[ServiceContract]
    public interface IMyService
    {
        [OperationContract]
        Category MyMethod(int i);

        [OperationContract]
        void Save(Category i);
    }

    [ServiceBehavior(IncludeExceptionDetailInFaults = true)]
    public class MyService : IMyService
    {
        public MyService(IRepository<object> repository) { }

        public Category MyMethod(int i) { Console.WriteLine("Parameter i = " + i); return new Category(); }

        public void Save(Category category) { Console.WriteLine("Category saved"); }
    }

namespace WCFServer
{
    public class Category
    {
        private IList<Category> childCategories = new List<Category>();
        private Category parentCategory;

        public IEnumerable<Category> ChildCategories { get { return this.childCategories; } }

        public virtual Category ParentCategory
        {
            get { return this.parentCategory; }
            set { this.parentCategory = value; }
        }
    }
}

class Program
    {
        static void Main(string[] args)
        {
            var service = new MyServiceClient();
            var c = service.MyMethod(5);
            var cat = new Category();
            service.Save(cat);//  Исключение!!!
            Console.WriteLine("Stop");
            Console.ReadKey();
        }
    }