title: When to use LINQ's .ToList() or .ToArray()
body: <p>After running this code:</p>
  
  <pre><code>var input = new List&lt;T&gt;( ... );
  var result = input.Select( t =&gt; new U(t) );
  
  U first1 = null;
  foreach ( U u1 in result )
      if ( first1 == null )
          first1 = u1;
  
  U first2 = null;
  foreach ( U u2 in result )
      if ( first2 == null )
          first2 = u2;
  </code></pre>
  
  <p>Then 'first1 == first2' evaluates to false even though both U's wrap the same T.  I haven't tested it yet, but I think it can be made to evaluate to true by chaining a .ToList() or .ToArray() onto the Select() call.</p>
  
  <p>In real code, which is much more complex than this simple illustration, what is a good rule of thumb to use for deciding if .ToList() or .ToArray() should be appended?  My initial thoughts are either any referenced expression that may be iterated more than once or, to be safer in case potential iterations are not obvious, any referenced expression whose result will never change.</p>