title: Why does this Using() give me an error?
body: <p>I am trying to open an (hundreds actually) excel file(s). I open the application but want to use the Using() functionality around each of the workbooks I open. Why is this resulting in an error?</p>
  
  <pre><code>using (Excel.Workbook wbXL = appXL.Workbooks.Open(_sourceFullPath, Type.Missing, Excel.XlFileAccess.xlReadOnly))
  {
      //stuff with wbXL
  }
  </code></pre>
  
  <p>using gets the red underline and says "'Microsoft.Office.Interop.excel.Workbook':Type used in a using statement must be implicitly convertible to 'System.IDisposable'.</p>
  
  <p>How to make this work?</p>