Question

Does dot net have an interface like IEnumerable with a count property? I know about interfaces such as IList and ICollection which do offer a Count property but it seems like these interfaces were designed for mutable data structures first and use as a read only interface seems like an afterthought – the presence of an IsReadOnly field and mutators throwing exceptions when this property is true is IMO ample evidence for this.

For the time being I am using a custom interface called IReadOnlyCollection (see my own answer to this post) but I would be glad to know of other alternative approaches.

Answer 1

The key difference between the ICollection family and the IEnumerable family is the absence of certainty as to the count of items present (quite often the items will be generated/loaded/hydrated as needed) – in some cases, an Enumerable may not ever finish generating results, which is why the Count is missing.

Deriving and adding a Count is possible depending on your requirements, but it goes against this spirit, which is the purpose of ICollection – a collection of stuff that’s all there.

Another way might be to use the System.Linq.Enumerable.Count method, i.e.

using System.Linq;  class X {    void Y(IEnumerable<int> collection)    {       int itemCount = collection.Count();    } } 

or use the (System.Linq.Enumerable) .ToList() to pull all the items from the enumerator into a Collection and work from there.

(Also to answer your comment before having 50 rep:- the ‘.Count()’ bit is a call to an extension method on the extension class System.Linq.Enumerable – the extension method is available on all things that derive from IEnumerable because the code has a ‘using System.Linq’ which brings the extension methods in all classes in that namespace into scope – in this case its in the class Enumerable. If you’re in VS, pressing F12 will bring you to the definition of S.L.Enumerable. BTW C# In Depth is a fantastic book for learning LINQ properly – its a page turner thats really helps you get the whole picture compared to learning the bits of LINQ piece by piece)