为什么 Html.DropDownListFor 需要额外的转换？
在我的控制器中，我创建了一个 SelectListItems 列表，并将其存储在 ViewData 中。当我在视图中读取 ViewData 时，它会给出有关类型不正确的错误。如果我手动转换类型，它可以工作，但似乎这应该自动发生。有人可以解释一下吗？
控制器：
enum TitleEnum { Mr, Ms, Mrs, Dr };
var titles = new List<SelectListItem>();
foreach(var t in Enum.GetValues(typeof(TitleEnum)))
titles.Add(new SelectListItem()
{ Value = t.ToString(), Text = t.ToString() });
ViewData["TitleList"] = titles;
视图：
// Doesn't work
Html.DropDownListFor(x => x.Title, ViewData["TitleList"])
// This Works
Html.DropDownListFor(x => x.Title, (List<SelectListItem>) ViewData["TitleList"])
In my controller I create a list of SelectListItems and store this in the ViewData. When I read the ViewData in my View it gives me an error about incorrect types. If I manually cast the types it works but seems like this should happen automatically. Can someone explain?
Controller:
enum TitleEnum { Mr, Ms, Mrs, Dr };
var titles = new List<SelectListItem>();
foreach(var t in Enum.GetValues(typeof(TitleEnum)))
titles.Add(new SelectListItem()
{ Value = t.ToString(), Text = t.ToString() });
ViewData["TitleList"] = titles;
View:
// Doesn't work
Html.DropDownListFor(x => x.Title, ViewData["TitleList"])
// This Works
Html.DropDownListFor(x => x.Title, (List<SelectListItem>) ViewData["TitleList"])
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评论（5）
因为
ViewData是一个Dictionary。还有什么方法可以在带键的集合中存储多种类型的对象呢？任何从ViewData检索而无需转换的内容都将被编译器视为基本Object。Because
ViewDatais aDictionary<string, Object>. How else could you store objects of multiple types in a keyed collection? Anything retrieved fromViewDatawithout casting will be treated by the compiler as a baseObject.如果我没记错的话，ViewData 是一个对象数组/集合。这就是为什么需要额外的演员阵容。
If I remember correctly, ViewData is an array/collection of objects. This is why the extra cast is needed.
显然，编译器不会自动执行从 object 类型的对象到另一种类型的转换。我希望 ViewData 的结果在编译时是对象类型。示例如下：
Apparently the compiler will not perform a cast from an object of type object to another type automatically. I expect that results from ViewData are of type object at compile time. Example follows:
这是因为一种称为“静态类型”的功能。有些人喜欢它，有些人讨厌它。
It's because of a feature called "static typing". Some love it, others hate it.
如果您更改
为
并再次尝试：
如果这有效，我会同意 Femaref....不过是个好问题。
what if you change
to
and again try it with:
if this works, the I would agree with Femaref....good question though.