无法推断C#中的类型,必须明确设置吗？

/// <sumMary>
        /// Binds all dataObjects e.g. IPersonList,IDepartmentList,ITopicList... and creates a visual list of elements to display in the ElementTextBox
        /// </sumMary>
        /// <typeparam name="T">Type of dataObject in the dataObjects list</typeparam>
        /// <typeparam name="TProperty">value for the Type specified by the TResult paramter</typeparam>
        /// <param name="dataObjects">entity from database the user wants to show in the ElementTextBox</param>
        /// <param name="SELEctorDisplaymember">The property like FirstName that is shown as the elements text</param>
        /// <param name="SELEctorSortMember">The property like SortId that is used to pre-sort the dataObjects so the elements appear in the order before they were saved</param>
        public void BindElements<T,TProperty>(IEnumerable<T> dataObjects,Func<T,TProperty> SELEctorDisplaymember,TProperty> SELEctorSortMember)
        { 
            if (dataObjects != null)
            {
                var sortedDataObjects = from d in dataObjects
                                        orderby SELEctorSortMember(d) ascending
                                        SELEct d;

                Paragraph para = new Paragraph();

                foreach (T item in dataObjects)
                {
                    TProperty displaymemberValue = SELEctorDisplaymember(item);
                    InlineUIContainer uiContainer = ElementList.CreateElementContainer(displaymembervalue);
                    para.Inlines.Add(uiContainer);
                } 

                FlowDocument flowDoc = new FlowDocument(para);
                ElementList.Document = flowDoc;
            }            
        }

这工作：ElementUserControl.bindElements(customers,c => c.CustomerId);

但是当我添加第3个参数时：

ElementUserControl.bindElements(customers,c => c.CustomerId,c => c.sortId);

它不再起作用了？