//使用扩展方法删除集合companies中的allcompanys的数据
var allcompanys = companies.Where(x =>
nations.Where(z => z.OnArea == "Asia").
Any(z => cities.Where(o => o.OnNation == z.Chinese).Any(o => o.Chinese == x.OnCity))).ToList();
companies.ForEach(x=> { allcompanys.Contains(x); }) ;
//这里已经找到了两个集合companies和allcompanys,想定义一个扩展方法RemoveList来直接删除,成为这样的companies.RemoveList(allcompanys);