我正在使用自定义帮助器来创建可以使用HtmlAttribute的select元素。我在@Alexander Puchkov的答案中使用了与我发现的here相同的代码（我将其发布以供参考）。
它工作正常，除了DDL助手加载了一个选项作为选定项目之外，所选项目的值为空/空。 （即在编辑页面上，DDL加载了创建时设置的选项，而不是＆＃39; - 请选择选项 - ＆＃39;）， The highlighted attribute in the picture shows the problem, the value should not be empty, but should show 'Medium'...
因此文本显示正确但元素没有值。关于这个问题来自哪里的任何想法？
以下是帮助者的完整代码：
/// <summary>
/// A selectListItem with an extra property to hold HtmlAttributes
/// <para>Used in conjunction with the sdDDL Helpers</para>
/// </summary>
public class SdSelectListItem : SelectListItem
{
public object HtmlAttributes { get; set; }
}
/// <summary>
/// Generate DropDownLists with the possibility of styling the 'option' tags of the generated 'select' tag
/// </summary>
public static class SdDdlHelper
{
public static MvcHtmlString sdDDLFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression, IEnumerable<SdSelectListItem> selectList,
string optionLabel, object htmlAttributes)
{
if (expression == null)
throw new ArgumentNullException("expression");
ModelMetadata metadata = ModelMetadata.FromLambdaExpression<TModel, TProperty>(expression, htmlHelper.ViewData);
return SelectInternal(htmlHelper, metadata, optionLabel, ExpressionHelper.GetExpressionText(expression), selectList,
false /* allowMultiple */, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
}
public static MvcHtmlString sdDDLFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression, IEnumerable<SdSelectListItem> selectList,
object htmlAttributes)
{
if (expression == null)
throw new ArgumentNullException("expression");
ModelMetadata metadata = ModelMetadata.FromLambdaExpression<TModel, TProperty>(expression, htmlHelper.ViewData);
//-> The below line is my problem (specifically the 'null' param), it set to null if no option label is passed to the method...So if I use this overload, the DDL will load (or re-load) with the default value selected, not the value binded to the property - And if I use the above overload, and set Model.action_priority as the optionLabel, then I get what is shown in the picture...
return SelectInternal(htmlHelper, metadata, null, ExpressionHelper.GetExpressionText(expression), selectList,
false /* allowMultiple */, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
}
#region internal/private methods
private static MvcHtmlString SelectInternal(this HtmlHelper htmlHelper, ModelMetadata metadata, string optionLabel, string name,
IEnumerable<SdSelectListItem> selectList, bool allowMultiple,
IDictionary<string, object> htmlAttributes)
{
string fullName = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(name);
if (String.IsNullOrEmpty(fullName))
throw new ArgumentException("No name");
if (selectList == null)
throw new ArgumentException("No selectlist");
object defaultValue = (allowMultiple)
? htmlHelper.GetModelStateValue(fullName, typeof(string[]))
: htmlHelper.GetModelStateValue(fullName, typeof(string));
// If we haven't already used ViewData to get the entire list of items then we need to
// use the ViewData-supplied value before using the parameter-supplied value.
if (defaultValue == null)
defaultValue = htmlHelper.ViewData.Eval(fullName);
if (defaultValue != null)
{
IEnumerable defaultValues = (allowMultiple) ? defaultValue as IEnumerable : new[] { defaultValue };
IEnumerable<string> values = from object value in defaultValues
select Convert.ToString(value, CultureInfo.CurrentCulture);
HashSet<string> selectedValues = new HashSet<string>(values, StringComparer.OrdinalIgnoreCase);
List<SdSelectListItem> newSelectList = new List<SdSelectListItem>();
foreach (SdSelectListItem item in selectList)
{
item.Selected = (item.Value != null)
? selectedValues.Contains(item.Value)
: selectedValues.Contains(item.Text);
newSelectList.Add(item);
}
selectList = newSelectList;
}
// Convert each ListItem to an <option> tag
StringBuilder listItemBuilder = new StringBuilder();
// Make optionLabel the first item that gets rendered.
if (optionLabel != null)
listItemBuilder.Append(
ListItemToOption(new SdSelectListItem()
{
Text = optionLabel,
Value = String.Empty,
Selected = false
}));
foreach (SdSelectListItem item in selectList)
{
listItemBuilder.Append(ListItemToOption(item));
}
TagBuilder tagBuilder = new TagBuilder("select")
{
InnerHtml = listItemBuilder.ToString()
};
tagBuilder.MergeAttributes(htmlAttributes);
tagBuilder.MergeAttribute("name", fullName, true /* replaceExisting */);
tagBuilder.GenerateId(fullName);
if (allowMultiple)
tagBuilder.MergeAttribute("multiple", "multiple");
// If there are any errors for a named field, we add the css attribute.
System.Web.Mvc.ModelState modelState;
if (htmlHelper.ViewData.ModelState.TryGetValue(fullName, out modelState))
{
if (modelState.Errors.Count > 0)
{
tagBuilder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
}
}
tagBuilder.MergeAttributes(htmlHelper.GetUnobtrusiveValidationAttributes(fullName, metadata));
return MvcHtmlString.Create(tagBuilder.ToString(TagRenderMode.Normal));
}
internal static string ListItemToOption(SdSelectListItem item)
{
TagBuilder builder = new TagBuilder("option")
{
InnerHtml = HttpUtility.HtmlEncode(item.Text)
};
if (item.Value != null)
{
builder.Attributes["value"] = item.Value;
}
if (item.Selected)
{
builder.Attributes["selected"] = "selected";
}
builder.MergeAttributes(HtmlHelper.AnonymousObjectToHtmlAttributes(item.HtmlAttributes));
return builder.ToString(TagRenderMode.Normal);
}
internal static object GetModelStateValue(this HtmlHelper htmlHelper, string key, Type destinationType)
{
System.Web.Mvc.ModelState modelState;
if (htmlHelper.ViewData.ModelState.TryGetValue(key, out modelState))
{
if (modelState.Value != null)
{
return modelState.Value.ConvertTo(destinationType, null /* culture */);
}
}
return null;
}
#endregion
}
}
以下是我在View中调用它的方法（使用Razor）：
@Html.sdDDLFor(x => Model.action_priority, Model.actionPriorityDDL(), Model.action_priority, new
{
@id = "_Action_Priority_DDL",
@class = "form-control"
})
最后这里是Model.actionPriorityDDL（）方法：
public List<SdSelectListItem> actionPriorityDDL()
{
action_priority_DDL = new List<SdSelectListItem>();
action_priority_DDL.Add(new SdSelectListItem
{
Value = StringRepository.ActionPriority.high,
Text = StringRepository.ActionPriority.high,
HtmlAttributes = new
{
@class = "lbl-action-priority-high"
}
}
);
action_priority_DDL.Add(new SdSelectListItem
{
Value = StringRepository.ActionPriority.medium,
Text = StringRepository.ActionPriority.medium,
HtmlAttributes = new
{
@class = "lbl-action-priority-medium"
}
}
);
action_priority_DDL.Add(new SdSelectListItem
{
Value = StringRepository.ActionPriority.low,
Text = StringRepository.ActionPriority.low,
HtmlAttributes = new
{
@class = "lbl-action-priority-low"
}
}
);
action_priority_DDL.Add(new SdSelectListItem
{
Value = StringRepository.ActionPriority.psar,
Text = StringRepository.ActionPriority.psar,
HtmlAttributes = new
{
@class = "lbl-action-priority-psar"
}
}
);
return action_priority_DDL;
}
答案 0 :(得分：0)
这是由于MVC框架中的前一个错误（http://aspnet.codeplex.com/workitem/8311;可在此处访问：https://web.archive.org/web/20131208041521/http://aspnet.codeplex.com/workitem/8311）在循环中使用DropDownListFor帮助程序时发生的，即模型属性具有索引器（如你的例子，生成的select元素有一个属性name =“actionList [0] .action_priority”）
复制一些代码来解决这个问题具体来说，在这个方法中
private static MvcHtmlString SelectInternal(this HtmlHelper htmlHelper, ModelMetadata metadata, string optionLabel, string name,
IEnumerable<ExtendedSelectListItem> selectList, bool allowMultiple,
IDictionary<string, object> htmlAttributes)
替换此
if (selectList == null)
throw new ArgumentException("No selectlist");
使用：
bool usedViewData = false;
// If we got a null selectList, try to use ViewData to get the list of items.
if (selectList == null)
{
selectList = htmlHelper.GetSelectData(name);
usedViewData = true;
}
现在，替换此
if (defaultValue == null)
defaultValue = htmlHelper.ViewData.Eval(fullName);
使用：
if (defaultValue == null && !String.IsNullOrEmpty(name))
{
if (!usedViewData)
{
defaultValue = htmlHelper.ViewData.Eval(name);
}
else if (metadata != null)
{
defaultValue = metadata.Model;
}
}
最后，您还需要添加此方法：
private static IEnumerable<SelectListItem> GetSelectData(this HtmlHelper htmlHelper, string name)
{
object o = null;
if (htmlHelper.ViewData != null)
{
o = htmlHelper.ViewData.Eval(name);
}
if (o == null)
{
throw new InvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.HtmlHelper_MissingSelectData,
name,
"IEnumerable<SelectListItem>"));
}
IEnumerable<SelectListItem> selectList = o as IEnumerable<SelectListItem>;
if (selectList == null)
{
throw new InvalidOperationException(
String.Format(
CultureInfo.CurrentCulture,
MvcResources.HtmlHelper_WrongSelectDataType,
name,
o.GetType().FullName,
"IEnumerable<SelectListItem>"));
}
return selectList;
}
将SelectListItem替换为您的自定义类（例如SdSelectListItem或ExtendedSelectListItem，或您为其命名的任何内容）