在WPF中如何添加菜单项事件？

In my WPF application, I add a menu then add several menu items under it. For example, one of my menu item is "Main Item", then I add subItem1, subItem2 and subItem3 under "Main Item". I want to click subItem1 and do something(e.g. MessageBox.show a message). Why I cannot find the event for this subItem1? How can I add the click event for subItem1? I find the property for subItem1 under the collection property for "Main Item", but can only see property, cannot see event list. How can I add click event for subItem1? Thank you!In my WPF application, I add a menu then add se