泛型中的协变

IEnumerable<Base> 的变量，如下面的代码所示。

IEnumerable<Sons> d = new List<Sons>();
IEnumerable<Base> b = d;

实事上，这正是我之前一篇文章上提到的，派生类集合可以转换为基类集合，这在.net术语中叫到泛型的转协变。
例如，在一个LINQ TO SQL的复杂查询时，你的代码可能是这样

IQueryable<UserAddress> linq1 = from data in new Repository<UserAddress>().GetModel()
                                            select new UserAddress
                                           {
                                               UserID = data.UserID,
                                               ID = data.ID,
                                               Address = data.Address,
                                           };

注意，这段代码会出错，因为ＬＩＮＱ不能返回自己本身的实体，所以必须为返回它的派生类型
  IQueryable<UserAddress> linq1 = from data in new Repository<UserAddress>().GetModel()
                                            select new UserAddress_Ext
                                             {
                                                 UserID = data.UserID,
                                                 ID = data.ID,
                                                 Address = data.Address,
                                             };
这种写法是没有问题的，呵呵，这其实就叫做泛型的协变