Html.DropDownList（）赋默认值：
页面代码如下：
<%
List<SelectListItem> list = new List<SelectListItem> {
new SelectListItem { Text = "启用", Value = "0",Selected = true},
new SelectListItem { Text = "禁用", Value = "1" } };
%>//list储存dropdownlist的默认值
<%=Html.DropDownList("state",list,Model.state) %> //state为实体的属性，默认选中"启用"
Html.DropDownList（）从数据库读取值：
页面代码如下：
<%= Html.DropDownList("Category", ViewData["Categories"] as SelectList,"--请选择--",new { @class = "my-select-css-class" } )%>
Controllers代码：
public ActionResult Create()
{
List<Category> categories = categoryService.GetAll();
ViewData["Categories"] = new SelectList(categories, "Id", "Name");
return View();
}