使用可为空的枚举属性和带有空字符串值的附加选项呈现单选按钮

Question

I am trying to display radio buttons on page where one of the radio buttons has to be selected otherwise an error should be displayed. The three options available are:

• Manual
• Automatic
• Unsure

The option "Unsure" does not make sense to add to the GearboxTransmission enum as it is merely indicating that the user does not know what option they want to select at this moment in time.

I have the following enum:

public enum GearboxTransmission
{
    Automatic,
    SemiAutomatic,
    Manual
}

Model:

public class BookingsIndexViewModel
{
    [Required(ErrorMessage = "Please select the preferred gearbox transmission to learn with.")]
    public GearboxTransmission? GearboxTransmission { get; set; }
}

And HTML:

<div class="form-group">
    <fieldset>
        <legend>Gearbox transmission</legend>
        <div class="radio">
            <label>
                @Html.RadioButtonFor(m => m.GearboxTransmission, GearboxTransmission.Manual) Manual
            </label>
        </div>
        <div class="radio">
            <label>
                @Html.RadioButtonFor(m => m.GearboxTransmission, GearboxTransmission.Automatic) Automatic
            </label>
        </div>
        <div class="radio">
            <label>
                @Html.RadioButtonFor(m => m.GearboxTransmission, string.Empty) Unsure
            </label>
        </div>
        @Html.ValidationMessageFor(m => m.GearboxTransmission, null, new { @class = "text-danger" })
    </fieldset>
</div>

As I have set the GearboxTransmission property to nullable I would expect that by creating a radio button with the value of an empty string when the page is posted back the GearboxTransmission property would be null, indicating that the option "Unsure" was selected.

What actually happens is the model view state is invalid and the validation message "Please select the preferred gearbox transmission to learn with." appears due to the required attribute on the GearboxTransmission property.

I would like it so that you have to specify an option, and if you don't you get the error "Please select the preferred gearbox transmission to learn with." unless you select the "Unsure" radio button in which case the form is submitted.

Answer 1

Your code doesn't seem to be wrong as such, although I can't really tell without running it. Also, you're also aware of what's wrong with this - your validation 'requires' that there is a selection made.

'Required' doesn't really check that a radio button is clicked, rather it checks whether the user selected a value.

Clearly, you can't use validation to display an error message based on not selecting a value, when the value of one of your options is empty (you follow?).

There are 2 answers to this:

A) Forget about the error message and default the radios to 'unsure' although that could well lower the quality of the data capture. B) Use an actual value for 'unsure', Add it to your GearboxTransmission Enum and flow it through Model and View etc. This would allow your empty selection to work.

If you really have to end up with an Enum value without 'Unsure', use a second one for model binding, but copy the Model Enum to the DB Enum in the controller.

Hope this helps.