c# - "Operation is not valid due to the current state of the object." Error when trying to create new List< AudioTrack> from JsonFile in WP8 -

i'm trying make music app , have 2 project in solution: main project , audioplaybackagent (apa) project. when chose song main project (which contained in list of song) , copied whole list json file in isolate storage , read apa project.

problem is, error when tried create new audiotrack items in json file

code create file (from main project)

public static void createonlinelist(string id, observablecollection<basicitemmodel> list)     {         using (isolatedstoragefile file = isolatedstoragefile.getuserstoreforapplication())         {             string filename = "onlinelist.json";              jobject obj = new jobject();                             jarray arr = new jarray();             (int dem = 0; dem < list.count(); dem++)             {                 if (list[dem].id == id)                      obj.add("index", dem);                 jobject item = new jobject();                                     item.add("name", list[dem].name);                 item.add("artist", list[dem].artist);                 item.add("url", list[dem].url);                 arr.add(item);             }             obj.add("list", arr);             string jsonstring = jsonconvert.serializeobject(obj);             using (isolatedstoragefilestream stream = file.openfile(filename,filemode.openorcreate,fileaccess.write))             {                 using (streamwriter writer = new streamwriter(stream))                 {                     writer.writeline(jsonstring);                                         }             }         }     } 

code read file (from apa project)

public static void readonlinelist()     {         using (isolatedstoragefile file = isolatedstoragefile.getuserstoreforapplication())         {             string filename = "onlinelist.json";             string jsonstring = "";             using (isolatedstoragefilestream stream = file.openfile(filename, filemode.open, fileaccess.read))             {                 using (streamreader writer = new streamreader(stream))                 {                     jsonstring = writer.readline();                 }             }             jobject obj = new jobject();             obj = jobject.parse(jsonstring);             audioplayer.currenttracknumber = int.parse(obj["index"].tostring());             list<jtoken> arr = obj["list"].tolist();             if (audioplayer.playlist == null) audioplayer.playlist = new list<audiotrack>();             else audioplayer.playlist.clear();             foreach (var item in arr)             {                 audiotrack track = new audiotrack()                 {                     source = new uri(item["url"].tostring(), urikind.absolute),                     title = item["name"].tostring(),                     artist = item["artist"].tostring(),                     album = null                 };                      //// error of line when trying create track                 audioplayer.playlist.add(track);             }         }     } 

the audiotrack object has strange behaviour , there 2 ways in can insert/edit data audiotrack.

in case accessing properties , generate error getting.

you can 1 of 2 things:

option 1 - insert data in constructor

audiotrack track = new audiotrack(new uri(item["url"].tostring(), urikind.absolute), item["artist"].tostring(), null, null); 

option 2 - use beginedit() , endedit() methods

audiotrack track = new audiotrack(); track.beginedit(); track.source = new uri(item["url"].tostring(), urikind.absolute); track.title = item["name"].tostring(); track.artist = item["artist"].tostring(); track.endedit(); 

i hope helps , answers question.