DropDownListFor的选定值可用于ViewModel，但不能用于整数

Question

When I select a template in my dropdownlist and press the OK button to make a form post the DeleteTemplateListViewModel object is created and filled with the current templateId and the ModelState is Valid.

When I select the default value "Select a template" in my dropdownlist and press the OK button to make a form post the DeleteTemplateListViewModel object is created and and the ModelState is InValid.

So far so good.

But when I change the Delete action to accept an integer which is the selectedTemplateId the ModelState is Valid for a selected templateId. When I select the default value "Select a template" defined in the DropDownListFor I get in my output window this exception:

A first chance exception of type 'System.ArgumentException' occurred in System.Web.Mvc.dll

How can I make it work with so the ModelState is false/Invalid for the default value using an integer as action parameter instead of a viewModel ?

@model ITMS.Web.Models.DeleteTemplateListViewModel

@using (Html.BeginForm("Delete", "Template"))
{ 
    @Html.ValidationSummary(false)      
    @Html.DropDownListFor(x => x.SelectedTemplateId, 
                               new SelectList(Model.DisplayList, "TemplateId", "Name"),
                               "Select a template", 
                               new { @class = "listviewmodel" })
}


public class DeleteTemplateListViewModel
{
    [Required(ErrorMessage = "No template selected.")]
    public int SelectedTemplateId { get; set; }
    public string Name { get; set; }
    public IEnumerable<TemplateViewModel> DisplayList { get; set; }
}

WORKS

[HttpPost]
public ActionResult Delete(DeleteTemplateListViewModel viewModel)
{
    if (ModelState.IsValid)
    {
        int templateId = viewModel.SelectedTemplateId;
        _templateDataProvider.DeleteTemplate(templateId);
        return new JsonNetResult(new { success = true, templateId = templateId });
    }
    return LoadDeleteTemplates();
}

DOES NOT WORK

[HttpPost]
public ActionResult Delete(int selectedTemplateId)
{
    if (ModelState.IsValid)
    {
        _templateDataProvider.DeleteTemplate(selectedTemplateId);
        return new JsonNetResult(new { success = true, templateId = selectedTemplateId});
    }
    return LoadDeleteTemplates();
}

Answer 1

Well, an int can't really get a non-value. Change your Delete method to accept and int? .

I don't know what kind of model validation MVC can do an an int? with no additional data, but you should check see if it's null or not.