如何知道的HtmlHelper数据上ViewBag？ [英] How HtmlHelper know data is on the ViewBag?

问题描述

我在看关于的HtmlHelper 的下拉的 https://www.youtube.com/watch?v=79aYSOcmpV8

大约8分钟，他正在读DB更换一些硬code值。

  

要从控制器通过部门列表，将它们存储在ViewBag

 公众的ActionResult指数（）
{
    //连接到数据库
    SampleDBContext DB =新SampleDBContext（）;
    //检索部门和建设的SelectList
    ViewBag.Departments =新的SelectList（db.Departments，ID，姓名）;    返回查看（）;
}
 

最后一步。

  

现在，在指数来看，访问部门从ViewBag

列出

  @ Html.DropDownList（部门，选择部门）
 

我不喜欢看到的视图强类型的模型东西。

结果

那么，如何在助手知道部门是指在ViewBag值？

解决方案

当您添加值 ViewBag ，它也被添加到了 ViewContext 中的ViewData 属性时生成视图。您使用等同于传递一个空的SelectList在

的的DropDownList（）超载

  @ Html.DropDownList（部门，NULL，选择部）
 

在这种情况下，在内部，助手搜索的ViewData 属性查找匹配的关键这是一个的IEnumerable＆LT; SelectListItem＆GT; （其中部门是）。你可以看到的<一个在私有静态MvcHtmlString SelectInternal（）法的相关code href=\"https://aspnetwebstack.$c$cplex.com/SourceControl/latest#src/System.Web.Mvc/Html/SelectExtensions.cs\"相对=nofollow>来源$ C ​​$ C

  //如果我们得到了一个空的选择列表，尝试使用的ViewData来获得的项目清单。
如果（选择列表== NULL）
{
    选择列表= htmlHelper.GetSelectData（名）;
    ....
}
 

请注意，在教程中的例子是一个贫穷的方式，用神奇的字符串，并要求您在使用POST方法访问值的Request.Form [部门] 。一个更好的方法是使用一个视图模型，并强烈绑定到你的浏览模式，例如：

 公共类MyViewModel
{
    公众诠释SelectedDepartment {搞定;组; }
    公共IEnumerable的＆LT; SelectListItem＆GT; DepartmentList {搞定;组; }
    ...
}
 

和GET方法将是

 公众的ActionResult的Create（）
{
    MyViewModel模式=新MyViewModel
    {
        DepartmentList =新的SelectList（db.Departments，ID，姓名）;
    };
    返回查看（模型）;
}
 

和视图

  @model MyViewModel
....
@ Html.DropDownListFor（M = GT; m.SelectedDepartment，Model.DepartmentList，选择部）
 

和发布形式回

  [HttpPost]
公众的ActionResult创建（MyViewModel模型）
 

I was watching a tutorial about HtmlHelper for DropDown https://www.youtube.com/watch?v=79aYSOcmpV8

Around min 8, he is reading the db to replace some hardcode values.

To pass list of Departments from the controller, store them in "ViewBag"

public ActionResult Index()
{
    // Connect to the database
    SampleDBContext db = new SampleDBContext();
    // Retrieve departments, and build SelectList
    ViewBag.Departments = new SelectList(db.Departments, "Id", "Name");

    return View();
}

Last step.

Now in the "Index" view, access Departments list from "ViewBag"

@Html.DropDownList("Departments", "Select Department") 

I dont see anything like strong type model on the view.

So how the Helper know Departments refers to a value in the ViewBag?

解决方案

When you add a value to ViewBag, it is also added to the ViewData property of ViewContext when the view is generated. The DropDownList() overload that your using in equivalent to passing a null SelectList in

@Html.DropDownList("Departments", null, "Select Department")

in which case, internally, the helper searches the ViewData property to find a matching key which is an IEnumerable<SelectListItem> (which "Departments" is). You can see the relevant code in the private static MvcHtmlString SelectInternal() method of the source code

// If we got a null selectList, try to use ViewData to get the list of items.
if (selectList == null)
{
    selectList = htmlHelper.GetSelectData(name);
    ....
}

Note that the example in the tutorial is a poor approach, using 'magic' strings and requiring you to access the value in the POST method using Request.Form["Departments"]. A far better approach is to use a view model and strongly bind to your view model, for example

public class MyViewModel
{
    public int SelectedDepartment { get; set; }
    public IEnumerable<SelectListItem> DepartmentList { get; set; }
    ...
}

and the GET method would be

public ActionResult Create()
{
    MyViewModel model = new MyViewModel
    {
        DepartmentList = new SelectList(db.Departments, "Id", "Name");
    };
    return View(model);
}

and in the view

@model MyViewModel
....
@Html.DropDownListFor(m => m.SelectedDepartment, Model.DepartmentList, "Select Department")

and post the form back to

[HttpPost]
public ActionResult Create(MyViewModel model)

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