dynamic load the DLL that implements the IOrderRule interfaces. http://www.jeremybytes.com/Demos.aspx#PR

DynamicOrderRuleLoader.cs

using OrderRules.Interface;
using System;
using System.Collections.Generic;
using System.IO;
using System.Reflection;

namespace OrderRules.RuleChecker
{
    public static class DynamicOrderRuleLoader
    {
        public static List<IOrderRule> LoadRules(string assemblyPath)
        {
            var ruleCatalog = new List<IOrderRule>();

            if (!Directory.Exists(assemblyPath))
                return ruleCatalog;

            IEnumerable<string> assemblyFiles = Directory.EnumerateFiles(
                assemblyPath, "*.dll", SearchOption.TopDirectoryOnly);

            foreach(string assemblyFile in assemblyFiles)
            {
                Assembly assembly = Assembly.LoadFrom(assemblyFile);
                foreach(Type type in assembly.ExportedTypes)
                {
                    if(type.IsClass &&
                        typeof(IOrderRule).IsAssignableFrom(type))
                    {
                        IOrderRule rule = Activator.CreateInstance(type) as IOrderRule;
                        ruleCatalog.Add(rule);
                    }
                }
            }

            return ruleCatalog;
        }
    }
}