请教个反射怪异的问题


                        var types = Assembly.LoadFrom((new FileInfo(file)).FullName).GetTypes();
                        if (types == null)
                            continue;
                        foreach (var t in types)
                        {
                            var intef = t.GetInterface("GPLib.IParaSelector");
                            if (intef != null)
                            {
                                var tt = Activator.CreateInstance(t);//成功
                                var tt0 = tt as UserControl;//成功
                                var ttt = tt as GPLib.IParaSelector;//null
                                //var obj = (GPLib.IParaSelector)Activator.CreateInstance(t);
                                //ParaTypes.Add(obj.TypeGUID, t);
                            }
                        }
转换为接口的时候失败，也不知道什么原因。
接口的dll在exe目录美错啊，奇怪。实在找不出问题在哪。
拜求。