public interface IMasterPadService
    {
        [OperationContract]
        [WebInvoke(UriTemplate = "/GetTask", ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.WrappedRequest)]
        OperationInfo GetTask(int taskCode);
    }

    [DataContract]
    public class OperationInfo
    {
        ...
    }

    [DataContract]
    public class OperationIdleInfo : OperationInfo
    {
        ...
    }

[DataContract]
    [KnownType(typeof(OperationIdleInfo))]
    public class OperationInfo
    {

[ServiceContract(Namespace = "http://EEMServices")]
    [ServiceKnownType(typeof(OperationIdleInfo))]
    public interface IMasterPadService
    {