using System.Collections.Generic;
using System.Linq;
using EPiServer.Shell.ObjectEditing;

namespace Project.Business.Selection
{
    public class ResourceIconSelection : ISelectionFactory
    {
        public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
        {
            var iconsList = new SortedDictionary<string, string>();

            iconsList.Add("Link", "link");
            iconsList.Add("Media", "media");
            iconsList.Add("PDF", "pdf");

            return iconsList
                .Select(x => new SelectItem() { Text = x.Key, Value = x.Value })
                .ToArray();
        }
    }
}

////Next declare property in your model. For select use 'SelectOne' and for list of checkboxes use 'SelectMany'.

[SelectOne(SelectionFactoryType = typeof(ResourceIconSelection))]
[Display(
    Name = "Icon",
    GroupName = SystemTabNames.Content,
    Order = 10)]
public virtual string Icon { get; set; }