ExpandPropertyAttribute.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;

namespace ZumoExtensions
{
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
    public class ExpandPropertyAttribute : ActionFilterAttribute
    {
        string propertyName;

        public ExpandPropertyAttribute(string propertyName)
        {
            this.propertyName = propertyName;
        }

        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            base.OnActionExecuting(actionContext);
            var uriBuilder = new UriBuilder(actionContext.Request.RequestUri);
            var queryParams = uriBuilder.Query.TrimStart('?').Split(new[] { '&' }, StringSplitOptions.RemoveEmptyEntries).ToList();
            int expandIndex = -1;
            for (var i = 0; i < queryParams.Count; i++)
            {
                if (queryParams[i].StartsWith("$expand", StringComparison.Ordinal))
                {
                    expandIndex = i;
                    break;
                }
            }

            if (expandIndex < 0)
            {
                queryParams.Add("$expand=" + this.propertyName);
            }
            else
            {
                queryParams[expandIndex] = queryParams[expandIndex] + "," + propertyName;
            }

            uriBuilder.Query = string.Join("&", queryParams);
            actionContext.Request.RequestUri = uriBuilder.Uri;
        }
    }

}