title: .NET Xml serializer optional attributes
body: <p>Given the following code...</p>
  
  <pre><code>[XmlType("Field")]
  public class SearchField
  {
      [XmlAttribute("alias")]
      public string Alias;
  
      [XmlAttribute("entity")]
      public string Entity;
  }
  </code></pre>
  
  <p>Alias is an optional field for us, but the deserializer throws when the "alias" attribute is missing from the xml.  How do you make it optional?  Is a schema required?</p>