title: DataContractSerializer How to emit clean Xml for generic list of interfaces
body: <p>I have this object structure:</p>
  
  <pre><code>[DataContract]
  [KnownType(typeof(ChildClassA))]
  [KnownType(typeof(ChildClassB))]
  public class MainClass
  {          
      [DataMember]
      public string PropA { get; set; }       
      [DataMember]
      public IList&lt;IMyInterface&gt; GenericInterfaceList { get; set; }       
      //....
  }    
  public interface IMyInterface
  {
      int Id { get; set; }        
      //....
  }    
  [DataContract]
  public abstract class BaseClass :  IMyInterface
  {
      [DataMember]
      public int Id { get; set; }     
      //....
  }    
  [DataContract]
  public class ChildClassA : BaseClass
  {
      //....
  }    
  [DataContract]
  public class ChildClassB : BaseClass
  {
      //.....
  }
  </code></pre>
  
  <p>I want that <code>DataContractSerializer</code> emits an Xml without <code>a:anyType i:type=</code> for my generic list of interfaces:</p>
  
  <pre><code>&lt;MainClass xmlns="MYNAMESPACE" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"&gt;
      &lt;PropA&gt; dfgdsfg gsd&lt;/PropA&gt;
      &lt;GenericInterfaceList xmlns:a="http://schemas.microsoft.com/2003/10/Serialization/Arrays"&gt;
          &lt;ChildClassA&gt;
              &lt;Id&gt;1&lt;/Id&gt;
          &lt;/ChildClassA&gt;
          &lt;ChildClassB&gt;
              &lt;Id&gt;2&lt;/Id&gt;
          &lt;/ChildClassB&gt;
      &lt;/GenericInterfaceList&gt;
  &lt;/MainClass&gt;
  </code></pre>
  
  <p>instead of:</p>
  
  <pre><code>&lt;MainClass xmlns="MYNAMESPACE" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"&gt;
      &lt;PropA&gt; dfgdsfg gsd&lt;/PropA&gt;
      &lt;GenericInterfaceList xmlns:a="http://schemas.microsoft.com/2003/10/Serialization/Arrays"&gt;
          &lt;a:anyType i:type="ChildClassA"&gt;
              &lt;Id&gt;1&lt;/Id&gt;
          &lt;/a:anyType&gt;
          &lt;a:anyType i:type="ChildClassB"&gt;
              &lt;Id&gt;2&lt;/Id&gt;
          &lt;/a:anyType&gt;
      &lt;/GenericInterfaceList&gt;
  &lt;/MainClass&gt;
  </code></pre>
  
  <p>I tryed with a custom <code>DataContractResolver</code> but it is not the right tool. Cannot find a way to shrink that Xml.</p>