Why IReadOnlyCollection has ElementAt but not IndexOf
<p>I am working with a <code>IReadOnlyCollection</code> of objects.</p>

<p>Now I'm a bit surprised, because I can use <code>linq</code> extension method <code>ElementAt()</code>. But I don't have access to <code>IndexOf()</code>.</p>

<p>This to me looks a bit illogical: I can get the element at a given position, but I cannot get the position of that very same element.</p>

<p>Is there a specific reason for it?</p>

<p>I've already read -> <a href="https://stackoverflow.com/questions/1290603/how-to-get-the-index-of-an-element-in-an-ienumerable">How to get the index of an element in an IEnumerable?</a> and I'm not totally happy with the response.</p>