为什么foreach(HttpCookie cookie in Request.Cookies)会出错
既然不能转换,那cookie究竟是什么类型?我们用代码测试一下:
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原来cookie全变成了System.String类型, 上面的cookie变量的值就是Request.Cookies中每个Cookie的Name。下面的两段代码,结果是一样的:
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那我们如何枚举Request.Cookies?通过索引,我也不用多说了,大家都知道。
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而Syste.Net.CookieCollection不存在这个问题,那HttpCookieCollection与CookieCollection有什么区别呢?我们比较一下它们的基类与接口:
HttpCookieCollection 继承了NameObjectCollectionBase,NameObjectCollectionBase实现了ICollection, IEnumerable, ISerializable, IDeserializationCallback接口
CookieCollection实现了ICollection, IEnumerable接口
我想应该在NameObjectCollectionBase中可以找到答案。
首先我们复习一下foreach的工作原理,先看下面的代码:
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在编译的时候,C#编辑器会对每一个foreach 区域进行转换,转换成下面的代码:
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对于foreach(HttpCookie cookie in Request.Cookies), 转换的结果应该是这样:
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因为上面的enumerator.Current返回的是System.String类型,所以会出现“指定的转换无效”的错误。
用Reflector查看NameObjectCollectionBase的源代码,可以发现 HttpCookieCollection.GetEnumerator()调用的是NameObjectCollectionBase中的GetEnumerator(),而GetEnumerator()返回的是NameObjectCollectionBase内部的一个类NameObjectKeysEnumerator的实例,这个类实现了IEnumerator。
上面的enumerator.Current实际上是调用的NameObjectKeysEnumerator的Current属性,而Current属性中又调用了NameObjectCollectionBase的BaseGetKey。头都昏了! 为了找到问题的真正原因,只能继续。BaseGetKey的代码是这样的:
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答案终于找到,NameObjectCollectionBase返回的是NameObjectEntry(这也是NameObjectCollectionBase的一个内部类)的一个成员Key, 这个Key就是string类型的。
问题的原因虽然找到,但为什么要这样设计,暂时还搞不明白,只能等以后慢慢研究。当然,如果有高人解惑就最好了。
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