I have a WebApi method that returns an Interface and, of course, when XML is requested the DataContractSerializer refuses to serialize the concrete type. I thought I could specify known types to get around this but I cannot get it to work.

Interface:

 public interface ISheetInfo
  {
    Guid _id { get; set; }
    string Name { get; set; }
    Guid OwnerId { get; set; }
  }

Class:

  [BsonIgnoreExtraElements]
  [DataContract]
  public class SheetInfo : ISheetInfo
  {
    [BsonId]
    [DataMember]
    public Guid _id { get; set; }
    [DataMember]
    public string Name { get; set; }
    [DataMember]
    public Guid OwnerId { get; set; }
  }

Web.config:

<system.runtime.serialization>
    <dataContractSerializer>
      <declaredTypes>
        <add type="LL.Web.Interfaces.ISheetInfo,LL.Web.Interfaces">
          <knownType type="LL.Repositories.Mongo.SheetInfo,LL.Repositories.Mongo" />
        </add>
      </declaredTypes>
    </dataContractSerializer>
  </system.runtime.serialization>

Anyone know what is, or might be, wrong? (References in web.config are correct)
Exception is:
Type 'LL.Repositories.Mongo.SheetInfo' with data contract name 'SheetInfo:http://schemas.datacontract.org/2004/07/LL.Repositories.Mongo' is not expected.