title: creating a compound index in c#
body: <p>I want to create a compound index where one key should be in ascending, the second key in descending order.</p>
  
  <p>How can I do this?</p>
  
  <p>I have a string containing the property names the user selected.</p>
  
  <pre><code>collection.EnsureIndex(IndexKeys.Descending(selectedProperties[0]),
                  IndexKeys.Ascending(selectedProperties[1])),
  IndexOptions.......
  </code></pre>
  
  <p>does not work</p>