Using Drop Down Lists with enums in ASP.NET MVC

ASP.NET MVC is a very powerful, yet quite complex (if not complicated) web development framework. There’s a heaps of various namespaces, classes and functions. It’s bloody hard to figure out which particular class or function overload you need to use for your specific task.

This article is one of the ‘DropDownList series’ articles that should help you in dealing with DropDownList / SelectList / SelectListItem related problems. Check out other DropDownList articles here:

• Using simple DropDownLists in ASP.NET MVC
• DropDownListFor with Dictionaries in ASP.NET MVC and why SelectList wants to kill you
• How to select a default value in DropDownList from a database

Seemingly simple things, such as creating a humble drop down list with elements from a given enum can be quite perplexing, and lots of people get stuck there, not even knowing how to start. In this article I will show in just a few simple steps how to do the following:

• put a drop down list on a form
• populate the drop down list with values from an enum, containing a list of industries
• make sure text in the drop down list is easy to read, such as Construction, Architecture & Interior Design, not ConstructionArchitectureAndInteriorDesign
• have a default value in the drop down list that prompts user to select something, “Please select your industry”

Check out my previous article Using simple Drop Down Lists in ASP.NET MVC for the detailed breakdown on how to create a simple drop down list on a form, populate it with values from a controller, send selected value back and render the drop down with the value selected.

Solution walkthrough

Let’s put together a fictitious user profile page which has text boxes for First and Last names and Industry selection drop down list. It’s going to look like this:

Contents of the drop down list will be based on the following enum:

public enum Industry
{
    [Description("Accounting")]
    Accounting,
    [Description("Administration & Secretarial")]
    AdministrationAndSecretarial,
    [Description("Advertising, Media, Arts & Entertainment")]
    AdvertisingMediaArtsAndEntertainment,
    //
    // ...more like that...
    //
    [Description("Trades & Services")]
    TradesAndServices,
    [Description("Voluntary, Charity & Social Work")]
    VoluntaryCharityAndSocialWork
}

ASP.NET MVC 5.1 – a shortcut

If you are amongst the more lucky ones and happen to use 5.1 or a later version of the ASP.NET MVC, there’s a really neat shortcut – check out this EnumDropDownListFor helper function. (By the way, isn’t that wonderful how cryptic Microsoft’s doco is? Fear not though.)

Here’s all you really need to do to put a drop down list on a form:

<div class="form-group">
    @Html.LabelFor(m => m.Industry)
    @Html.EnumDropDownListFor(model => model.Industry, // Model field that will be used to store user selection
                              "- Please select your industry -", // Text for the first 'default' option
                              new { @class = "form-control" })  @* A class name to assign to "select" tag *@
</div>

Similarly to the simple drop down list, you get user’s selection in the model.Industry field on POST. The helper function will also do all the heavy lifting to create proper <option> elements under the <select> tag, which will have their text set to the corresponding value from [Display(Name="...")] attribute.

ASP.NET MVC 5 & 4 – the hard way

In case of an earlier version of the MVC you need to do some work to get those pesky enum values and their descriptions out. You can use @Html.DropDownListFor helper function, although it needs a list of SelectListItem objects, so we need to convert the members of the enum into instances of the that class. Here’s a way of doing this:

private IEnumerable<SelectListItem> GetSelectListItems()
{
    var selectList = new List<SelectListItem>();

    // Get all values of the Industry enum
    var enumValues = Enum.GetValues(typeof(Industry)) as Industry[];
    if (enumValues == null)
        return null;

    foreach (var enumValue in enumValues)
    {
        // Create a new SelectListItem element and set its 
        // Value and Text to the enum value and description.
        selectList.Add(new SelectListItem
        {
            Value = enumValue.ToString(),
            // GetIndustryName just returns the Display.Name value
            // of the enum - check out the next chapter for the code of this function.
            Text = GetIndustryName(enumValue)
        });
    }

    return selectList;
}

Show selected enum value on a page

Both of the solutions above help you to get a user-selected value in the model on postback, but how can you actually render that value, in its readable form, on another page? Instead of using the enum value, we want to render the value specified in those [Display(Name="...")] attributes.

That’s how to get the text description out:

private string GetIndustryName(Industry value)
{
    // Get the MemberInfo object for supplied enum value
    var memberInfo = value.GetType().GetMember(value.ToString());
    if (memberInfo.Length != 1)
        return null;

    // Get DisplayAttibute on the supplied enum value
    var displayAttribute = memberInfo[0].GetCustomAttributes(typeof(DisplayAttribute), false)
                           as DisplayAttribute[];
    if (displayAttribute == null || displayAttribute.Length != 1)
        return null;

    return displayAttribute[0].Name;
}

And put it on the model:

public ActionResult ViewProfile()
{
    // Get user profile information from the session
    var model = Session["UserProfileModel"] as UserProfileModel;
    if (model == null)
        return RedirectToAction("UserProfile");

    // Get the description of the currently selected industry from the 
    // [Display] attribute of Industry enum
    model.IndustryName = GetIndustryName(model.Industry);

    return View(model);
}

What if my problem is different?

That’s all good, I hear you saying, but what if I have more than just one enum? Say I also want to use fields like ‘Gender’, ‘State’ and ‘Time Zone’ on my profile page. Wouldn’t it be nicer if instead of having to declare a new, slightly different function for each of those enums you could create just a generic function, that deals with any enum, as long as it adheres to the convention above?

That’s exactly the job for C# generics, which is a powerful language construct that saves you some typing. What’s more important, it makes you think harder about commonalities and differences in your code.

That’s how we can make the retrieval of Display[Name="..."] value to work on any enum:

private string GetEnumDisplayName<T>(T value) where T: struct
{
    // Get the MemberInfo object for supplied enum value
    var memberInfo = value.GetType().GetMember(value.ToString());
    if (memberInfo.Length != 1)
    return null;

    // Get DisplayAttibute on the supplied enum value
    var displayAttribute = memberInfo[0].GetCustomAttributes(typeof(DisplayAttribute), false)
                           as DisplayAttribute[];
    if (displayAttribute == null || displayAttribute.Length != 1)
    return null;

    return displayAttribute[0].Name;
}

As you can see from above, there’s almost no difference between earlier GetIndustryName and this new GetEnumDisplayName, except for the piece where we broaden the usage of this function to any enum via the generic specification: <T>(T value) where T: struct. All it really does is to say: “accept an argument of type T as long this type is a struct” (and enums are structs in CLR).

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