How can I validate the result in an ASP.NET MVC editor template?

Question

I have created an editor template for representing selecting from a dynamic dropdown list and it works as it should except for validation, which I have been unable to figure out. If the model has the [Required] attribute set, I want that to invalidate if the default option is selected.

The view model object that must be represented as the dropdown list is Selector:

public class Selector
{
    public int SelectedId { get; set; }
    public IEnumerable<Pair<int, string>> Choices { get; private set; }
    public string DefaultValue { get; set; }

    public Selector()
    {
        //For binding the object on Post
    }

    public Selector(IEnumerable<Pair<int, string>> choices, string defaultValue)
    {
        DefaultValue = defaultValue;
        Choices = choices;
    }
}

The editor template looks like this:

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %>
<select class="template-selector" id="<%= ViewData.ModelMetadata.PropertyName %>.SelectedId" name="<%= ViewData.ModelMetadata.PropertyName %>.SelectedId">
<%
    var model = ViewData.ModelMetadata.Model as QASW.Web.Mvc.Selector;
    if (model != null)
    {
            %>
    <option><%= model.DefaultValue %></option><%
        foreach (var choice in model.Choices)
        {
            %>
    <option value="<%= choice.Value1 %>"><%= choice.Value2 %></option><%
        }
    }
     %>
</select>

I sort of got it to work by calling it from the view like this (where Category is a Selector):

<%= Html.ValidationMessageFor(n => n.Category.SelectedId)%>

But it shows the validation error for not supplying a proper number and it does not care if I set the Required attribute.

Answer 1

Why is not your editor template strongly typed?

<%@ Control Language="C#" 
    Inherits="System.Web.Mvc.ViewUserControl<QASW.Web.Mvc.Selector>" %>

Why not use the DropDownListFor helper:

<%= Html.DropDownListFor(
    x => x.SelectedId, 
    new SelectList(Model.Choices, "Value1", "Value2")
)%>

To avoid the magic strings you could add a ChoicesList property to your view model:

public IEnumerable<SelectListItem> ChoicesList 
{
    get
    {
        return Choices.Select(x => new SelectListItem
        {
            Value = x.Value1.ToString(),
            Text = x.Value2
        });
    }
}

and bind your helper to it:

<%= Html.DropDownListFor(x => x.SelectedId, Model.ChoicesList) %>

Answer 2

I found a solution where validation is done against hidden fields using custom validation rules, here. Using this approach you can easily add custom validation to arbitrary types.