight plugin.

  • The plugin is triggered by the <object> element.
  • It loads the "source" XAML file. If there isn't, then nothing is warned.
  • It looks for the implementation class from "x:class", which looks like : "SilverlightProject1.Page;assembly=ClientBin/SilverlightProject1.dll"
    • If there isn't, then Javascript warning is raised.
    • If the referenced type is not an expected type (only Canvas?), then Javascript warning is raised.
  • Create a GTK+ canvas for Silverlight object (indicated by "source")

Hosting APIs

System.Silverlight.dll and Microsoft.Scripting.Silverlight.dll are involved.

Microsoft.Scripting.Silverlight is smaller:

  • ErrorHandler : static aggregated class of error handling which dispatch managed exceptions to browser.
  • HtmlDocumentMemberInjector
  • HtmlElementMemberInjector
  • IDlrNameScope
  • SilverlightPAL
  • SilverlightScriptHost
  • XamlDlrScriptHost


  • it is nothing to do with JavaScriptSerializer (methods and events are ignored in the serializer).
  • For now simple string and primitive types are supported as method parameters. (See below for details.)

How to enable ScriptableObject in managed code

  • Add ScriptableAttribute on the class, and property, method and events.
  • Call WebApplication.Current.RegisterScriptableObject(nameOnJS, instance).

How to use ScriptableObject in client Javascript

  • registered scriptable objects are accessible via document.getElementById ("SilverlightControl").Content as its properties.
  • Properties are created as Javascript objects.
  • Events are created as event triggers which expects two arguments (sender, eargs).
  • functions are created as functions as they are.
  • Types are strictly limited: bool, string, char, numeric types except for Decimal are OK. No other types are allowed (object, Guid, DateTime, IDictionary<string,object>, any custom types). Only In parameters allowed.

Hosted environment

  • Every ScriptableObject seems to check its hosting environment. For example, HtmlPage.Document causes an error (NRE at alpha1) outside hosted environment (for example console app).

ScriptableObject injection

See also:

List for some objects/methods that need interaction from managed code to DOM:

  • ScriptableObject.GetProperty<T>(string name)
    • T must be derived from ScriptableObject. For example, HtmlPage.Document.GetProperty<object>("documentElement")
  • ScriptableObject.SetProperty(string,object)
  • HtmlObject.AttachEvent(string name,EventHandler handler)
  • HtmlObject.DetachEvent(string name,EventHandler handler)
  • generic way to invoke methods (e.g. getElementByTagName(), createElement() etc.)

Error handling

In agclr, System.Windows.ErrorEventHandler and ErrorEventArgs are used to handle errors at managed side.

Microsoft.Scripting.Silverlight.ErrorHandler is (in a sense) the closest API to the browser. It is tied to ErrorEventArgs in agclr and "default_error_handler" in silverlight.js, which

  • expects ParserError and RuntimeError as errorType (ParserErrorEventArgs/RuntimeErrorEventArgs in agclr)
  • expects errorCode, errorType and errorMessage (ErrorEventArgs properties)

Misc notes

  • WebApplication.Current is created and used to serve RegisterScriptableObject().
    • NPN_CreateObject() is likely used.
  • I'm not sure if SilverlightScriptHost and SilverlightPAL are created on silverlight apps. ScriptDomainManager.CurrentManager.PAL is not an instance of SilverlightPAL on silverlight apps.
  • static property HtmlPage.Document accesses to the document. It somehow causes NRE on non-silverlight app.

Implementation status notes

Already written as DllImport (note that they are not in moon/plugin yet):

  • WebApplication
    • InvokeMethodInternal(IntPtr xpp, IntPtr obj, string name,object[] args)
    • GetPropertyInternal(IntPtr xpp,IntPtr obj, string name)
    • SetPropertyInternal(IntPtr xpp,IntPtr obj, string name, object value); They are commonized accessors and used by several classes such as HtmlElement.AppendChild().
  • BrowserInformation
    • LoadBrowserInformation(BrowserInformation)
  • BrowserRuntimeSettings
    • LoadBrowserRuntimeSettings(BrowserRuntimeSettings)
  • HtmlObject
    • AttachEvent(IntPtr xpp, IntPtr obj, string name, EventHandler h)
    • AttachEvent(IntPtr xpp, IntPtr obj, string name, EventHandler<HtmlEventArgs> h)
    • DetachEvent(IntPtr xpp, IntPtr obj, string name, EventHandler h)
    • DetachEvent(IntPtr xpp, IntPtr obj, string name, EventHandler<HtmlEventArgs> h)
  • HtmlPage
    • Navigate (IntPtr npp, string uri)
    • Navigate (IntPtr npp, string uri, string target, string features) (returns a handle)
    • NavigateToBookmark (IntPtr xpp, string bookmark)
    • Submit (IntPtr xpp, string formId)

Just rough ideas:

  • WebApplication.Current is likely hold a pointer to NPP instance, a pointer to PluginInstance, and so on. Use AppDomain.GetData() to return pointers here.
  • WebApplication.RegisterScriptableObject() has to create a NPObject. Also, access to the members must be reflected to the managed object (and vice versa).
  • WebApplication.StartupArguments. Not sure what should be set here.
  • HtmlTimer. Probably it generates call to setTimeout() and calls Javascript eval(), depending on Enabled and Interval. Not sure where it is used.
  • HtmlPage properties:
    • Cookies, string
    • CurrentBookmark, string
    • Document, IntPtr to NPObject?
    • DocumentUri, string
    • QueryString, string or dictionary
    • Window, IntPtr to PluginInstance->window
  • HtmlElement.SetStyleAttribute() and RemoveStyleAttribute() ; not sure how it is done.