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    <p>Granted <code>IEnumerable</code> is an interface used in collections but the underlying implementation is an <strong>object</strong> and you have set that <code>collection</code> variable to point to <code>null</code>, hence the exception.</p> <p>If on the other hand you initialized <code>collection</code> with:</p> <pre><code>IEnumerable&lt;string&gt; collection = Enumerable.Empty&lt;string&gt;(); </code></pre> <p>OR</p> <pre><code>IEnumerable&lt;string&gt; collection = new List&lt;string&gt;(); </code></pre> <p>You would have an empty list <strong>object</strong> that you could act on. The <code>ArgumentNullException</code> exception is thrown because the collection <strong>argument</strong> is in fact <code>null</code> and that is what <code>ToArray()</code> is trying to act on. So logically, to me anyways, this was the only design choice.</p> <p><strong>Edit</strong></p> <p>On the other hand, in practice, I've made the conscious decision to always return a valid <code>IEnumerable&lt;T&gt;</code> when the return type on my class methods are supposed to return an <code>IEnumerable&lt;T&gt;</code>.</p> <p>For example; a method signature that looks like <code>IEnumerable&lt;T&gt; GetAll()</code> would always return a valid enumerable and If there was nothing to return then I would return <code>return Enumerable.Empty&lt;T&gt;();</code></p> <p>The difference here to me is that <code>GetAll()</code> is not acting on a collection <strong>argument</strong>. You could really look at this like the collection is really nothing more than a parameter to the method and if you passed in a null parameter to a regular method you would probably throw an <code>ArgumentNullException</code>.</p> <hr> <p><strong>The short answer</strong> is that your <code>collection</code> variable is an <em>expected</em> <strong>argument</strong> (or parameter) to the <code>ToArray()</code> method and it is <code>null</code> so it makes sense to throw an <code>ArgumentNullException</code>.</p>
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    1. PODesign choice: Any good reason for 'ToArray()' LINQ extension to throw an exception for null collections?
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