wpf 在Popup内的TextBox 输入法 不能切换

切换输入法 输入不了中文

[DllImport("User32.dll")]
public static extern IntPtr SetFocus(IntPtr hWnd);

IntPtr GetHwnd(Popup popup)
{
    HwndSource source = (HwndSource)PresentationSource.FromVisual(popup.Child);
    return source.Handle;
}

private void ShowPopupButtonClick(object sender, RoutedEventArgs e)
{
    ThePopup.IsOpen = true;
    IntPtr handle = GetHwnd(ThePopup);
    SetFocus(handle);
}