public class DefaultPriceProvider : PriceProvider 

class MyPage : System.Web.UI.Page
{
    private void CheckProviderTypesInCache()
    {
        // Check if a cache item for PriceResolverTypes is already created
        if (((HttpContext.Current.Cache["PriceProviderTypes"] == null)))
        {
            // Create a new arraylist
            ArrayList PriceProviderTypes = new ArrayList();

            // and fill it with ResolverTypes Types
            foreach (Type t in AddInManager.GetAddInClasses(typeof(Dynamicweb.eCommerce.Prices.PriceProvider)))
            {
                if ((!object.ReferenceEquals(t, typeof(Dynamicweb.eCommerce.Prices.DefaultPriceProvider))))
                {
                    PriceProviderTypes.Add(t);
                }
            }

            PriceProviderTypes.Add(typeof(Dynamicweb.eCommerce.Prices.DefaultPriceProvider));

            // Create a new cache item with for 5 min. expiration.
            // TODO: Change to inherited cache dependency object when switching to .net 2,0. Use Appdomain.OnLoad to flush the dependency.
            HttpContext.Current.Cache.Add("PriceProviderTypes", PriceProviderTypes, null, DateTime.Now.AddSeconds(300), TimeSpan.Zero, System.Web.Caching.CacheItemPriority.High, null);
        }
    }
}