c# - I can't manage to save my serialized .json in wp8.1 -

i'm attempting write .json file in c# using json.net, can't manage save actual file. current code:

list<article> bm = new list<article>(); bm.add(new article()   {     id = movieid2.text,     name = title.text,     imagepath = image.text,     year = year.text   }); string json = jsonconvert.serializeobject(bm.toarray()); system.io.file.writealltext("bookmarks.json", json); 

but told file class doesn't exist (yet have declared system.io, , msdn confirms windows phone 8.1 supports class: https://msdn.microsoft.com/en-us/library/system.io.file(v=vs.110).aspx)

i have tried various other things, nothing works :/