title: With LINQ, get count of items that satisfy criteria in grouping
body: <p>I'm probably going to butcher the terminology if I try too hard, so it'll be easier to describe via code:</p>
  
  <p><code>
  var fooGroup = fooList.GroupBy(x =&gt; x.SomeID);<br>
  //fooGroup is now of type IEnumerable&lt;IGrouping&lt;int, Foo&gt;&gt;<br>
  var someCount = fooGroup.Count(y =&gt; y.Where(f =&gt; f.Bar == "Bar"));
  </code></p>
  
  <p>The above will not compile due to this error: "Cannot convert lambda expression to delegate type <code>System.Func&lt;System.Linq.IGrouping&lt;int,Foo&gt;,bool&gt;</code> 
  because some of the return types in the block are not implicitly convertible to the delegate return type"</p>
  
  <p>I assume the answer is pretty easy, but I can't quite wrap my head around how to do this.</p>