title: how to post like with facebook c# sdk
body: <p>How do you post a like to a facebook app using the c# sdk ?
  I tried with this code:</p>
  
  <pre><code>...
  FacebookClient app = new FacebookClient(accessToken);
  Dictionary&lt;string,object&gt; parameters = new Dictionary&lt;string, object&gt;();
  app.PostAsync(string.Format("{0}/likes", appId), parameters);
  ...
  </code></pre>
  
  <p>but I get ArgumentNullException: stream cannot be null</p>