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.NET Forum / Languages / C# / May 2008Interfaces and Inheritance
Hi all. I have bean studying the use of Interfaces, and from what I understand if you create a class that inherits from an interface, that class must poses a member that represents each member of the derived interface. Well this all seams sensible because otherwise it would defy the principles behind inheritance, I was studying the class “System.Collections.Generic.List<T>”. This class states that it derives from the interface “System.Collections.Icollection” Therefore I would assume that it would possess all the members that the interface contains. This is where I become very confused. “System.Collections.Generic.List<T>” douse not poses the members: IsSynchronized SyncRoot Trying to find an answerer to this I ran the code <Code> System.Collections.ICollection i = new System.Collections.Generic.List<string>(); System.Windows.Forms.MessageBox.Show(i.IsSynchronized.ToString()); </Code> I found that it ran successfully and displayed the value “False”. This states to me that the class douse in fact poses the member “IsSynchronized” but fore some reason douse not give you access to it at design time. Could anybody please explain this to me? Thanks Jay Dee > I have bean studying the use of Interfaces, and from what I understand > if you create a class that inherits from an interface, that class must > poses a member that represents each member of the derived interface. Technically, a class does not inherit from an interface, it implements it. It's useful to make the distinction because the mechanisms are slightly different. > Well this all seams sensible because otherwise it would defy the > principles behind inheritance, [quoted text clipped - 4 lines] > Therefore I would assume that it would possess all the members that > the interface contains. It does. > This is where I become very confused. > “System.Collections.Generic.List<T>” douse not poses the members: > > IsSynchronized > SyncRoot Yes, it does! It implements the interface explicitly. This means you can only access these members from a reference of type ICollection... > Trying to find an answerer to this I ran the code > [quoted text clipped - 3 lines] > System.Windows.Forms.MessageBox.Show(i.IsSynchronized.ToString()); > </Code> ...as you demonstrate here. > Could anybody please explain this to me? It's called "explicit interface implementation". The MSDN explains it here: http://msdn.microsoft.com/library/ms173157.aspx The idea is to use explicit interface implementations when regular clients are not expected to be interested in the members; only clients accessing the class through the interface will be. A clearer example is ICollection.IsReadOnly. For most collections, you do not want to expose this property because the collection will *always* be read-only or read-write, as part of its type. In this case it makes sense to implement the property explicitly, so only consumers of ICollection (who do not know the specifics of the implementing class) see the property.  SignatureJ. http://symbolsprose.blogspot.com > I have bean studying the use of Interfaces, and from what I understand > if you create a class that inherits from an interface, that class must [quoted text clipped - 11 lines] > This is where I become very confused. > =3FSystem.Collections.Generic.List<T>=3F douse not poses the members: Yes it does - but it uses explicit interface implementation to define them. That means that to use those members, you have to use the reference exactly as an ICollection, rather than as the concrete type. Search for C# and "explicit interface implementation" for more details. SignatureJon Skeet - <skeet@pobox.com> Web site: http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet C# in Depth: http://csharpindepth.com > Hi all. > > I have bean studying the use of Interfaces, and from what I understand > if you create a class that inherits from an interface, that class must > poses a member that represents each member of the derived interface. A class do not inherit from an interface, it does implement it. That how a class can implement (and behave) like more than one interface AND inheriting from only one class. > Well this all seams sensible because otherwise it would defy the > principles behind inheritance, see the above remark > Could anybody please explain this to me? I think I know the answer, but I would like to see your code first, Hint, take a look at the difference between explicitely and implicit interface implementation >> Hi all. >> [quoted text clipped - 11 lines] > > see the above remark I believe the OP was referring to the Liskov Substitution Principle. And in that sense, classes are as much a subtype of any interfaces as of the base class(es). They don't, however, inherit any behavior from interfaces. Many languages have only one means of subtyping, which is usually called "inheritance". .NET distinguishes between behavioral inheritance (derivation) and interface inheritance (implementation), however. >> Could anybody please explain this to me? > > I think I know the answer, but I would like to see your code first, > Hint, take a look at the difference between explicitely and implicit > interface implementation Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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