title: Please explain why I am able to instantiate the "Application" interface in Excel VSTO
body: <p>I have the following C# code in my application which works just fine.  It launches a new instance of Excel.</p>
  
  <pre><code>private readonly Microsoft.Office.Interop.Excel.Application _application;
  _application = new Microsoft.Office.Interop.Excel.Application();
  _application.Visible = true;
  </code></pre>
  
  <p>I only recently noticed that <a href="http://msdn.microsoft.com/en-us/library/microsoft.office.interop.excel.application.aspx" rel="noreferrer">Application</a> is an interface type.  What exactly is going on and how is it possible?</p>