DropDownList不选择SelectList中的选定项

Question

I am working on an ASP.NET MVC3 application and I cannot get a DropDownListFor to work with my property in a particular editor view.

My model is a Person and a person has a property that specifies it's "Person Type". PersonType is a class that contains a name/description and an ID. I can access the available PersonType s within the application through my static/shared class called ApplicationSettings .

In the Edit Template view for my Person I have created a SelectList for debugging purposes:

@ModelType MyNamespace.Person
@Code
    Dim pTypesSelectList As New SelectList(MyNamespace.ApplicationSettings.PersonTypes, "ID", "Name", Model.PersonType.ID)
End Code

I am then providing this SelectList as a parameter to the DropDownListFor that is bound to the PersonType property of my Person .

I am also printing the Selected property of each item in the SelectList for debugging purposes:

<div style="text-align: center; margin: 5px 0 0 0;">
    <div>
        @Html.LabelFor(Function(model) model.PersonType)
    </div>
    <div>
        @Html.DropDownListFor(Function(model) model.PersonType, pTypesSelectList)
        @Html.ValidationMessageFor(Function(model) model.PersonType)

        <br />
        <br />
        <!-- The following is debugging code that shows the actual value-->
        @Model.Type.Name
        <br />
        @Model.Type.ID
        <br />
        <br />

        <!--This section is to show that the select list has properly selected the value-->
        @For Each pitem In pTypesSelectList
            @<div>
                @pitem.Text selected: @pitem.Selected
            </div>
        Next
    </div>
</div>

The view is bound to a Person whose PersonType property is "Person Type # 2" and I expect this to be selected; however the HTML output of this code looks like this:

<div style="text-align: center; margin: 5px 0 0 0;">
    <div>
    <label for="PersonType">PersonType</label>
    </div>
    <div>
        <select id="PersonType" name="PersonType">
            <option value="7e750688-7e00-eeee-0000-007e7506887e">Default Person Type</option>
            <option value="87e5f686-990e-5151-0151-65fa7506887e">Person Type # 1</option>
            <option value="a7b91cb6-2048-4b5b-8b60-a1456ba4134a">Person Type # 2</option>
            <option value="8a147405-8725-4b53-b4b8-3541c2391ca9">Person Type # 3</option>
        </select>
        <span class="field-validation-valid" data-valmsg-for="PersonType" data-valmsg-replace="true"></span>
        <br />
        <br />
        <!-- The following is debugging code that shows the actual value-->
        Person Type # 2
        <br />
        a7b91cb6-2048-4b5b-8b60-a1456ba4134a
        <br />
        <br />
        <!--This section is to show that the select list has properly selected the value-->
        <div>
            Default Person Type selected: False
        </div>
        <div>
            Person Type # 1 selected: False
        </div>
        <div>
            Person Type # 2 selected: True
        </div>
        <div>
            Person Type # 3 selected: False
        </div>
    </div>
</div>

As you can see the printed Selected properties for the items in the SelectList shows that the 3rd item is "Selected". But what is driving me crazy is that the option that corresponds with this is Not Selected.

Answer 1

Generally, the Selected property in SelectList will be totally ignored by the HTML helpers unless there's no other option. If DropDownListFor can find the value by other means, it will insist on using that value.

In this case, it will use the value of model.PersonType ( .ToString() ) - but that's not what you want, judging by the model.PersonType.ID you pass to the SelectList .

More info in the answer here .

Workaround

One easy workaround that should work would be to set:

ViewData["PersonType"] = model.PersonType.Id. 

The helper looks in ModelState first if it exists - ie on POST. This should work already, since ModelState["PersonType"] will be populated with the actual selected value that was posted.

After ModelState it will look in ViewData - with ViewData["PersonType"] first, and only then ViewData.Model.PersonType . In other words, you can "override" the value on your model with a value set directly on ViewData .

Better (IMO) solution

The more general, "better practice", way to solve it (which also avoids having a custom model binder in order to translate the POST'ed ID back to PersonType ) is to use a ViewModel instead of working with full models in your view:

• Have a PersonTypeID property - instead of PersonType .
• Populate it with PersonType.ID
• use this in your view
  • VB.NET: Html.DropDownListFor(Function(model) model.PersonTypeID) , or
  • C#: Html.DropDownListFor(model => model.PersonTypeID)
• When form is POST'ed, translate the ViewModel (including PersonTypeID => PersonType ) back into the actual model in your POST Action.

This may seem like more work, but generally there tend to be many occasions in a project where you need more view-specific representations of your data to avoid too much inline Razor code - so translating from business objects to view models, while it may seem redundant and anti-DRY at times, tends to spare you of a lot of headaches.

Answer 2

Are you sure that your ModelState for "PersonType" key before rendering the view is empty? As JimmiTh commented is going to search for the value in the ModelState first. It happened to me too, you can try @Html.DropDownList("PersonTypeFake", Function(model) model.PersonType, pTypesSelectList) and it should select the right option.