title: How to make (.cs) code files expand / collapse underneath another code file in Visual Studio solution explorer?
body: <p>In Visual Studio 2008/2010 it's possible to place a series of simple .cs code files underneath another .cs file in solution explorer. This behavior is similar to how you see files arranged when adding a WPF UserControl to the solution, where instead a single .cs file can collapse/expand underneath the .xaml file.</p>
  
  <p>How do you make the solution arrange files this way? I've already looked in some of the obvious places, like the file properties pane, the solution and .csproj property pages.</p>