Generic ToSelectListItem method

Question

I have about 10 very simple tables in database:

create table EmploymentStatus
(
    Id int identity(1,1) Primary Key,
    Name nvarchar(100) NOT NULL
)

These tables are used for DropDownLists on registration view, like this:

@Html.DropDownListFor(m => m.EmploymentStatus, Model.EmploymentStatusList, Resource.EmploymentStatus, new { @class = "form form-control" })

DropDownListFor requires a IEnumerable<SelectListItem> selectList as a collection. So, I created a method:

public static List<SelectListItem> ToSelectItemList(IQueryable<EmploymentStatu> statuses)
{
    return statuses.Select(m => new SelectListItem
    {
        Text = m.Name,
        Value = m.Id.ToString()
    }).ToList();
}

The problem is that I have to create a method for each of ten entities. Is there a good way of writing some generic method, like this:

public static List<SelectListItem> ToSelectItemList(IQueryable<T> collection)
{
    return collection.Select(m => new SelectListItem
    {
        Text = m.Name,
        Value = m.Id.ToString()
    }).ToList();
}

Roman Marusyk · Answer 1 · 2017-11-28T14:49:39.030

8

You can define an interface for all entities that you are going to use for DropDownLists like

public interface IDropDownItem
{
   int Id {get; set;}
   string Name {get; set;}
}

then

public static List<SelectListItem> ToSelectItemList<T>(IEnumerable<T> collection) 
   where T : IDropDownItem
{
    return collection.Select(m => new SelectListItem
    {
        Text = m.Name,
        Value = m.Id.ToString()
    }).ToList();
}

It would be better to make ToSelectItemList as extension method:

public static class EnumerableExtensions
{
    public static List<SelectListItem> ToSelectItemList<T>(this IEnumerable<T> collection)
        where T : Program
    {
        return collection.Select(m => new SelectListItem
        {
            Text = m.Name,
            Value = m.Id.ToString()
        }).ToList();
    }
}

edited Nov 28 '17 at 14:49

answered Nov 28 '17 at 12:41

Roman Marusyk

23,328
24
73
116

ok, should i inherit this interface in entity class? Because every time I will update my model from db this will be missed – kir.gera Nov 28 '17 at 13:29
1

the approach using delegate is more better in my opinion as suggested by @Titian – Ehsan Sajjad Nov 28 '17 at 13:30
@kir.gera yes you should implement the interface. His solution has a flaw though, you should change `IQueryable` to `IEnumerable` otherwise you will get an error. You can't do LINQ to EF on interfaces, you can do it on an `IEnumerable` as that will get handled in memory and no SQL has to be genearetd – Titian Cernicova-Dragomir Nov 28 '17 at 13:36
Also I would have made the method an extension method (and I did in my answer) – Titian Cernicova-Dragomir Nov 28 '17 at 13:38
@kir.gera Yes, you should. It is additional work but you will get a nice code where you're going to call it `m => m.EmploymentStatus.ToSelectItemList()`. And it makes that method more testable. As I write before (and Titian) it would be better to make it as an extension method. See updates – Roman Marusyk Nov 28 '17 at 14:49

Titian Cernicova-Dragomir · Accepted Answer · 2017-11-28T12:51:32.937

There are two options:

If you can implement an interface on the entity

public interface ISelectable
{
    string Name { get; }
    int Id { get; }
}
public static List<SelectListItem> ToSelectItemList<T>(this IEnumerable<T> collection)
    where T: ISelectable
{
    return collection.Select(m => new SelectListItem
    {
        Text = m.Name,
        Value = m.Id.ToString()
    }).ToList();
}

Using delegates

public static List<SelectListItem> ToSelectItemList<T>(this IEnumerable<T> collection, Func<T, string> nameGetter, Func<T, int> idGetter)
{
    return collection.Select(m => new SelectListItem
    {
        Text = nameGetter(m),
        Value = idGetter(m).ToString()
    }).ToList();
}

Usage :

m.EmploymentStatus.ToSelectItemList(e => e.Name, e => e.Id);

The second option is more verbose to use, but you get a ton more flexibility, since you don't have to clutter your data model with useless interface implementation, and you are free to use any property names for Name or Id

score 3 · Answer 3 · answered Nov 28 '17 at 12:57

3

DropDownListFor requires a IEnumerable<SelectListItem> selectList as a collection

There's already a class for that, namely SelectList:

var selectListItems = new SelectList(items, "Id", "Name");

No need for new extension methods whatsoever. I thought SelectList() also has an overload with expressions for the key/value members instead of strings, but I may have found that somewhere else.

See also ASP.NET MVC Dropdown List From SelectList.

answered Nov 28 '17 at 12:57

CodeCaster

147,647
23
218
272

1

I didn't know about the class, generally it is better to use built-in classes, however, I don't think properties as magic uncheckable strings is a great idea, would still prefer a more type safe option. – Titian Cernicova-Dragomir Nov 28 '17 at 13:41

Generic ToSelectListItem method

3 Answers3