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copied!<blockquote> <p>Of course, I don't want to force all itens to be created unnecessarily</p> </blockquote> <p>In which case you need to allow the method to create them on demand, and if objects are created on demand (and without some form of cache) they will be <em>different</em> objects (at least in the sense of being different references—the default definition of equality for non-value objects).</p> <p>If your objects are inherently unique (i.e. they don't define some value based equality) then each call to <code>new</code> will create a different object (whatever the constructor parameters).</p> <p>So the answer to</p> <blockquote> <p>but how can I explicitly define that some method gets an IEnumerable as parameter that creates a single result set dispite of how many times it calls "First()" method.</p> </blockquote> <p>is "you can't" except by creating one set of objects and repeatedly returning the same set, <em>or</em> by defining equality to be something different.</p> <hr> <p>Additional (based on comments). If you really want to be able to replay (for want of a better term) the same set of objects without building the whole collection you could cache want has already been generated and replay that first. Something like:</p> <pre><code>private static List<MyData> cache = new List<MyData>(); public IEnumerable<MyData> GetData() { foreach (var d in cache) { yield return d; } var position = cache.Count; while (maxItens < position) { MyData next = MakeNextItem(position); cache.Add(next); yield return next; } } </code></pre> <p>I expect it would be possible to build such a caching wrapper around an iterator as well (<code>while</code> would become <code>foreach</code> over underlying iterator, but you would need to cache that iterator or <code>Skip</code> to the require position if the caller iterated beyond the cahing <code>List</code>).</p> <p><strong>NB</strong> any caching approach would be hard to make thread safe.</p>

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