MVC架构下，关于如何设置DropDownList的默认选中项以及修改后填充以后的设置选中项

SelectList Constructor (IEnumerable, String, String, Object)

.NET Framework 4

Initializes a new instance of the SelectList class by using the specified items for the list, the data value field, the data text field, and a selected value.

Namespace:   System.Web.Mvc
Assembly:   System.Web.Mvc (in System.Web.Mvc.dll)

 

---

C#

public SelectList(

	IEnumerable items,

	string dataValueField,

	string dataTextField,

	Object selectedValue

)

Parameters

items

Type: System.Collections.IEnumerable
The items.

dataValueField

Type: System.String
The data value field.

dataTextField

Type: System.String
The data text field.

selectedValue

Type: System.Object
The selected value.

使用这个SelectList构造函数，第四个参数就是选中项的值：
http://msdn.microsoft.com/en-us/library/dd492553.aspx

先定义自定义用户配置类     （情形1：默认选中项参数为string类型）

    [Serializable]
    public class ProfileInformation
    {      

       public string SubscriptionType { get; set; }

       public static SelectList GetSubscriptionList(String SubscriptionType)
        {
            List<SelectListItem> subscriptionList = new List<SelectListItem>()
            {
                new SelectListItem() { Value = "HTML", Text = "Subscribe to HTML Version" },
                new SelectListItem() { Value = "Plain", Text = "Subscribe to Plain Text Version" },
                new SelectListItem() { Value = "None", Text = "No Thanks" }
            };
            return new SelectList(subscriptionList, "Value", "Text", SubscriptionType);
        }

}

Controller 控制器方法

        public ActionResult UserProfile()
        {
            string id = HttpContext.User.Identity.Name.ToString();
            ProfileBase profileBase;
            if (!String.IsNullOrEmpty(id))
            {
                profileBase = ProfileBase.Create(id);
            }
            else
            {
                profileBase = HttpContext.Profile as ProfileBase;
            }
            ProfileInformation profile =( ProfileInformation) profileBase.GetPropertyValue("ProfileInformation") ;
            ViewData["subscriptionType"] = ProfileInformation.GetSubscriptionList(profile.SubscriptionType);
            if (profile == null)
            {
                ProfileInformation profileInformation = new ProfileInformation()
                {

                };
                profileBase.SetPropertyValue("ProfileInformation", profileInformation);
            }
           
            return View(profile);
        }

        [AcceptVerbs("POST")]
        public ActionResult UserProfile(ProfileInformation profileInformation)
        {
            ProfileBase profileBase = HttpContext.Profile as ProfileBase;
          
            profileBase.SetPropertyValue("ProfileInformation", profileInformation);

            //profileInformation.GenderType = Request.Form["GenderType"];
            profileBase.Save();  //通过设置视图页面的强类型为ProfileInformation并从控制器为视图传入一个ProfileInformation类型的值profile，POST操作中可直接保存新的profile值，<%= Html.DropDownList("SubscriptionType")%>中的值将自动匹配ProfileInformation的属性SubscriptionType
           
            ProfileInformation profile =(ProfileInformation) profileBase.GetPropertyValue("ProfileInformation") ;
            ViewData["subscriptionType"] = ProfileInformation.GetSubscriptionList(profileInformation.SubscriptionType.ToString());
            TempData["SuccessMessage"] = "Your profile information has been saved";
            ViewData["PageTitle"] = "My Profile";
            return View(profileInformation);
        }

标注为红色的为SelectList所设置的选中项所传人的值，若传入的值SubscriptionType为"" ,则SelectList采用默认选择项即第一项，否则采用传入的值SubscriptionType所对应的项为选中项     (注意：string类型不具有可空类型，默认为"",不为null，而int类型可设为可空类型，int？)

View    视图部分  (视图页面设置强类型为YouXieKu.Models.ProfileInformation)

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/UserCenter.Master" Inherits="System.Web.Mvc.ViewPage<YouXieKu.Models.ProfileInformation>" %>

                <span class="label UserEditlabel">订阅类型：</span>
                <div class="fl">
                <%= Html.DropDownList("SubscriptionType")%>    //绑定键为subscriptionType的ViewData值  也可写作<%= Html.DropDownList("subscriptionType")%>  此处大小写无关    注意：若要设置填充修改后的选中项的话，此处不可写作<%= Html.DropDownList("SubscriptionType",ViewData["subscriptionType"] as SelectList)%>，否则将无法填充修改后的选择项而

是默认绑定SelectList的第一项默认选择项
                </div>

静态类静态方法  （情形2： 默认选中项参数为int？ 可空类型）

        public static SelectList GetDepartmentList(int? department)
        {
            TreeHelper departmentTree = new TreeHelper();
            List<SelectListItem> departmentList = new List<SelectListItem>();
            foreach (var item in departmentTree.departments)
            {
                departmentList.Add(new SelectListItem() { Value = item.DepartmentID.ToString(), Text = item.Title });  // (departmentTree.departments);
            }
            return new SelectList(departmentList, "Value", "Text",department);
        }

Controller  控制器方法 

       ViewData["departmentID"] = Product.GetDepartmentList(product.DepartmentID);

更方便的生成ViewData["departmentID"]的方式 如下：

ViewData["Categories"] = new SelectList(CategoriesDal.GetAllCategories(), "Id", "Name", art.CategoryId);

View      视图部分

                <%= Html.DropDownList("DepartmentID")%>    //绑定键为DepartmentID的ViewData值  也可写作<%= Html.DropDownList("departmentID")%>  此处大小写无关注意：若 要设置填充修改后的选中项的话，此处也不可写作<%= Html.DropDownList("DepartmentID",ViewData["departmentID"] as SelectList)%>，否则将无法填充修改后的选择项而是默认绑定SelectList的第一项默认选择项

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