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);
 }