private static void RemoveWndProcUsage(HwndHost hwndHost)
{
int refCount = (int)hwndHost.GetValue(WindowHookRefCountProperty);
refCount--;
hwndHost.SetValue(WindowHookRefCountProperty, refCount);
if (refCount == 0) {
HwndHostExtensionsWindowHook hook = (HwndHostExtensionsWindowHook)hwndHost.GetValue(WindowHookProperty);
hook.Dispose();
hwndHost.ClearValue(WindowHookProperty);
}
}