asp.net mvc Remote远程验证

/// <summary>
        /// 课程编号
        /// </summary>
        [MaxLength(50)]
        [Remote("IsUnique_Available", "Course")]
        [Required(ErrorMessageResourceName = "Course_CourseNum", ErrorMessageResourceType = typeof(ErrorMessage))]
        public string CourseNum { get; set; }

#region 验证课程编号不唯一
        public JsonResult IsUnique_Available(string CourseNum)
        {
            Course model = o_Res.GetCourseAny(a => a.CourseNum == CourseNum);
            if (model == null)
            {
                return Json(true, JsonRequestBehavior.AllowGet);
            }
            else
            {
                string suggestedUID = String.Format(CultureInfo.InvariantCulture, "课程编号:{0} 已经存在！", CourseNum);
                return Json(suggestedUID, JsonRequestBehavior.AllowGet);
            }
        }
        #endregion

<td colspan="3">
                    @Html.TextBoxFor(a => a.CourseNum, new { maxlength = 50 }) @Html.ValidationMessageFor(model => model.CourseNum)
                </td>

<td colspan="3">
                    @Html.TextBoxFor(a => a.CourseNum, new { maxlength = 50 , disabled = true }) @Html.ValidationMessageFor(model => model.CourseNum)
                </td>