title: C# Bot framework: How to set a field optional in the form flow Dialog
body: <p>I am trying to build a bot which have a form flow dialog in it. I have to make a field in my form builder to <code>Optional</code>. I done by giving the <code>[Optional]</code> annotation in my class attribute. But its not working. By Form keeps prompting for the field. How can i control this?</p>
  
  <pre><code>form = new FormBuilder&lt;myClass&gt;()
                      .Message("Welcome")
                      .Field(nameof(xxx))
                      .Field(nameof(yyy),//make this field optional, if value is there then validate. But should not prompt if no value.                        
                          validate: async (state, value) =&gt;
                          {
                          }
  </code></pre>
  
  <p>My Class</p>
  
  <pre><code>[Serializable]
  public class myClass
  {
      [Prompt("I didnt get you")]       
      public string xxx{ get; set; }
  
      [Optional]       
      public string yyy{ get; set; }
  
  }
  </code></pre>