title: C# XML nullable attribute
body: <p>I'd like to have an enum attribute to be optional.
  <br>I guess the way to achieve that would be for it to be nullable.</p>
  
  <pre><code>[XmlAttribute(AttributeName = "action")]
  public EAction? Action
  {
      get;
      set;
  }
  </code></pre>
  
  <p>But this won't work.</p>
  
  <p>With my current knowledge on the matter the only valid nullable tpye I could use is string.
  <br/>But if it's possible I'd rather not use the trick where you just hide the Enum with a string.</p>
  
  <pre><code>[XmlIgnore]
  public EAction? Action
  {
      get;
      set;
  }
  
  [XmlAttribute(AttributeName = "action")]
  public string _Action
  {
      get { return this.Action.ToString();
      set { ... }
  }
  </code></pre>
  
  <p>Am I asking for the impossible?</p>