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

Question

I'm currently attempting to write to a .json file in c# using JSON.NET, but I can't manage to save the actual file. This is my 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 I am told that the File Class doesn't exist (yet I have declared System.IO, and MSDN confirms that Windows Phone 8.1 supports this class: https://msdn.microsoft.com/en-us/library/system.io.file(v=vs.110).aspx)

I have tried various other things, but nothing works :/