Question

Scenario:

The Create action method will pass an uninitialized instance of Person to the View.
The View will show a dropdown control with “–Select–” highlighted.
Since the property Sex is nullable, I cannot invoke the extension method.

How to fix this problem?

Model:

namespace MvcApplication1.Models
{
    public enum Sex { Male, Female };
    public class Person
    {
        public int Id { get; set; }
        public string Name { get; set; }

        [Required(ErrorMessage="Please select either Female or Male.")]
        public Sex? Sex { get; set; }
    }
}

Controller:

    public ActionResult Create()
    {
        var p = new Person();
        ViewBag.SelectList = p.Sex.Value.GetSelectList();//Source of error!
        return View(p);
    }

Partial View:

@model MvcApplication1.Models.Person
@Html.HiddenFor(model => model.Id)
<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>
    @Html.DropDownListFor(model => model.Sex, ViewBag.SelectList as SelectList,"--Select--")
</div>

Extension:

 public static class Utilities
    {
        public static SelectList GetSelectList<TEnum>(this TEnum obj)
        {
            var values = from TEnum x in Enum.GetValues(typeof(TEnum))
                         select new { Text = x.ToString(), Value = x };
            return new SelectList(values, "Value", "Text", obj);
        }
    }

Answer 1

Why don’t you make your extension method to accept a Nullable as a parameter? (This extension method looks like a code smell to me)

public static class Utilities
{
    public static object GetSelectList<TEnum>(this TEnum? obj)
        where TEnum : struct 
    {
        var values = from TEnum x in Enum.GetValues(typeof(TEnum))
                     select new { Text = x.ToString(), Value = x };
        return new SelectList(values, "Value", "Text", obj);
    }
}

and use without extracting value:

ViewBag.SelectList = p.Sex.GetSelectList();