Creating a Dropdownlist in Optimizely CMS: Populate ISelectionFactory with values from another block's properties

Create a Block to hold selection options

using EPiServer.Cms.Shell.UI.ObjectEditing.EditorDescriptors;
using EPiServer.PlugIn;
using EPiServer.Shell.ObjectEditing;
using System.ComponentModel.DataAnnotations;
namespace MyApp.Models.Blocks
{
    [ContentType(DisplayName = "Select Options Block", Description = "")]
    public class SelectOptionsBlock : BlockData
    {
        [CultureSpecific]     
        [Display(Name = "Selection Options", Description = "List of options for the selection list.",  GroupName = SystemTabNames.Content, Order = 100)]
        [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor<SelectionOption>))]
        public virtual IList<SelectionOption> SelectionOptions { get; set; }
    }

    public class SelectionOption
    {
        public string Text { get; set; }
        public string Value { get; set; }
    }

    [PropertyDefinitionTypePlugIn]
    public class SelectionOptionProperty : PropertyList<SelectionOption> { }

}

Create a page where you want to have the selection list and add the following 2 properties

namespace MyApp.Models.Pages
{
    [ContentType(DisplayName = "SelectionList Page", Description = "Page with a selection list populated from a block.", GroupName = SiteGroupNames.Specialized)]
    public class SelectionListPage : PageData
    {
        [CultureSpecific]
        [Display(Name = "Select Options Block", Description = "", GroupName = SystemTabNames.Content, Order = 1)]
        [AllowedTypes(typeof(SelectOptionsBlock))]
        public virtual ContentReference SelectOptionsBlockContent { get; set; }

        [CultureSpecific]
        [Display(Name = "Select List", Description = "",  GroupName = SystemTabNames.Content,  Order = 1)]
        [SelectOne(SelectionFactoryType = typeof(ItemSelectionFactory))]
        public virtual string SelectList { get; set; }
    }
}

Now create a selection list factory

using EPiServer.ServiceLocation;
using EPiServer.Shell.ObjectEditing;
using SVP.Models.Blocks;
namespace MyApp.Business.SelectionFactories
{
    public class ItemSelectionFactory : ISelectionFactory
    {
        public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
        {
            var loader = ServiceLocator.Current.GetInstance<IContentLoader>();
            var list = new List<SelectItem>();  
            dynamic contentMetadata = metadata;
            var ownerContent = contentMetadata.OwnerContent as IContent;            
            //create a dynamic object of current content type
            dynamic parentItem = ownerContent;
            //check if the block containing the select options is added to the current content (page or block)
            if (parentItem.SelectOptionsBlockContent != null)
            {
                //Get the block containing select options
                var optionsBlock = loader.Get<SelectOptionsBlock> parentItem.SelectOptionsBlockContent);
                //check if the optionsBlock has any items
                if (optionsBlock.SelectionOptions != null && optionsBlock.SelectionOptions.Count > 0)
                {
                    foreach (var listItem in optionsBlock.SelectionOptions)
                    {
                        list.Add(new SelectItem() { Text = listItem.Text, Value = listItem.Value });
                    }
                }
            }
            return list;
        }
    }
}

From the CMS, create a new block of type SelectOptionsBlock and add the necessary select options and publish it.

Select this newly created block in your page that you have created to have the selection list and publish the page.