[CollectionDataContract(ItemName = "string", Namespace = "")]
public class MessageType : List<string>
{
}

[MessageContract(WrapperName = "updateBillExt", WrapperNamespace = "http://rsdn.ru/")]
public class updateBillInfo
{
    [MessageBodyMember(Name = "login", Namespace = "", Order = 0)]
    public string Login { get; set; }
    [MessageBodyMember(Name = "password", Namespace = "", Order = 1)]
    public string Password { get; set; }
    [MessageBodyMember(Name = "txn", Namespace = "", Order = 2)]
    public string Txn { get; set; }
    [MessageBodyMember(Name = "status", Namespace = "", Order = 3)]
    public int Status { get; set; }
    [MessageBodyMember(Name = "Message", Namespace = "", Order = 4)]
    public MessageType Message { get; set; }
}

[ServiceContract(Namespace = "http://rsdn.ru/", Name = "MyService", SessionMode = SessionMode.NotAllowed)]
public interface IMyService
{
    [OperationContract(Name = "updateBillExt")]
    void updateBillExt(updateBillInfo request);
}