Question

I wish to show a DropDownList in a view and therfore include in my model (ExampleAddSetupDto) sent to a view a list of entries to populate the dropdownlist. That works fine, but if I have a validation error and redisplay the view with in incoming model my list is now null.

My Action is given below (note: the problem occurs if ModelState.IsValid fails). Also the Action method second parameter may look odd as I am using Autofac to inject the right service into the method).

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Add(ExampleAddSetupDto add, IServiceAddCommit<IExampleAddSetupDto, IExampleAddCommitDto> service)
    {

        if (ModelState.IsValid)
        {
            var response = service.Create(add);
            if (response.IsValid)
            {
                TempData["message"] = "You successfully added a new Example Entry";
                return View("AddSuccess", response);
            }

            //else errors, so copy the errors over to the ModelState
            response.CopyErrorsToModelState(ModelState, add);
        }

        // Some validation error, so redisplay same view
        return View(add);

    }

My model looks like this:

public class ExampleAddSetupDto : IExampleAddSetupDto
{

    [StringLength(50, MinimumLength = 2)]
    public string Name { get; set; }

    public int Option1Id { get; set; }

    public int Option2Id { get; set; }

    //-----------------------------
    //now the properties for the drop down lists 

    public IList<Option1> PosibleEntriesForOption1 { get; set; }
    public IList<Option2> PosibleEntriesForOption2 { get; set; }

}

My View is:

@model ServiceLayer.Example.DTOs.ExampleAddSetupDto

@{
    ViewBag.Title = "Add";
}

<h2>Add</h2>

<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>

@using (Html.BeginForm())
{  
@Html.AntiForgeryToken()
@Html.ValidationSummary(true)
<fieldset>
    <legend>Add an Example item</legend>

    <div class="editor-label">
        @Html.LabelFor(model => model.Name)
    </div>
    <div class="editor-field">
        @Html.EditorFor(model => model.Name)
        @Html.ValidationMessageFor(model => model.Name)
    </div>
    <div class="editor-field">
        @Html.Label("Option1")
        @Html.DropDownListFor(model => model.Option1Id, new SelectList(Model.PosibleEntriesForOption1, "Option1Id", "OptionText"))
        @Html.ValidationMessageFor(model => model.Option1Id)
    </div>
    <div class="editor-field">
        @Html.Label("Option2")
        @Html.DropDownListFor(model => model.Option2Id, new SelectList(Model.PosibleEntriesForOption2, "Option2Id", "OptionText"))
        @Html.ValidationMessageFor(model => model.Option2Id)
    </div>
    <p>
        <input type="submit" value="Create" />
    </p>
</fieldset>

}

I understand that I need to return the Model.PosibleEntriesForOption in with the form. I tried using the Html.HiddenFor helper in the view to return the list, i.e.

@Html.HiddenFor(model => model.PosibleEntriesForOption1)

but this throws the error
‘The value ‘System.Collections.Generic.List`1[DataClasses.ExampleClasses.Option1]’ is invalid.’.

Clearly I am missing something here and I would appreciate some advice on how to return the lists so that redisplaying the model won’t cause an error.

Answer 1

If you are forced to persist the entire list between the two requests, for whatever reason, the best way I see to do this would be to use:

TempData["EnterUniqueKeyHere"] = PossibleEntriesForOption1;

to store it, and then:

PossibleEntriesForOption1 = TempData["EnterUniqueKeyHere"] as IList<Option1>;

to retrieve it.

Note that anything stored in TempData will be removed automatically after a single request.