将枚举值传递到ASP.NET MVC3中的ListBoxFor

Question

I've read up a good bit on creating a SelectList from an Enum to populate a drop down list, and found many solutions. Here are a few examples

public static IEnumerable<SelectListItem> GetEnumSelectList<T>()
{
    return (Enum.GetValues(typeof(T)).Cast<T>().Select(
        enu => new SelectListItem() { Text = enu.ToString(), Value = enu.ToString() })).ToList();
}

public static SelectList ToSelectList<TEnum>(this TEnum enumObj) 
    where TEnum : struct, IComparable, IFormattable, IConvertible
{
    var values = from TEnum e in Enum.GetValues(typeof(TEnum))
                 select new { Id = e, Name = e.ToString() };
    return new SelectList(values, "Id", "Name", enumObj);
}

Both of these (and most others) return the names of the enum values for both Text and Value .

Example

public enum WeatherInIreland
{
     NotGreat = 0,
     Bad = 1,
     Awful = 2
} 

Results from the first two methods

<select id="Weather" name="Weather">
    <option value="NotGreat">NotGreat</option>
    <option value="Bad">Bad</option>
    <option value="Awful">Awful</option>
</select>

However, I wanted to return the name for Text and the int value for the Value .

<select id="Weather" name="Weather">
    <option value="0">NotGreat</option>
    <option value="1">Bad</option>
    <option value="2">Awful</option>
</select>

This is the code I came up with.

public static System.Web.Mvc.SelectList ToSelectList<TEnum>(this TEnum enumObj) where TEnum : struct, IComparable, IFormattable, IConvertible
{
    List<SelectListItem> selist = new List<SelectListItem>();
    foreach (int value in Enum.GetValues(typeof(TEnum)))
    {
        TEnum type = (TEnum)Enum.Parse(typeof(TEnum), value.ToString());
        selist.Add(new SelectListItem { Value = value.ToString(), Text = type.ToString() });
    }
    return new System.Web.Mvc.SelectList(selist, "Value", "Text");
}

As you can see it's a modified version of the last method above. It works, but it is ugly. I have to iterate over the Enum values as ints and then separately parse each one to return the name. Surely there's a better way of doing this?

Answer 1

You have to write it this way:

public static System.Web.Mvc.SelectList ToSelectList<T>(this Enum TEnum) where T : struct, IComparable, IFormattable, IConvertible
{

    return new SelectList( Enum.GetValues(typeof(T)).OfType<T>()
                                                .Select(x =>
                                                   new SelectListItem
                                                   {
                                                       Text = x.ToString(),
                                                       Value = ((T)x).ToString()
                                                   }));
}

REVISED ONE:

public static System.Web.Mvc.SelectList ToSelectList<TEnum>(this TEnum obj) where TEnum : struct, IComparable, IFormattable, IConvertible
{

  return new SelectList(Enum.GetValues(typeof(TEnum))
                                      .OfType<Enum>()
                                      .Select(x =>
                                      new SelectListItem
                                         {
                                          Value = (Convert.ToInt32(x)).ToString(),
                                          Text = Enum.GetName(typeof(TEnum),x)
                                         }
                                         ),"Value","Text");

}

I tested it on my machine and it works fine.

Answer 2

Unfortunately there's not really a better way to do this. In a perfect world, Enum would implement IEnumerable and you could iterate through the collection. Instead you can only either iterate through Enum.GetValues or Enum.GetNames . Whichever you choose, you'll still need to call Enum.GetName or Enum.GetValue to get the other piece of data.

Similiar StackOverflow question: C# Iterating through an enum? (Indexing a System.Array)

You can however make the code a bit more readable and linq-y :

 return
      new SelectList(
          Enum.GetValues(typeof (TEnum))
              .Select(e => new SelectListItem
              {
                 Text = Enum.GetName(typeof (TEnum), e),
                 Value = e.ToString()
              })
              .ToList(),
          "Value",
          "Text");

Answer 3

We made a helper at work which does this really well

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

        Type underlyingType = Nullable.GetUnderlyingType(realModelType);
        if (underlyingType != null)
        {
            realModelType = underlyingType;
        }
        return realModelType;
    }

    private static readonly SelectListItem[] SingleEmptyItem = new[] { new SelectListItem { Text = "", Value = "" } };

    public static string GetEnumDescription<TEnum>(TEnum value)
    {
        FieldInfo fi = value.GetType().GetField(value.ToString());

        DescriptionAttribute[] attributes = (DescriptionAttribute[])fi.GetCustomAttributes(typeof(DescriptionAttribute), false);

        if ((attributes != null) && (attributes.Length > 0))
            return attributes[0].Description;
        else
            return value.ToString();
    }

    public static MvcHtmlString EnumDescriptionDropDownListFor<TModel, TEnum>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TEnum>> expression)
    {
        return EnumDescriptionDropDownListFor(htmlHelper, expression, null);
    }

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

        IEnumerable<SelectListItem> items = from value in values
                                            select new SelectListItem
                                            {
                                                Text = GetEnumDescription(value),
                                                Value = value.ToString(),
                                                Selected = value.Equals(metadata.Model)
                                            };

        // If the enum is nullable, add an 'empty' item to the collection
        if (metadata.IsNullableValueType)
            items = SingleEmptyItem.Concat(items);

        return htmlHelper.DropDownListFor(expression, items, htmlAttributes);
    }
}

Then you simply call it on your view for the enum in the Model or ViewModel (whichever your using)

@Html.EnumDescriptionDropDownListFor(model => model.Type)    

Also if you put a Description attribute on each enum option you also get a nice user friendly string, if not then it just uses the regular string value of the enum.

[Description("MOT Checker")]    

Finally if you change the value in the select list to the following it will output the int value instead of the string also (which is what what you want i believe?);

Value = Convert.ToInt32(value).ToString(),

<option value="1">e1</option>
<option value="2">e2</option>
<option value="3">e3</option>
<option value="4">e4</option>
<option value="5">e5</option>
<option value="6">e6</option>

Hope this helps.