在视图中使用DropDownListFor时如何解决空值？

如何解决在视图中使用DropDownListFor时如何解决空值？

有一个问题我已经进行了整整一天半的故障排除，我需要一些帮助。
代码看起来不错，但是问题在于它返回了Null。

我可以在View Side上获得该值的属性。
它抛出：

没有Viewnum IEnumerable。

这是我到目前为止所做的，并且需要一些帮助，我的View使用Multiple Model，但是它的正确类名具有正确的属性。

 [HttpPost]
       public ActionResult CourseList() 
        {
            var course = new List<string>() 
            {  "Private(copyrighted)","Public Domain","Creative Commons Licences","CC Attribution","CC Attribution Share ALike","CC Attribution Non-Commercial","CC Attribution Non-Commercial Share Alike","CC Attribution Non Directive","CC Attribution Non-Commercial No Directives" 
            };
            // List of items here.
            List<SelectListItem> select_list = new List<SelectListItem>();

            var query = from li in course
                        select
                        new SelectListItem
                        {
                            Value = li.ToString(),Text = li.ToString(),};
            select_list = query.ToList();
            ViewBag.course = select_list;
            eNtsaCourseList eCourse = new eNtsaCourseList();

            return View(eCourse);

        }

  // eNtsaRegCourses.
    public class RegCoursesviewmodel
    {
        public eNtsaCourses Courses { get; set; }
        public eNtsaCourseList eCourses { get; set; } // I am using this property for my View to access dropdownlist.
        public eNtsaDashboardviewmodel Dashboard { get; set; }
        public List<eNtsaCourseList> lsteNtsaDashboard { get; set; }
        public RegCoursesviewmodel MainModel { get; set; }

    }
 public class eNtsaCourseList
    {
        public string CourseLicence { get; set; }
        public string Text { get; set; }

        public string Value { get; set; }
        public string CourseList { get; set; }
    }
                     @using (Html.BeginForm("CourseList","Home",FormMethod.Post))
                                        {
                                        <div class="form-group row">
                                            <label for="Content-Licence" class="col-sm-3 col-form-label">Content Licence</label>
                                            <div class="col-sm-5">
                                                @Html.DropDownListFor(m => m.eCourses.CourseList,ViewBag.course as List<SelectListItem>)
                                              </div>
                                            </div>

                                        }