级联变动

public JsonResult getDepartmentJson(int CompanyId = 0)
        {
            List<SelectListItem> SelectItems = new List<SelectListItem>();

            IEnumerable<Department> SerieslistsJieGuo = db.department.Where(o => o.CompanyId == CompanyId).ToList();
            foreach (Department br in SerieslistsJieGuo)
            {
                SelectItems.Add(new SelectListItem { Text = br.DepartmentName, Value = br.Id.ToString() });
            }

            return Json(SelectItems, JsonRequestBehavior.AllowGet);
        }

  //查询出所有公司
            ViewBag.Company = new SelectList(db.company.OrderBy(o=>o.Id),"Id","CompanyName");
            //默认显示Id排序第一个公司的部门
            int firstCompanyId = db.company.OrderBy(o => o.Id).FirstOrDefault().Id;
            ViewBag.Department = new SelectList(db.department.Where(o=>o.CompanyId==firstCompanyId), "Id", "DepartmentName");

//前台页面

<script language="javascript" type="text/javascript">

    $(document).ready(function () {
        $("#Company").change(function () { getSeriesSelectList() });
    });

    function getSeriesSelectList() {
        $("#Department").empty();
        if ($("#Company").val() != "") {
            var url = "/Home/getDepartmentJson?CompanyId=" + $("#Company").val();
            $.getJSON(url, function (data) {
                $.each(data, function (i, item) {
                    $("<option></option>").val(item["Value"]).text(item["Text"]).appendTo("#Department");
                });
            });
        }
    }
</script>
@Html.DropDownList("Company")
@Html.DropDownList("Department")