The enableHtmlAccess parameter is set on the Silverlight plug-in. It enables managed code in the .xap file to access the JavaScript and DOM code on the host page. This parameter can be set only during plug-in initialization, and is read-only afterward. For same-domain applications, the parameter is set to true by default, and you do not have to explicitly set its value in code. For cross-domain applications, the parameter is set to false by default, and you have to explicitly enable it, as shown in the following host page HTML code.

<div id="silverlightControlHost">
    <object data="data:application/x-silverlight-2,"
            type="application/x-silverlight-2"
            width="300" height="100"
        <param name="source"
                 value="http://www.northwindtraders.com/MySample.xap"/>
        <param name="enableHtmlAccess" value="true" />
    </object>
</div>

When the enableHtmlAccess parameter is set to true, as shown in the previous example, the following HtmlPage properties are enabled:

• HtmlPage..::.Document

• HtmlPage..::.Window

• HtmlPage..::.Plugin

• HtmlPage..::.BrowserInformation

Caution:

When a host page enables HTML access, it gives the Silverlight-based application complete access to its DOM, including the ability to modify content, insert and evaluate JavaScript code, and so on. This access should be allowed only if the host page trusts the Silverlight-based application. HTML access is enabled by default for same-domain Silverlight-based applications, because there is implicit trust between HTML and same-domain Silverlight content. However, access must be specifically enabled for cross-domain applications.

When the enableHtmlAccess parameter is set to false (the default for cross-domain applications), the previous properties are disabled, and they throw exceptions instead of returning values. Therefore, Silverlight code is prevented from directly obtaining references to DOM or JavaScript objects. In addition, the enableHtmlAccess parameter cannot be set to true after the Silverlight Web object has been initialized.

enableHtmlAccess Workarounds

When the enableHtmlAccess parameter is set to false, direct access to JavaScript or DOM elements and objects is not possible. However, individual, specific access can be programmatically re-established in the following cases:

• Silverlight code exposes one or more scriptable entry points that accept ScriptObject references as input parameters.

• Silverlight code explicitly registers the scriptable entry points by calling the RegisterScriptableObject method.

• Access to scriptable entry points is not disabled with the ExternalCallersFromCrossDomain attribute.

• JavaScript code accesses the plug-in's Content property, obtains a reference to one of the scriptable entry points, and passes a DOM object or JavaScript object reference as an input parameter.

Note:
These conditions cannot occur by accident. The Silverlight managed code and the JavaScript code must each be written specifically to allow mutual access.

How to Retrieve the enableHtmlAccess Value

The ExternalCallersFromCrossDomain attribute controls JavaScript and DOM access to scriptable objects that are defined in cross-domain .xap files.

The attribute has no effect in same-domain .xap files. Access is disabled by default in cross-domain .xap files.

The attribute is set on the root element of the application manifest, as shown in the following Silverlight deployment code.

<Deployment xmlns="http://schemas.microsoft.com/client/2007"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    EntryPointAssembly="MyAppAssembly"
    EntryPointType="MyNamespace.MyApplication"
    ExternalCallersFromCrossDomain="ScriptableOnly"
>
<Deployment.Parts>
    <AssemblyPart Source="MyAppAssembly.dll” />
    <AssemblyPart Source="MyUserControl.dll" />
  </Deployment.Parts>
</Deployment>

The ExternalCallersFromCrossDomain attribute accepts two values: ScriptableOnly and NoAccess.

When the attribute is set to ScriptableOnly, the host's native JavaScript can only access scriptable objects that the Silverlight application code registers with the runtime. It cannot query or set properties on any other objects. Additionally, it will not receive event notifications.

Caution:
The ScriptableOnly value will allow a cross-domain .xap file to programmatically call any scriptable objects that are explicitly exposed by the application.

When the ExternalCallersFromCrossDomain attribute is set to NoAccess, scriptable entry points and creatable types are not available to JavaScript or DOM through Content and createObject. Although managed calls to RegisterScriptableObject, RegisterCreateableType, and so on will appear to succeed (exceptions to managed code are not thrown), the registered entry points and types will not be available through Content or createObject.

Application Manifest

ExternalCallersFromCrossDomain Workarounds

• When the ExternalCallersFromCrossDomain attribute is set to NoAccess, direct access to Silverlight managed code is not possible. However, individual, specific access can be programmatically re-established if the following conditions are true:

• The Silverlight plug-in's enableHtmlAccess property is set to true.

• Silverlight managed code calls a JavaScript function and passes one or more managed objects as input parameters to the Invoke, InvokeSelf, and SetProperty methods.

• The managed instances passed in the previous step have scriptable properties, methods, or events, and the objects have been registered for scriptable access by using the RegisterScriptableObject method.

Note:
These conditions cannot occur by accident. They require explicit steps by the cross-domain Silverlight-based application to pass managed objects to the host's JavaScript.

How to Retrieve the ExternalCallersFromCrossDomain Value

Reference

Other Resources