WPF MVVM 如何在 ViewModel 中关闭界面窗口

<Button Width="80" 
        Command="{Binding CancelCmd}"
        CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}">
</Button>

private CommandBase cancelCmd;
public CommandBase CancelCmd
{
    get
    {
        if (cancelCmd == null)
        {
            cancelCmd = new CommandBase(CloseApp, CancelCanExcute);
        }
        return cancelCmd;
    }
}
void CloseApp(object o)
{
    var win = o as System.Windows.Window;
    win.Close();
}
bool CancelCanExcute(object o) { return true; }