Example Umbraco Macro Parameter editor. This editor allows you to pick a descendant node based on a specific document type alias. This particular parameter editor supported picking a descendant node to feed a macro to render a playlist. The document type Question has a rich text editor property bodyText. The content editors can create children "…

DescendantPickerBase

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.UI.WebControls;
using umbraco.interfaces;
using Umbraco.Core;
using Umbraco.Web;

namespace Customer.MacroParameters
{
    public abstract class DescendantPickerBase : DropDownList, IMacroGuiRendering
    {
        protected abstract IEnumerable<string> AllowedDocumentTypes { get; }

        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            int documentId;
            if (int.TryParse(UmbracoContext.Current.HttpContext.Request.QueryString["umbpageid"], out documentId))
            {
                // Convert the list of allowed document types to lower case
                var lowerCaseAllowedAliases = AllowedDocumentTypes.Select(
                    alias => string.IsNullOrEmpty(alias) ? alias : alias.ToLowerInvariant());

                // Get all descendants so we can filter by the allowed node types
                // Add the descendants that are in the white listed aliases to the drop down
                ApplicationContext.Current.Services.ContentService.GetDescendants(documentId)
                    .Where(content => lowerCaseAllowedAliases.Contains(content.ContentType.Alias.ToLowerInvariant()))
                    .OrderBy(content => content.Name)
                    .ToList()
                    .ForEach(content => Items.Add(new ListItem(content.Name, content.Id.ToString()))); 
            }
        }

        #region IMacroGuiRendering Implementation
        public bool ShowCaption
        {
            get { return true; }
        }

        public string Value
        {
            get
            {
                return this.SelectedValue;
            }
            set
            {
                this.SelectedValue = value;
            }
        }
        #endregion
    }
}

DescendantPlaylistPicker

using System.Collections.Generic;

namespace Customer.MacroParameters
{
    public class DescendantPlaylistPicker : DescendantPickerBase
    {
        private readonly IEnumerable<string> allowedDocumentTypes = new List<string> { "playlist" }; 

        protected override IEnumerable<string> AllowedDocumentTypes
        {
            get { return allowedDocumentTypes; }
        }
    }
}