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copied!<p>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.</p> <p>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.</p> <p>Another way might be to use the System.Linq.Enumerable.Count method, i.e.</p> <pre><code>using System.Linq; class X { void Y(IEnumerable<int> collection) { int itemCount = collection.Count(); } } </code></pre> <p>or use the (System.Linq.Enumerable) .ToList() to pull all the items from the enumerator into a Collection and work from there.</p> <p>(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)</p>

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