asp.net-mvc - SelectListItem 中的选定属性永远不起作用 (DropDownListFor)

Question

我在选择 DropDownList 的值时遇到问题。我一直在阅读所有类似的帖子，但找不到解决方案。

实际方法对我来说似乎非常好，因为我可以检查 SelectList 内的字段:

var selectList = new List<SelectListItem>(
    from variable in someKindOfCollection
    select new SelectListItem
        {
            Selected = variable.Property == selection,
            Text = variable.Property,
            Value = variable.Property
        });

据说，这给了我完全的控制权。构建 selectList 后，我​​可以使用调试器检查变量。一切正常，其中之一已标记为“选定”属性。

然后我使用 DropDownListFor 以便在 View 上显示:

@Html.DropDownListFor(
    g => g.SomePropertyInModel , selectList, new { @class = "cssClass" })

但是它不起作用，永远不会...“渲染”下拉列表，但没有选择任何内容。

非常感谢:)

新示例首先我想道歉。我一直在隐藏信息，当然完全是无意的。所有代码都发生在 Razor For 循环内:

@foreach (var loopVariable in Model.Collection)
{
    if (Model.SomeCondition != null)
    {
        selection = someValue;
    }

    var selectList = new List<SelectListItem>(
      from variable in someKindOfCollection
      select new SelectListItem
    {
        Selected = variable.Property == selection,
        Text = variable.Property,
        Value = variable.Property
    });

    @Html.DropDownListFor(
        g => g.SomePropertyInModel , selectList, new { @class = "cssClass" })

}

那么，事实上 selectList 是一个局部变量导致了这种行为？抱歉，我没想到是这样的。

Answer 1

我想你也遇到了和我一样的问题。我查看了源代码来找到我的解决方案，看来这对我来说是一个错误。下面的 DropDownListFor 应该有所帮助，关键是将选定的值传递到 html 帮助器中。希望这会有所帮助。

public static class SelectExtensions {
    public static IHtmlString DropDownListFor<TModel, TProperty>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression, IEnumerable<SelectListItem> selectList, string selectedValue, string optionLabel, object htmlAttributes = null) {
       return DropDownListHelper(helper, ExpressionHelper.GetExpressionText(expression), selectList, selectedValue, optionLabel, htmlAttributes);
    }

    /// <summary>
    /// This is almost identical to the one in ASP.NET MVC 3 however it removes the default values stuff so that the Selected property of the SelectListItem class actually works
    /// </summary>
   private static IHtmlString DropDownListHelper(HtmlHelper helper, string name, IEnumerable<SelectListItem> selectList, string selectedValue, string optionLabel, object htmlAttributes) {
        name = helper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(name);

        // Convert each ListItem to an option tag
        var listItemBuilder = new StringBuilder();

        // Make optionLabel the first item that gets rendered
        if (optionLabel != null)
            listItemBuilder.AppendLine(ListItemToOption(new SelectListItem() { Text = optionLabel, Value = String.Empty, Selected = false }, selectedValue));

        // Add the other options
        foreach (var item in selectList) {
            listItemBuilder.AppendLine(ListItemToOption(item, selectedValue));
        }

        // Now add the select tag
        var tag = new TagBuilder("select") { InnerHtml = listItemBuilder.ToString() };
        tag.MergeAttributes(HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
        tag.MergeAttribute("name", name, true);
        tag.GenerateId(name);

        // If there are any errors for a named field, we add the css attribute
        ModelState modelState;

        if (helper.ViewData.ModelState.TryGetValue(name, out modelState)) {
            if (modelState.Errors.Count > 0)
                tag.AddCssClass(HtmlHelper.ValidationInputCssClassName);
        }

        // Add the unobtrusive validation attributes
        tag.MergeAttributes(helper.GetUnobtrusiveValidationAttributes(name));

        return tag.ToHtmlString(TagRenderMode.Normal);
    }

    private static string ListItemToOption(SelectListItem item, string selectedValue) {
        var tag = new TagBuilder("option") { InnerHtml = HttpUtility.HtmlEncode(item.Text) };

        if (item.Value != null)
            tag.Attributes["value"] = item.Value;

        if ((!string.IsNullOrEmpty(selectedValue) && item.Value == selectedValue) || item.Selected)
            tag.Attributes["selected"] = "selected";

        return tag.ToString(TagRenderMode.Normal);
    }
}