title: WorksheetFunction.CountA RuntimeBinderException error
body: <p>New to VSTO, know Excel's model very well... I have </p>
  
  <pre><code>using Excel = Microsoft.Office.Interop.Excel;
  </code></pre>
  
  <p>at the head of my class, and the following simple call, which compiles correctly and even appears correct in intellisense</p>
  
  <pre><code>double usedRows = (double)Excel.WorksheetFunction.CountA(oWks.Columns[1]);
  </code></pre>
  
  <p>but fails at runtime with this exception</p>
  
  <blockquote>
    <p>'Microsoft.Office.Interop.Excel.WorksheetFunction' does not contain a
    definition for 'CountA'</p>
  </blockquote>
  
  <p>about which I can find little further information. What am I missing?</p>