Html.DropDownList（）赋默认值：页面代码如下：  <%                 List<SelectListItem> list = new List<SelectListItem> {                new SelectListItem { Text = "启用", Value = "http://www.mamicode.com/0",Selected = true},                new SelectListItem { Text = "禁用", Value = "http://www.mamicode.com/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();}  •原型一：public static string DropDownList(this HtmlHelper htmlHelper, string name){     IEnumerable<SelectListItem> selectData = http://www.mamicode.com/htmlHelper.GetSelectData(name);"001", Value = "http://www.mamicode.com/1", Selected = false });items.Add(new SelectListItem() {Text = "002", Value = "http://www.mamicode.com/2", Selected = false });ViewData["items"] = items; 简化后：  var items = new List<SelectListItem>(){    (new SelectListItem() {Text = "001", Value = "http://www.mamicode.com/1", Selected = false}),    (new SelectListItem() {Text = "002", Value = "http://www.mamicode.com/2", Selected = false})};将items值给ViewData：ViewData["items"] = items; 在aspx中这样使用：<%= Html.DropDownList("items") %> 生成的代码中，items将作为<select>标签的name和id值。 •原型二：public static string DropDownList(this HtmlHelper htmlHelper, string name, IEnumerable<SelectListItem> selectList){    return htmlHelper.DropDownList(name, selectList, null);}使用方法：<%= Html.DropDownList("items", new List<SelectListItem>{    (new SelectListItem() {Text = "001", Value = "http://www.mamicode.com/1", Selected = false}),    (new SelectListItem() {Text = "002", Value = "http://www.mamicode.com/2", Selected = false})})%>在这里，不需要ViewData传入值，第一个参数items作为标签的name和id的值。items也可以是任意的字符串。•原型三public static string DropDownList(this HtmlHelper htmlHelper, string name, string optionLabel){    IEnumerable<SelectListItem> selectData = http://www.mamicode.com/htmlHelper.GetSelectData(name);>