32-bit Java Accessibility on a 64-bit machine

I have a 32-bit app that makes use of Java Accessibility (WindowsAccessBridge-32.dll, via the Java Access Bridge), and works perfectly on a 32-bit machine, but fails on an x64 machine.

I believe I have tracked it down to one of the first calls after Windows_run:

getAccessibleContextFromHWND(hwnd, out vmId, out context)

defined as follows:

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("WindowsAccessBridge-32.dll", CallingConvention = CallingConvention.Cdecl)]
public extern static bool getAccessibleContextFromHWND(IntPtr hwnd, out Int32 vmID, out IntPtr acParent);

This call works fine on the 32-bit system, returning True, populating both vmId (with some 5-digit value, which), and context – whereas on the 64-bit system, it returns True, populates ‘context’, but returns ‘0’ for vmId.

If I assume that 0 is valid (even though it’s a random 5-digit number resembling a pointer on the 32-bit system), the next call still fails:

AccessibleContextInfo aci = new API.AccessibleContextInfo();
if (!getAccessibleContextInfo(vmId, context, ref aci))
  throw new Exception();

where:

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("WindowsAccessBridge-32.dll", CallingConvention = CallingConvention.Cdecl)]
public extern static bool getAccessibleContextInfo(Int32 vmID, IntPtr ac, ref AccessibleContextInfo info);

(I’m omitting the AccessibleContextInfo struct for brevity, but I can provide it if necessary).

I know that the libraries are working, because both JavaMonkey and JavaFerret work correctly. Furthermore, call to isJavaWindow works, returning ‘true’, or ‘false’ as appropriate, and I am linking to the correct DLL (WindowsAccessBridge-32).

Can anyone suggest what may be wrong here?

Answer

It appears that the problem is in the type of AccessibilityContext:

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("WindowsAccessBridge-32.dll", CallingConvention = CallingConvention.Cdecl)]
public extern static bool getAccessibleContextFromHWND(IntPtr hwnd, out Int32 vmID, out IntPtr acParent);

AccessibilityContext (acParent above), which I had incorrectly mapped as an IntPtr, is actually an Int32 when using the “legacy” WindowsAccessBridge.dll library (used under x86), and an Int64 when using the WOW64 WindowsAccessBridge-32.dll library.

So the upshot is, the code has to differ between x86 and WOW x64, and must be compiled separately for each. I do this by #define’ing WOW64 during x64 builds, always referencing the Int64 methods, and using “shim” methods on x86:

#if WOW64 // using x64

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("WindowsAccessBridge-32.dll", CallingConvention = CallingConvention.Cdecl)]
public extern static bool getAccessibleContextFromHWND(IntPtr hwnd, out Int32 vmID, out Int64 acParent);

#else // using x86

[return: MarshalAs(UnmanagedType.Bool)]
[DllImport("WindowsAccessBridge.dll", EntryPoint = "getAccessibleContextFromHWND", CallingConvention = CallingConvention.Cdecl)]
private extern static bool _getAccessibleContextFromHWND(IntPtr hwnd, out Int32 vmID, out Int32 acParent);

public static bool getAccessibleContextFromHWND(IntPtr hwnd, out Int32 vmID, out Int64 acParent)
{
  Int32 _acParent;

  bool retVal = _getAccessibleContextFromHWND(hwnd, out vmID, out _acParent);
  acParent = _acParent;

  return retVal;
}

#endif

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