How to use Html.GetUnobtrusiveValidationAttributes

Question

I am trying to work around the fact that when they wrote asp.net MVC 3 they forgot to include code to add the unobtrusive validation attributes to select lists and their "fix" for this is to include it in MVC 4, which is no bloody use to anyone using MVC 3.

My proposed work around is to use Html.GetUnobtrusiveValidationAttributes() to add them myself, just like any other custom attributes, but i can't work out the correct syntax for calling the method. There are 2 overloads, one takes a string and the other takes a string and a ModelMetaData class. I understand the metadata param, I presume I just pass in ViewData.ModelMetadata but what should the string be? The MSDN documentation says it is "the specified HTML name attribute" which makes no sense to me. The HTML name attribute of what? The select list? Why would it need that and how does that help it know what property on my model i want the validation for? Looking at examples of usage they all seem to pass in the name of the property on my model that i want the validation attributes for, which makes sense. Unfortunately I can't get the method to return anything but an empty collection no matter what i pass in.

My model class is called Event and my property is called EventTypeID. I am using a slightly different viewmodel class as the basis for the view because i need to display a list of Events and also also allow a new event to be entered on the same view so i have a simple viewmodel class as below:

public class EventViewModel
{
    public Model.Event NewEvent { get; set; }
    public IEnumerable<Model.Event> Events { get; set; }
}

The dropdown list is mapped to the property like: @Html.DropDownListFor(model => model.NewEvent.EventTypeID what do I pass as the string to Html.GetUnobtrusiveValidationAttributes(string) or Html.GetUnobtrusiveValidationAttributes(string, ModelMetadata) to get the attributes for this property. I have tried:

Html.GetUnobtrusiveValidationAttributes("EventTypeID")
Html.GetUnobtrusiveValidationAttributes("EventTypeID",ViewData.ModelMetadata)
Html.GetUnobtrusiveValidationAttributes("NewEvent.EventTypeID")
Html.GetUnobtrusiveValidationAttributes("NewEvent.EventTypeID",ModelMetadata)

They all return an empty collection.

I know that my model is correct because if i change the call from Html.DropDownListFor to Html.TextBoxFor then the validation "just works" without me having to do anything other than add the validation attributes to my model class.

EDIT:

Just tried turning client side validation off, the validation works fine server side for all select lists.

Answer 1

For those still looking for an answer, this works for me:

public static IDictionary<string, object> UnobtrusiveValidationAttributesFor<TModel, TProperty>(this HtmlHelper<TModel> html, Expression<Func<TModel, TProperty>> propertyExpression)
{
    var propertyName = html.NameFor(propertyExpression).ToString();
    var metadata = ModelMetadata.FromLambdaExpression(propertyExpression, html.ViewData);
    var attributes = html.GetUnobtrusiveValidationAttributes(propertyName, metadata);
    return attributes;
}

Note that I'm using .Net MVC 4, you don't have the html.NameFor method in MVC 3. However, I believe this can be done in MVC 3 with the following method:

var propertyName = ExpressionHelper.GetExpressionText(propertyExpression);

Answer 2

Here is a link to an answer I posted, showing an HtmlHelper I wrote to provide unobtrusive validation for dropdownlists: MVC 3 dropdownlist validation not working for complex view model

UPDATE

Are you trying to get the attributes in an HtmlHelper, or in-line in your view?

Assuming you are trying to get the attributes in your view, that is the problem.

First, you need to understand that ModelMetadata does not represent a single object available across your entire model. Rather, it represents the metadata for a particular element, be it your model, or any property within the model. A better descriptive name would be ObjectMetadata, since ModelMetadata is the metadata for a specified object, be it a model, a nested model, or a specific property.

ModelMetadata in the view is only the metadata for the top-level model. You must get the ModelMetadata for the property to which the dropdownlist is bound. If you use a helper, then the helper is passed the correct ModelMetadata as a matter of course. If you use your view, you need to engage in some gymnastics to get the correct ModelMetadata, see for example my answer here: Validating and editing a “Changeable”/optional type in Asp.net MVC 3

Answer 3

You can use it inline

Example for select element

<select name="@Html.NameFor(m=> m.MyProperty)"
        id="@Html.IdFor(m=> m.MyProperty)"
        @Html.Raw(string.Join(" ", Html.GetUnobtrusiveValidationAttributes(Html.NameFor(m => m.MyProperty).ToString()).Select(x => x.Key.ToString() + "=\"" + x.Value + "\"")))
>