r">public</span>&#160;<span class="Modifier">static</span>&#160;<span class="ValueType">double</span>? Average(<span class="Keyword">this</span> IEnumerable&lt;<span class="ValueType">int</span>?&gt; source)     <br />    <br /><span class="Modifier">public</span>&#160;<span class="Modifier">static</span>&#160;<span class="ValueType">double</span>? Average&lt;TSource&gt;(     <br />&#160;&#160;&#160; <span class="Keyword">this</span> IEnumerable&lt;TSource&gt; source,     <br />&#160;&#160;&#160; Func&lt;TS