在 Window 关闭时，无法将 Visibility 设置为 Visible，也无法调用 Show、ShowDialog、Close 或 WindowInteropHelper.EnsureHandle。

private static async void Window_Closing(object sender, CancelEventArgs e)
{

　　e.Cancel = true;

　　//继续执行别的代码：

　　......

　　......

　　var window = sender as Window;

　　window.ForceClose();

}

/// <summary>
/// 强制关闭窗体
/// <para>在Window的Closing事件执行Close会报如下错误：</para>
/// <para>Cannot set Visibility to Visible or call Show, ShowDialog, Close, or WindowInteropHelper.EnsureHandle while a Window is closing.</para>
/// <para>使用此方法可解决问题并成功关闭窗体</para>
/// </summary>
/// <param name="window"></param>
public static async void ForceClose(this Window window)
{
　　await Dispatcher.CurrentDispatcher.InvokeAsync(() => window.Close(), DispatcherPriority.Normal);
}

参考连接