blocks|key|967985|text|您正在将集合设置为ViewData字典中的项，并尝试将其作为模型上的属性进行检索。一个简单的解决方法是以与设置相同的方式引用它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|967986|++++<%25var+basetype+=+ViewData["basetype"]+as+IEnumerable<SelectListItem>;%25>
++++<div+class="editor-label">
++++++++<%25:+Html.Label("basetype")+%25>
++++</div>
++++<div+class="editor-field">
++++++++<%25:+Html.DropDownList("basetype",+basetype)+%25>
++++++++<%25:+Html.ValidationMessage("basetype")+%25>
++++</div>|code-block|syntax|javascript|967987|或者，下面的代码使用强类型视图：|967988|public+class+ViewModel+{
+++//Model+properties
+++public+IEnumerable<SelectListItem>+basetype+{get;set;}
}

public+ActionResult+Create()
++++{
++++++++var+db+=+new+DB();
++++++++IEnumerable<SelectListItem>+basetypes+=+db.Basetypes.Select(b+=>+new+SelectListItem+{+Value+=+b.basetype,+Text+=+b.basetype+});
++++++++return+View(new+ViewModel+{+basetype=basetypes+});
++++}|967989|然后，在您的强类型视图中：|967990|++++<div+class="editor-label">
++++++++<%25:+Html.LabelFor(model+=>+model.basetype)+%25>
++++</div>
++++<div+class="editor-field">
++++++++<%25:+Html.DropDownListFor(model=>model.basetype)+%25>
++++++++<%25:+Html.ValidationMessageFor(model+=>+model.basetype)+%25>
++++</div>|967991|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

You are setting the collection as an item in ViewData dictionary and trying to retreive it as property on the model. A simple fix would be to reference it the same way as you set it:

<pre><code> &lt;%var basetype = ViewData["basetype"] as IEnumerable&lt;SelectListItem&gt;;%&gt;
 &lt;div class="editor-label"&gt;
 &lt;%: Html.Label("basetype") %&gt;
 &lt;/div&gt;
 &lt;div class="editor-field"&gt;
 &lt;%: Html.DropDownList("basetype", basetype) %&gt;
 &lt;%: Html.ValidationMessage("basetype") %&gt;
 &lt;/div&gt;
</code></pre>

Alternatively, the below code uses a strongly typed view:

<pre><code>public class ViewModel {
 //Model properties
 public IEnumerable&lt;SelectListItem&gt; basetype {get;set;}
}

public ActionResult Create()
 {
 var db = new DB();
 IEnumerable&lt;SelectListItem&gt; basetypes = db.Basetypes.Select(b =&gt; new SelectListItem { Value = b.basetype, Text = b.basetype });
 return View(new ViewModel { basetype=basetypes });
 }
</code></pre>

Then, in your strongly typed view:

<pre><code> &lt;div class="editor-label"&gt;
 &lt;%: Html.LabelFor(model =&gt; model.basetype) %&gt;
 &lt;/div&gt;
 &lt;div class="editor-field"&gt;
 &lt;%: Html.DropDownListFor(model=&gt;model.basetype) %&gt;
 &lt;%: Html.ValidationMessageFor(model =&gt; model.basetype) %&gt;
 &lt;/div&gt;
</code></pre>

blocks|key|1182241|text|尝试添加一个字符串作为您的下拉列表的名称作为第一个参数，并从您的viewdata中获取该项：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1182242|+<%25=+Html.DropDownList("SomeDropdownName",+(IEnumerable<SelectListItem>)ViewData["basetype"])+%25>|code-block|syntax|javascript|1182243|这也是一个你可以使用的扩展方法，这样下拉列表就会以类似于你做其他控件的方式来设置：|1182244|++++++++public+static+string+DropDownList<TModel,+TProperty>(this+HtmlHelper<TModel>+htmlHelper,+Expression<Func<TModel,+TProperty>>+expression,+IEnumerable<SelectListItem>+selectList,+string+optionLabel)
++++++++where+TModel+:+class
++++{
++++++++string+inputName+=+ExpressionHelper.GetInputName(expression);
++++++++return+htmlHelper.DropDownList(inputName,+selectList,+optionLabel);
++++}|1182245|例如|1182246|<%25=+Html.DropDownList(x+=>+x.BaseType,+(IEnumerable<SelectListItem>)ViewData["basetype"],+"")%25>|1182247|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Try adding a string for the name of your dropdown list as the first parameter, and get the item out of your viewdata:

<pre><code> &lt;%= Html.DropDownList("SomeDropdownName", (IEnumerable&lt;SelectListItem&gt;)ViewData["basetype"]) %&gt;
</code></pre>

Here is also an extension method you can use so the dropdown list is set up in a similar style to how you have done your other controls:

<pre><code> public static string DropDownList&lt;TModel, TProperty&gt;(this HtmlHelper&lt;TModel&gt; htmlHelper, Expression&lt;Func&lt;TModel, TProperty&gt;&gt; expression, IEnumerable&lt;SelectListItem&gt; selectList, string optionLabel)
 where TModel : class
 {
 string inputName = ExpressionHelper.GetInputName(expression);
 return htmlHelper.DropDownList(inputName, selectList, optionLabel);
 }
</code></pre>

For example

<pre><code>&lt;%= Html.DropDownList(x =&gt; x.BaseType, (IEnumerable&lt;SelectListItem&gt;)ViewData["basetype"], "")%&gt;
</code></pre>

blocks|key|968044|text|我刚刚遇到这个问题，这篇文章帮助我解决了这个问题--+http://odetocode.com/Blogs/scott/archive/2010/01/18/drop-down-lists-and-asp-net-mvc.aspx|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|968045|最可能的原因是您的集合在po之后被重新填充|968046|entityMap|0|LINK|mutability|MUTABLE|url|http://odetocode.com/Blogs/scott/archive/2010/01/18/drop-down-lists-and-asp-net-mvc.aspx^0|R|2G|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

I've just come across this issue and this article helped me through it - <a href="http://odetocode.com/Blogs/scott/archive/2010/01/18/drop-down-lists-and-asp-net-mvc.aspx" rel="nofollow">http://odetocode.com/Blogs/scott/archive/2010/01/18/drop-down-lists-and-asp-net-mvc.aspx</a>

The most likely cause it that your collection is repopulated after the po

blocks|key|1182322|text|我也有同样的问题，最后我得到了答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1182323|问题是，在POST操作中，在提交表单后，ModelState无效，或者在try/catch中捕获错误，因此返回视图。但是这次视图没有正确设置ViewData["basetype"]。|offset|length|style|CODE|1182324|您需要再次填充它，可能使用与之前相同的代码，因此请重复此操作：|1182325|var+db+=+new+DB();
IEnumerable<SelectListItem>+basetypes+=+db.Basetypes.Select(
++++b+=>+new+SelectListItem+{+Value+=+b.basetype,+Text+=+b.basetype+});
ViewData["basetype"]+=+basetypes;|code-block|syntax|javascript|1182326|在[HttpPost]方法中的return+View(meal)之前。|1182327|这就解决了你的问题：|1182328|[HttpPost]
public+ActionResult+Create(Meal+meal)
{
++++if+(ModelState.IsValid)
++++{
++++++++try
++++++++{
++++++++++++//+TODO:+Add+insert+logic+here
++++++++++++var+db+=+new+DB();
++++++++++++db.Meals.InsertOnSubmit(meal);
++++++++++++db.SubmitChanges();
++++++++++++return+RedirectToAction("Index");
++++++++}
++++++++catch
++++++++{
++++++++++++var+db+=+new+DB();
++++++++++++IEnumerable<SelectListItem>+basetypes+=+db.Basetypes.Select(
+++++++++++++++b+=>+new+SelectListItem+{+Value+=+b.basetype,+Text+=+b.basetype+});
++++++++++++ViewData["basetype"]+=+basetypes;
++++++++++++return+View(meal);
++++++++}
++++}
++++else
++++{
++++++++var+db+=+new+DB();
++++++++IEnumerable<SelectListItem>+basetypes+=+db.Basetypes.Select(
++++++++++++b+=>+new+SelectListItem+{+Value+=+b.basetype,+Text+=+b.basetype+});
++++++++ViewData["basetype"]+=+basetypes;
++++++++return+View(meal);
++++}
}|1182329|我知道这个问题很老了，但我今天带着同样的问题来到这里，所以其他人可以稍后来这里……|1182330|entityMap^0|0|1Y|K|0|0|0|1|A|F|H|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@$D|10|E|11|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|12|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|13|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|14|8|@$D|15|E|16|F|G]|$D|17|E|18|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|19|8|@]|9|@]|A|$]]|$1|S|3|T|5|L|7|1A|8|@]|9|@]|A|$M|N]]|$1|U|3|V|5|6|7|1B|8|@]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1C|8|@]|9|@]|A|$]]]|X|$]]

I had same problem, and finally I got the answer...

The problem is that in the POST action, after submitting the form, the ModelState is not valid, or it's catching an error in try/catch, so the View is returned. But this time the View has not the <code>ViewData["basetype"]</code> correctly set.

You need to populate it again, probably with the same code used before, so repeat this:

<pre><code>var db = new DB();
IEnumerable&lt;SelectListItem&gt; basetypes = db.Basetypes.Select(
 b =&gt; new SelectListItem { Value = b.basetype, Text = b.basetype });
ViewData["basetype"] = basetypes;
</code></pre>

before the <code>return View(meal)</code> in the <code>[HttpPost]</code> method.

exactly this will solve your problem:

<pre><code>[HttpPost]
public ActionResult Create(Meal meal)
{
 if (ModelState.IsValid)
 {
 try
 {
 // TODO: Add insert logic here
 var db = new DB();
 db.Meals.InsertOnSubmit(meal);
 db.SubmitChanges();
 return RedirectToAction("Index");
 }
 catch
 {
 var db = new DB();
 IEnumerable&lt;SelectListItem&gt; basetypes = db.Basetypes.Select(
 b =&gt; new SelectListItem { Value = b.basetype, Text = b.basetype });
 ViewData["basetype"] = basetypes;
 return View(meal);
 }
 }
 else
 {
 var db = new DB();
 IEnumerable&lt;SelectListItem&gt; basetypes = db.Basetypes.Select(
 b =&gt; new SelectListItem { Value = b.basetype, Text = b.basetype });
 ViewData["basetype"] = basetypes;
 return View(meal);
 }
}
</code></pre>

I know this question is very old, but I came here today with the same problem, so other could come here later...

blocks|key|1182381|text|如果您使用Html.DropDownList()方法-如果您的ViewData/Viewbag项未设置，则可能会发生相同的错误，如@Peto所回答的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1182382|但在控制器设置项正确的情况下可能不会设置，但在主视图中，您可以使用带有新ViewDataDictionary值的部分viw调用。|1182383|如果您有@Html.Partial("Partianame",+Model,new+ViewDataDictionary()+{+/*+...+*/+})，那么您的分部视图将看不到ViewData和ViewBag数据，请删除new+ViewDataDictionary()参数|1182384|entityMap^0|5|J|0|0|4|21|2I|8|2R|7|34|O|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|K|$]]

If you use <code>Html.DropDownList()</code> method - same error may occur, if your ViewData/Viewbag item not set, as @Peto answered. 

But it may be not set in case of controller set item correctly, but in main view you use partial viw call with new ViewDataDictionary values.

if you have <code>@Html.Partial("Partianame", Model,new ViewDataDictionary() { /* ... */ })</code> then your partial view will not see <code>ViewData</code> and <code>ViewBag</code> data, remove <code>new ViewDataDictionary()</code> parameter

blocks|key|968150|text|如果SelectList为空，您将收到此错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|968151|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You will receive this error if the SelectList is null.

blocks|key|1182439|text|对于未来的读者，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1182440|我今天遇到了这个问题，但没能解决。毕竟，它变得非常简单。我使用的是表%2B视图。当我更新表(添加了一些列)时，我忘记了更新(删除并重新创建)视图，这给我带来了问题。希望对某些人有帮助。|1182441|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

To future readers,

I came across with the problem today and couldn´t fix it. It turned to be really simple after all. I was working with a table + view. When I updated the table (added a few colums) I forgot to update (drop and recreate) the view, which caused the problem for me. 
Hope it helps somebody.

blocks|key|1182471|text|对于将来的读者，如果你使用的是razor，试着把选择列表项的类型从List改为IEnumerable。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1182472|从…|1182473|@Html.DropDownListFor(m+=>+m.id,+ViewBag.SomeList+as+List<SelectListItem>)|code-block|syntax|javascript|1182474|至|1182475|@Html.DropDownListFor(m+=>+m.id,+ViewBag.SomeList+as+IEnumerable<SelectListItem>)|1182476|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

For future readers, if you are using razor, try to change type of selectlist item from List to IEnumerable.

From

<pre><code>@Html.DropDownListFor(m =&gt; m.id, ViewBag.SomeList as List&lt;SelectListItem&gt;)
</code></pre>

To

<pre><code>@Html.DropDownListFor(m =&gt; m.id, ViewBag.SomeList as IEnumerable&lt;SelectListItem&gt;)
</code></pre>

blocks|key|1182530|text|我今天收到了同样的错误，我的解决方案是使模型“有效”。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1182531|在我的例子中，用户点击“保存”提交后，我得到了模型状态:如果用户键入"0"，则模型状态无效，但如果用户键入"0.0“，则模型状态将有效。|1182532|所以我重写了"IsValid“方法以返回真甚至用户键入的"0”。|1182533|希望能有所帮助。|1182534|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

I got same error today and my solution is to make model "valid".

In my case, after user submit by clicking "save", I got model state: invalid if user key-in "0", but model state will be valid if user key-in "0.0".

So I override "IsValid" method to return true even user key-in "0".

Hope it helps.

I am trying to populate a dropdown list from a database mapped with Linq-2-SQL, using ASP.NET MVC 2, and keep getting this error.

I am so confused because I am declaring a variable of type <code>IEnumerable&lt;SelectListItem&gt;</code> on the second line, but the error makes me think this is not the case. I feel like this should be very simple, but I am struggling. Any help is appreciated.

Here are the interesting bits of my controller:

<pre><code>public ActionResult Create()
{
 var db = new DB();
 IEnumerable&lt;SelectListItem&gt; basetypes = db.Basetypes.Select(
 b =&gt; new SelectListItem { Value = b.basetype, Text = b.basetype });
 ViewData["basetype"] = basetypes;
 return View();
}
</code></pre>

And here are the interesting bits of my view:

<pre><code>&lt;div class="editor-label"&gt;
 &lt;%: Html.LabelFor(model =&gt; model.basetype) %&gt;
&lt;/div&gt;
&lt;div class="editor-field"&gt;
 &lt;%: Html.DropDownList("basetype") %&gt;
 &lt;%: Html.ValidationMessageFor(model =&gt; model.basetype) %&gt;
&lt;/div&gt;
</code></pre>

Here is the POST action when submitting the Form 

<pre><code>// POST: /Meals/Create
[HttpPost]
public ActionResult Create(Meal meal)
{
 if (ModelState.IsValid)
 {
 try
 {
 // TODO: Add insert logic here
 var db = new DB();
 db.Meals.InsertOnSubmit(meal);
 db.SubmitChanges();
 return RedirectToAction("Index");
 }
 catch
 {
 return View(meal);
 }
 }
 else
 {
 return View(meal);
 }
}
</code></pre>

Thanks.

The ViewData item that has the key 'MY KEY' is of type 'System.String' but must be of type 'IEnumerable<SelectListItem>'

数据库

我试图使用ASP.NET MVC2从一个用Linq- 2 -SQL映射的数据库中填充一个下拉列表，但一直收到这个错误。我很困惑，因为我在第二行声明了一个IEnumerable<SelectListItem>类型的变量，但这个错误让我认为情况并非如此。我觉得这应该是非常简单的，但我正在努力。任何帮助都是非常感谢的。下面是...

问具有键“”MY ViewData“”的密钥项的类型为“”System.String“”，但必须为“”IEnumerable<SelectListItem>“”类型“”
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问具有键“”MY ViewData“”的密钥项的类型为“”System.String“”，但必须为“”IEnumerable<SelectListItem>“”类型“”
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