ASP.NET MVC DropDownListFor not selecting value fr

问题:

I'm using ASP.NET MVC 3, and just ran into a 'gotcha' using the DropDownListFor HTML Helper.

I do this in my Controller:

ViewBag.ShippingTypes = this.SelectListDataRepository.GetShippingTypes();

And the GetShippingTypes method:

public SelectList GetShippingTypes()
{
    List<ShippingTypeDto> shippingTypes = this._orderService.GetShippingTypes();

    return new SelectList(shippingTypes, "Id", "Name");
}

The reason I put it in the ViewBag and not in the model (I have strongly typed models for each view), is that I have a collection of items that renders using an EditorTemplate, which also needs to access the ShippingTypes select list.

Otherwise I need to loop through the entire collection, and assign a ShippingTypes property then.

So far so good.

In my view, I do this:

@Html.DropDownListFor(m => m.RequiredShippingTypeId, ViewBag.ShippingTypes as SelectList)

(RequiredShippingTypeId is of type Int32)

What happens is, that the value of RequiredShippingTypeId is not selected in the drop down.

I came across this: http://web.archive.org/web/20090628135923/http://blog.benhartonline.com/post/2008/11/24/ASPNET-MVC-SelectList-selectedValue-Gotcha.aspx

He suggests that MVC will lookup the selected value from ViewData, when the select list is from ViewData. I'm not sure this is the case anymore, since the blog post is old and he's talking about MVC 1 beta.

A workaround that solves this issue is this:

@Html.DropDownListFor(m => m.RequiredShippingTypeId, new SelectList(ViewBag.ShippingTypes as IEnumerable<SelectListItem>, "Value", "Text", Model.RequiredShippingTypeId.ToString()))

I tried not to ToString on RequiredShippingTypeId at the end, which gives me the same behavior as before: No item selected.

I'm thinking this is a datatype issue. Ultimately, the HTML helper is comparing strings (in the Select List) with the Int32 (from the RequiredShippingTypeId).

But why does it not work when putting the SelectList in the ViewBag -- when it works perfectly when adding it to a model, and doing this inside the view:

@Html.DropDownListFor(m => m.Product.RequiredShippingTypeId, Model.ShippingTypes)

回答1:

The reason why this doesn't work is because of a limitation of the DropDownListFor helper: it is able to infer the selected value using the lambda expression passed as first argument only if this lambda expression is a simple property access expression. For example this doesn't work with array indexer access expressions which is your case because of the editor template.

You basically have (excluding the editor template):

@Html.DropDownListFor(
    m => m.ShippingTypes[i].RequiredShippingTypeId, 
    ViewBag.ShippingTypes as IEnumerable<SelectListItem>
)

The following is not supported: m => m.ShippingTypes[i].RequiredShippingTypeId. It works only with simple property access expressions but not with indexed collection access.

The workaround you have found is the correct way to solve this problem, by explicitly passing the selected value when building the SelectList.

回答2:

This might be silly, but does adding it to a variable in your view do anything?

var shippingTypes = ViewBag.ShippingTypes;

@Html.DropDownListFor(m => m.Product.RequiredShippingTypeId, shippingTypes)

回答3:

you can create dynamic viewdata instead of viewbag for each dropdownlist field for complex type. hope this will give you hint how to do that

      @if (Model.Exchange != null)
                                {
                                    for (int i = 0; i < Model.Exchange.Count; i++)
                                    {
                                        <tr>
                                            @Html.HiddenFor(model => model.Exchange[i].companyExchangeDtlsId)
                                            <td>
                                                 
                                                @Html.DropDownListFor(model => model.Exchange[i].categoryDetailsId, ViewData["Exchange" + i] as SelectList, " Select category", new { @id = "ddlexchange", @class = "form-control custom-form-control required" })
                                                @Html.ValidationMessageFor(model => model.Exchange[i].categoryDetailsId, "", new { @class = "text-danger" })
                                            </td>
                                            <td>
                                                @Html.TextAreaFor(model => model.Exchange[i].Address, new { @class = "form-control custom-form-control", @style = "margin:5px;display:inline" })
                                            
                                                @Html.ValidationMessageFor(model => model.Exchange[i].Address, "", new { @class = "text-danger" })
                                            </td>
                                        </tr>
                                    }

                                }

  ViewModel CompanyDetail = companyDetailService.GetCompanyDetails(id);
               if (CompanyDetail.Exchange != null)
                for (int i = 0; i < CompanyDetail.Exchange.Count; i++)
                {
                    ViewData["Exchange" + i]= new SelectList(companyDetailService.GetComapnyExchange(), "categoryDetailsId", "LOV", CompanyDetail.Exchange[i].categoryDetailsId);
                }

回答4:

I was just hit by this limitation and figured out a simple workaround. Just defined extension method that internally generates SelectList with correct selected item.

public static class HtmlHelperExtensions
{
    public static MvcHtmlString DropDownListForEx<TModel, TProperty>(
       this HtmlHelper<TModel> htmlHelper,
       Expression<Func<TModel, TProperty>> expression,
       IEnumerable<SelectListItem> selectList,
       object htmlAttributes = null)
    {
        var selectedValue = expression.Compile().Invoke(htmlHelper.ViewData.Model);
        var selectListCopy = new SelectList(selectList.ToList(), nameof(SelectListItem.Value), nameof(SelectListItem.Text), selectedValue);

        return htmlHelper.DropDownListFor(expression, selectListCopy, htmlAttributes);
    }
}

The best thing is that this extension can be used the same way as original DropDownListFor:

@for(var i = 0; i < Model.Items.Count(); i++)
{
   @Html.DropDownListForEx(x => x.Items[i].CountryId, Model.AllCountries)
}

回答5:

There is an overloaded method for @html.DropdownList for to handle this. There is an alternative to set the selected value on the HTML Dropdown List.

@Html.DropDownListFor(m => m.Section[b].State, 
              new SelectList(Model.StatesDropdown, "value", "text", Model.Section[b].State))

I was able to get the selected value from the model.

"value", "text", Model.Section[b].State this section the above syntax adds the selected attribute to the value loaded from the Controller