How to Host an MSAA Windowless ActiveX Control

Learn how to create a control container that can host windowless Microsoft ActiveX controls that implement Microsoft Active Accessibility. By following the steps described here, you can ensure that any Microsoft Active Accessibility-based windowless controls that are hosted in your control container are accessible to assistive technology (AT) client applications.

What you need to know



  • C/C++
  • Microsoft Win32 and Component Object Model (COM) programming
  • Windowless ActiveX controls
  • Microsoft Active Accessibility servers


Step 1: Provide the root IAccessible interface on behalf of the windowless control.

Whenever the system needs the IAccessible pointer for the root of a windowless control, the system queries the control container. To retrieve the pointer, the container calls the windowless control's implementation of the IServiceProvider::QueryService method.

If the control container has a Microsoft Active Accessibility implementation, it can return the windowless control's IAccessible pointer to the system.

If the control container has a Microsoft UI Automation implementation, call the UiaProviderFromIAccessible function to obtain an IRawElementProviderSimple interface pointer that represents the control, and then return the IRawElementProviderSimple interface pointer to the system.

Step 2: Respond to the WM_GETOBJECT message on behalf of the windowless control.

When a client application responds to a WinEvent raised by a windowless control, the control container receives a WM_GETOBJECT message on behalf of the control. The message includes the object ID of the windowless control that raised the event.

The control container responds by looking up the windowless control that "owns" the object ID, and then calling that control's IAccessibleHandler::AccessibleObjectFromID method. The AccessibleObjectFromID method returns the IAccessible interface pointer for the UI item, and the control container returns the pointer to the system, which forwards it to the client application.

Step 3: Implement the IAccessibleWindowlessSite interface.

  1. Implement the IAccessibleWindowlessSite::GetParentAccessible method.

    When a client application needs accessibility information about the parent of the windowless control's root provider, the system calls the windowless control's IAccessible::get_accParent method. The control responds by calling the control container's IAccessibleWindowlessSite::GetParentAccessible method, which provides the IAccessible pointer of the windowless control's parent.

  2. Implement the IAccessibleWindowlessSite::AcquireObjectIdRange, QueryObjectIdRange, and ReleaseObjectIdRange methods.

    Your control container must maintain a mapping of object ID ranges to windowless controls. The mapping enables the control container to identify the control that should respond to a particular request for an object ID. The following table shows an example of mapping object ID ranges to windowless controls.

    Range baseRange size Control
    1000500Control 1
    15001000Control 2
    2500 2000Control 1


    The control container must maintain the mapping of object ID ranges to windowless controls for itself and all windowless children. The object ID ranges do not need to be adjacent to one another. Also, to avoid denial-of-service attacks, the control container can place limits on the number of ranges that are granted to a particular control. However, it is useful to allow more than one range per control, to enable a control to grow.

Step 4: Optional: Implement the IAccessibleHostingElementProviders interface.

Implement the IAccessibleHostingElementProviders interface if your control container has a Microsoft Active Accessibility server implementation and the server is the root of an accessibility tree that includes windowless ActiveX controls that support UI Automation. The IAccessibleHostingElementProviders interface has a single method, GetEmbeddedFragmentRoots, which retrieves the IRawElementProviderFragmentRoot interface pointers of all UI Automation windowless ActiveX controls that are hosted by your control container.

Related topics

How to Host a UI Automation Windowless ActiveX Control
Windowless ActiveX Control Accessibility