Validating required selection in DropDownList

Question

My view model defines property which has to be displayed as combo box. Property definition is:

[Required]
public int Processor { get; set; }

I'm using DropDownListFor to render combo box:

<%=Html.DropDownListFor(r => r.Processor, Model.Processors, Model.Processor)%>

Model.Processors contains IEnumerable<SelectListItem> with one special item defined as:

var noSelection = new SelectListItem
  {
    Text = String.Empty,
    Value = "0"
  };

Now I need to add validation to my combo box so that user must select different value then 'noSelection'. I hoped for some configuration of RequiredAttribute but it doesn't have default value setting.

Solution

How about this:

[Required]
public int? Processor { get; set; }

And then:

<%= Html.DropDownListFor(
    x => x.Processor, Model.Processors, "-- select processor --"
) %>

And in your POST action

[HttpPost]
public ActionResult Index(MyViewModel model)
{
    if (ModelState.IsValid)
    {
        // the model is valid => you can safely use model.Processor.Value here:
        int processor = model.Processor.Value;
        // TODO: do something with this value
    }
    ...
}

And now you no longer need to manually add the noSelection item. Just use the proper DropDownListFor overload.

Comments

[Ladislav Mrnka]

Nice, I completely misunderstood that overload. Thanks.

[Akira Yamamoto]

This does not validate on client side :(

[Legends]

This works for me ... perfect. So, instead of adding the default value directly to the model, you add it directly via the overload.

[Ratan]

This worked for me too, but trying to figure out why? Any reason why this works and the other doesn't?