IAccessible Proxies

IAccessible proxies provide default accessibility information for standard UI elements: USER controls, USER menus, and common controls from COMCTL and COMCTL32. This default support is exposed through IAccessible objects created by Oleacc.dll and delivers Microsoft Active Accessibility support without additional server development work. The server can then use the Dynamic Annotation API to modify much of the information exposed by Oleacc.dll, but it does not have complete control.

Creating a Proxy

To determine whether a UI element natively supports the IAccessible interface, Oleacc.dll sends it a WM_GETOBJECT message. A nonzero return value means the element natively supports Microsoft Active Accessibility and provides its own IAccessible support. However, if the return value is zero, Oleacc.dll provides a proxy object for the UI element and attempts to return meaningful information on its behalf. For more information about WM_GETOBJECT, see How WM_GETOBJECT Works.

What Information Is Exposed

Oleacc.dll uses the UI element's Windows class name to determine what information should be exposed for each of its IAccessible properties and how to collect that information. For example, Oleacc.dll calls the GetWindowText function to retrieve the Name property for a standard push button, but calls this same function to retrieve the Value property for a standard edit control. In effect, Oleacc.dll is mapping each IAccessible method to an appropriate Microsoft Win32 or control-specific message or function call. By using this class name-based special casing, it can return meaningful information through IAccessible proxies without any Microsoft Active Accessibility support in the server.

Applications built with standard UI elements typically get full Microsoft Active Accessibility support without additional development work. The exceptions to this rule are controls that have been subclassed, that do not store their own strings (absence of the HASSTRINGS style), or that are owner-drawn. In these cases, Oleacc.dll cannot gather the information it needs because the information is stored outside the control. However in many of these scenarios, established workarounds, or the use of Dynamic Annotation, allow the server to cooperate with the proxies provided by Oleacc.dll.

Generic Proxy Objects

If Oleacc.dll does not recognize the class name of the UI element, it creates a generic proxy that exposes as much information as possible. At most, this includes the object's bounding rectangle, parent object, name (from WM_GETTEXT), and any children in the window hierarchy.