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.NET Forum / .NET Framework / New Users / August 2005Generic ReadOnlyCollection sealed
I'm trying to create a custom read only collection. I found the generic ReadOnlyCollection class and the documentation says "Implementers are encouraged to extend this base class instead of creating their own." OK, sounds good. The only problem is that it seems that most of the methods of ReadOnlyCollection<T> are sealed, which makes extending the class difficult. Have I missed something? Thanks. Ian. > I'm trying to create a custom read only collection. I found the generic > ReadOnlyCollection class and the documentation says "Implementers are > encouraged to extend this base class instead of creating their own." > OK, sounds good. The only problem is that it seems that most of the > methods of ReadOnlyCollection<T> are sealed, which makes extending the > class difficult. Have I missed something? Thanks. I guess you mean the methods are not virtual, right? In that case yes, you are right. But there's nothing wrong with that, it's the same pattern that's been used for all the other generic collection classes. This shouldn't normally be much of a problem because the generic collection class is typed already. You can derive your own type from it and extend it with additional methods, if you want, or you can use it as it is, because in contrast to the non-generic [ReadOnly]CollectionBase, this is not an abstract class. What exactly do you want to do in your derived class? Why is it a problem? Oliver Sturm  Signatureomnibus ex nihilo ducendis sufficit unum Spaces inserted to prevent google email destruction: MSN oliver @ sturmnet.org Jabber sturm @ amessage.de ICQ 27142619 http://www.sturmnet.org/blog Thanks, Oliver. What I want to be able to do is to provide a custom enumerator so that I can iterate through only specific objects in a "master collection" which the read-only collection wraps. To do this I need to overload IEnumerable<T>::GetEnumerator (and ICollection<T>::Count). Ian. > Thanks, Oliver. What I want to be able to do is to provide a custom > enumerator so that I can iterate through only specific objects in a > "master collection" which the read-only collection wraps. To do this I > need to overload IEnumerable<T>::GetEnumerator (and > ICollection<T>::Count). Why don't you just create an additional iterator instead of overriding the standard one? class MyCollection : ReadOnlyCollection<MyContent> { ... public IEnumerable<MyContent> MyNewIterator() { foreach (MyObject mo in this) if (mo.SatisfiesMyCriteria()) yield return mo; } ... } Actually, if I understand the scenario correctly that you are describing, you could just as well implement this additional iterator (or any number of them) directly in your "master collection". You can even pass in parameters to the iterator to make it more flexible... Oliver Sturm Signatureomnibus ex nihilo ducendis sufficit unum Spaces inserted to prevent google email destruction: MSN oliver @ sturmnet.org Jabber sturm @ amessage.de ICQ 27142619 http://www.sturmnet.org/blog Oliver, thanks for the suggestion. If I created a new method to return a different enumerator, I would have a schitzophrenic collection which is not a good solution IMHO. For the record, I rolled my own read only collection class. I had some trouble until I realized that you have to implement both of IEnumerable::GetEnumerator and IEnumerable<>::GetEnumerator using something like: virtual IEnumerator^ GetEnumerator() = IEnumerable::GetEnumerator() {...} virtual IEnumerator<T>^ GetGenericEnumerator() = IEnumerable<T>::GetEnumerator() {...} So much for not having to reinvent the wheel... Ian. > Oliver, thanks for the suggestion. If I created a new method to return > a different enumerator, I would have a schitzophrenic collection which > is not a good solution IMHO. Well, that's the solution devised by the .NET team, and I like it. An iterator is not about what a collection contains, but about enumerating elements in a collection in a specific way. So I don't think you have a schizophrenic collection, you just have a collection that knows how to distinguish between different types of its elements and enumerate each type accordingly. > For the record, I rolled my own read only collection class. I had some > trouble until I realized that you have to implement both of [quoted text clipped - 6 lines] > > So much for not having to reinvent the wheel... Under many circumstances it's enough to have one of those, if I'm not mistaken. It's also possible to implement the generic one "for real", meaning with the algorithm that's needed to select the correct data you need, and to just implement the non-generic method like this: IEnumerator IEnumerable.GetEnumerator() { foreach (T item in this) yield return item; } Oliver Sturm Signatureomnibus ex nihilo ducendis sufficit unum Spaces inserted to prevent google email destruction: MSN oliver @ sturmnet.org Jabber sturm @ amessage.de ICQ 27142619 http://www.sturmnet.org/blog Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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