Custom DropDownListFor for nullable types

CustomEnumDropDownListFor.cs

private static Type GetNonNullableModelType(ModelMetadata modelMetadata)
{
    Type realModelType = modelMetadata.ModelType;

    Type underlyingType = Nullable.GetUnderlyingType(realModelType);
    if (underlyingType != null)
    {
        realModelType = underlyingType;
    }
    return realModelType;
}
        
public static string GetEnumDescription<TEnum>(TEnum value)
{
    var field = value.GetType().GetField(value.ToString());
    var attributes = (DescriptionAttribute[])field.GetCustomAttributes(typeof(DescriptionAttribute), false);
    return attributes.Length > 0 ? attributes[0].Description : value.ToString();
}

public static MvcHtmlString CustomEnumDropDownListFor<TModel, TEnum>(
    this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TEnum>> expression, object htmlAttributes)
{
    var metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
    Type enumType = GetNonNullableModelType(metadata);
    var values = Enum.GetValues(enumType).Cast<TEnum>();

    var items =
        values.Select(
            value =>
                new SelectListItem
                {
                    Text = GetEnumDescription(value),
                    Value = value.ToString(),
                    Selected = false
                });
    var attributes = System.Web.Mvc.HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);
    return htmlHelper.DropDownListFor(expression, items, "Selecione...", attributes);
}