public ActionResult Edit(long id = 0)
        {
            Post post = db.Post.Find(id);
            if (post == null)
            {
                return HttpNotFound();
            }
            ViewBag.content = post.content;
            ViewBag.cates = new SelectList(db.PostCate, "id", "name", post.cate_id);
            ViewBag.updetp = new SelectList(db.Bank, "id", "name", post.updept_id);
            var ds = from d in db.Department
                        where d.updep == post.updept_id
                        select new { d.id, d.name };

            ViewBag.depts = new SelectList(ds, "id", "name", post.dept_id);
            return View("~/Views/Post/Edit.cshtml", post);
        }

<p>

                    <label for="cate_id">类别：</label>

                    @Html.DropDownListFor(model=>model.cate_id, ViewBag.cates as IEnumerable<SelectListItem>)

                    @Html.ValidationMessage("cate_id", "请选择公告类型")

                </p>

                <p>

                    <label for="updetp">分行：</label>

                    @Html.DropDownListFor(model=>model.updept_id, ViewBag.updetp as IEnumerable<SelectListItem>, String.Empty, new { style = "width:180px" })

                    @Html.ValidationMessage("updept_id", "请选择分行")

                </p>

                <p>

                    <label for="dept_id">机构：</label>

                    @Html.DropDownListFor(model => model.dept_id, ViewBag.depts as IEnumerable<SelectListItem>,string.Empty, new { style = "width:180px" })

            

                    @Html.ValidationMessage("dept_id", "请选择发布机构")

                </p>

关键点在于ViewBag.XX不能与字段名同名。否则无法设置默认值。应该是因为冲突。

例如：

@Html.DropDownListFor(model => model.dept_id, ViewBag.depts as IEnumerable<SelectListItem>,string.Empty, new { style = "width:180px" })

不能写成：

@Html.DropDownListFor(model => model.dept_id,ViewBag.dept_id ?as IEnumerable<SelectListItem>,string.Empty, new { style = "width:180px" })

ASP.NET MVC4.0 DropDownListFor 设置默认选项