how to use enum with DescriptionAttribute in asp.n

问题:

I am new to asp.net MVC. I am trying to use dropdown control on my view page, which populates from enum. I also want to add custom descriptions to dropdown values. I searched so many examples, but no one posted how to populated description on view page. Here is my code:

ViewModel:

public enum SearchBy
    {
        [Description("SID/PID")]
        SID = 1,
        [Description("Name")]
        Name,
        [Description("Birth Date")]
        DOB,
        [Description("Cause#")]
        Cause
    }

Index.cshtml

<div class="form-horizontal">
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group form-inline">
        @Html.LabelFor(model => model.searchBy, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EnumDropDownListFor(model => model.searchBy, "Search By", htmlAttributes: new { @class = "form-control" })
            @Html.TextBox("searchByVal", null, htmlAttributes: new { @placeholder = "SID / PID ", @class = "form-control" })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.FirstName, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.FirstName, new { htmlAttributes = new { @placeholder = "First Name", @class = "form-control" } })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.LastName, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.LastName, new { htmlAttributes = new { @placeholder = "Last Name", @class = "form-control" } })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.DOB, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.DOB, new { htmlAttributes = new { @placeholder = "Birth Date", @class = "form-control" } })
        </div>
    </div>

    <div class="form-group">
        @Html.LabelFor(model => model.CauseNumber, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.EditorFor(model => model.CauseNumber, new { htmlAttributes = new { @placeholder = "Cause#", @class = "form-control" } })
        </div>
    </div>

    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Search" class="btn btn-block btn-primary" />
        </div>
    </div>
</div>

It is not populating the with description fields as mentioned in my SearchBy enum. see the image here. http://postimg.org/image/phdxgocj7/ Please help me, where I am making mistake. Thank you

UPDATE: I got solution for this from Nico. And I researched little bit on this. I am updating this post with solution because it may useful to others, who are new to MVC http://weblogs.asp.net/jongalloway//looking-at-asp-net-mvc-5-1-and-web-api-2-1-part-1-overview-and-enums

Thank you all. Enjoy coding..

回答1:

The Html helper EnumDropDownListFor or EnumDropDownList does not take into consideration the Description attribute decorations on the enum members. However by reviewing the source code:

Enum Dropdown List Helper: https://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Web.Mvc/Html/SelectExtensions.cs

Enum Helper Classes: https://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Web.Mvc/Html/EnumHelper.cs

The enum helper classes above are used to convert an Enum to a List<SelectListItem>. From the code below:

// Return non-empty name specified in a [Display] attribute for the given field, if any; field's name otherwise
private static string GetDisplayName(FieldInfo field)
{
    DisplayAttribute display = field.GetCustomAttribute<DisplayAttribute>(inherit: false);
    if (display != null)
    {
        string name = display.GetName();
        if (!String.IsNullOrEmpty(name))
        {
            return name;
        }
    }

    return field.Name;
}

You can see that in the method GetDisplayName it checks for the existence of the DisplayAttribute on the enum member. If the display attribute exists then the name is set to the result of DisplayAttribute.GetName() method.

Putting this together we can modify the enum to use the DisplayAttribute instead of the DescriptionAttribute and setting the Name property to the value you wish to display.

public enum SearchBy
{
    [Display(Name = "SID/PID")]
    SID = 1,
    [Display(Name = "Name")]
    Name,
    [Display(Name = "Birth Date")]
    DOB,
    [Display(Name = "Cause#")]
    Cause
}

This gives you the result you wish.

Hope this helps.

回答2:

I created a helper class that tries different types of attributes. I needed it because I was using bootstrap with https://github.com/civicsource/enums and https://silviomoreto.github.io/bootstrap-select/

public static class EnumHelper<T>
    {
        static EnumHelper()
        {
            var enumType = typeof(T);
            if (!enumType.IsEnum) { throw new ArgumentException("Type '" + enumType.Name + "' is not an enum"); }
        }

        public static string GetEnumDescription(T value)
        {
            var fi = typeof(T).GetField(value.ToString());
            var attributes = (DescriptionAttribute[]) fi.GetCustomAttributes(typeof(DescriptionAttribute), false);
            return attributes.Length > 0 ? attributes[0].Description : value.ToString();
        }

        public static IEnumerable<SelectListItem> GetSelectList()
        {
            var groupDictionary = new Dictionary<string, SelectListGroup>();

            var enumType = typeof(T);
            var fields = from field in enumType.GetFields()
                         where field.IsLiteral
                         select field;

            foreach (var field in fields)
            {
                var display = field.GetCustomAttribute<DisplayAttribute>(false);
                var description = field.GetCustomAttribute<DescriptionAttribute>(false);
                var group = field.GetCustomAttribute<CategoryAttribute>(false);

                var text = display?.GetName() ?? display?.GetShortName() ?? display?.GetDescription() ?? display?.GetPrompt() ?? description?.Description ?? field.Name;
                var value = field.Name;
                var groupName = display?.GetGroupName() ?? group?.Category ?? string.Empty;
                if (!groupDictionary.ContainsKey(groupName)) { groupDictionary.Add(groupName, new SelectListGroup { Name = groupName }); }

                yield return new SelectListItem
                {
                    Text = text,
                    Value = value,
                    Group = groupDictionary[groupName],
                };
            }
        }
    }

And you call it like:

<div class="form-group">
   @Html.LabelFor(model => model.Address.State, htmlAttributes: new { @class = "control-label col-md-2" })
   <div class="col-sm-4">
      @Html.DropDownListFor(model => model.Address.State, EnumHelper<StateProvince>.GetSelectList(), new { @class = "selectpicker show-menu-arrow", data_live_search = "true" })
      @Html.ValidationMessageFor(model => model.Address.State, "", new { @class = "text-danger" })
   </div>
</div>

---

回答3:

Given :

public enum MyEnum
{
    [Description("This is the description if my member A")]
    A,
    [Description("This is the description if my member B")]
    B
}

I personally use this extension method to get a description from my enum:

 public static string GetDescription(this Enum 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();
            }
        }

To use it:

MyEnum.A.GetDescription();

Hope this help.

回答4:

There is no need to create a helper class if you're using .Net Framework 4.0 or newer.

You can just use the Display attribute in conjunction with EnumDropDownListFor

public enum SearchBy
{
    [Display(Name = "SID/PID")]
    SID = 1,
    [Display(Name = "Name")]
    Name,
    [Display(Name = "Birth Date")]
    DOB,
    [Display(Name = "Cause#")]
    Cause
}

In your View:

@Html.EnumDropDownListFor(model => model.SearchBy, "Search By", new { @class = "form-control" })

Microsoft documentation:

https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.dataannotations.displayattribute?view=netframework-4.8