title: How to apply an Extension Method on the object having the type of ExpandoObject?
body: <p>Here is my code:</p>
  
  <pre><code>public static class DynamicExtensions
  
      public static void Add(this ExpandoObject obj, string path){
          dynamic _obj = obj;
          if (_obj == null) throw new ArgumentNullException("obj");
          _obj.path = path;
      }
  }
  </code></pre>
  
  <p>But I got the error of "'System.Dynamic.ExpandoObject' does not contain a definition for 'Add'", when I call it in this way:</p>
  
  <pre><code>dynamic obj = new ExpandoObject();
  obj.Add("p1");
  </code></pre>
  
  <p>How to fix it?</p>
  
  <p>Thanks in advance!</p>