ASP.NET MVC DropDownList的使用

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一、控制器·

 public ActionResult Index()        {            if (ViewBag.courseID != null)            {            }            else            {                ViewBag.studentID = new SelectList(db.Students,"ID","LastName");                ViewBag.courseID = new SelectList(db.Courses, "CourseID", "Title");            }            return View();        }

二、Ajax

（1）服务器

        public ActionResult DDL(int studentID)        {            //string a = studentID;            //var courseID = db.Enrollments.Where(s => s.StudentID == studentID).Select(s=>s.CourseID);            //foreach (var cid in courseID)            //{            //    var course = db.Courses.Where(s => s.CourseID == cid);            //    IEnumerable<>            //}            //var q = from a in db.Enrollments            //        join b in db.Students on a.StudentID equals studentID            //        join c in db.Courses on a.CourseID equals c.CourseID            //        group new {            //           c=c            //        } by c.CourseID into  g            //        select new            //        {            //            CourseID = g.Key,            //            Title = g.FirstOrDefault,            //        };            var q = from a in db.Enrollments                    from b in db.Students                    from c in db.Courses                    where a.StudentID == studentID && a.CourseID == c.CourseID                    select new                    {                        CourseID = c.CourseID,                        Title = c.Title                    };            var result=q.Distinct().ToList();//去重Distinct()            //return RedirectToAction("Index");            return Json(q.Distinct().ToList());        }

（2）前端

    @using (Html.BeginForm("DDL", "Home"))    {    @Html.Label("课程：")        @*@Html.DropDownList("courseID")*@    @Html.DropDownList("studentID")    @Html.DropDownList("courseID",ViewBag.Course as IEnumerable<SelectListItem>, "请选择课程")        <input type="submit"value="提交" />    }

  <script type="text/javascript">        //$(function () {        //    $("#courseID").change(function () {        //        alert(this.id)        //    })        //})               //$(function () {        //    $("#courseID").change(function () {        //        alert($("#courseID").val())        //        //alert(this.val())        //    })        //})        //var value=$("#courseID").val();        //alert(value);        $(function () {            $("#studentID").change(function () {                $.ajax({                    url: "@Url.Action("DDL","Home")" + "?studentID= " + $(this).val(),                    type: 'POST',                    data: null,                    dataType: "json",                    success: function (data) {                        $("#courseID").empty();                      //  $("#courseID").append("<option value=0>Ajax</option>")                        var json = eval(data); //数组                        $.each(json, function (index, item) {                            //循环获取数据                                var CourseID = json[index].CourseID;                            var Title = json[index].Title;                            //  $("#list").html($("#list").html() + "<br>" + name + " - " + idnumber + " - " + sex + "<br/>");                            $("<option value='" + CourseID + "'>" + Title + "</option>").appendTo("#courseID");                        });                    }                });            });        });    </script>

@Html.DropDownList("studentID")，studentID即为ViewBag.studentID，studentID也是下拉框的id，下拉框的Value和Text会自动填充。

@Html.DropDownList("courseID",ViewBag.Course as IEnumerable<SelectListItem>, "请选择课程")，第一个参数是下拉框的id，第二个即ViewBag.Course，第三个参数为下拉框Text默认值。

Linq To SQL三张表联结查询，Distinct（）去重。

JQuery Ajax语法，使用Ajax实现DropDownList两级联动，在ASP.NET MVC中Ajax最好使用POST方式，否则会报500的错误码。

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