What’s in a name? (C#’s hidden support for structural typing)
togaroga.comC# is well known for its nominative type system. This means that C# identifies types and their relationships based on their names. That is why C# does not allow you to create two types with the same name, even if they have a different public interface – the C# compiler only cares about the name of the type. I suspect the reason this works this way is because `foreach` is a language construct - not a part of the .NET Framework Class Library.
On the other hand, `IEnumerable` is an interface defined in the class library that ships as part of the .NET runtime. Having the foreach construct require implementing IEnumerable will tie the compiler to a specific class definition. So instead, it only looks for a method with that name, which happens to be formalized in the class library in the form of an interface called IEnumerable. Edit: hm.. but I note that this doesn't work for the using(){} construct. This requires the object to implement an interface that is convertable to IDisposable instead of just looking for a `Dispose` method. I always found this somewhat inelegant. I suppose given all the constraints, its OK, but it feels like such a feature (such as the collection initializers) should be based on language constructs, not compiler special cases. Lucian Wischik gave a really nice talk recently about language design at Microsoft and he indicated that they are trying to do it in that order from now on: determine the feature that's needed, add the necessary plumbing that would allow programmers to implement it, and implement it as a reference implementation. How can I use this structural typing feature? AOP weavers like postsharp let you do some duck typing like stuff by injecting interfaces as post compile step. You could use Reflection to search for types based on structure. But there is no syntax for it. Actually Linq + Reflection is some of the most entertaining code to write C#. Example: http://github.com/ecoffey/Bebop/blob/master/Bebop/BebopAppli... we're saying there "Give me all the types out of a specific assembly that implement IResource". You could try reflection and/or dynamic. The latter requires .Net 4. The big one he doesn't cover is LINQ query syntax. There's a post on my blog ( http://contextfree.livejournal.com ) where I explain why it works this way ...