Windows Phone 8 append to JSON file - json

Question

I'm working on a Windows Phone 8 app.
I'm having issue appending to my JSON file.
It works fine if I keep the app open but once I close it and come back in it starts back writing from the beginning of the file.
Relevant code:
private async void btnSave_Click(object sender, RoutedEventArgs e)
{
// Create a entry and intialize some values from textbox...
GasInfoEntries _entry = null;
_entry = new GasInfoEntries();
_entry.Gallons = TxtBoxGas.Text;
_entry.Price = TxtBoxPrice.Text;
_GasList.Add(_entry);
//TxtBlockPricePerGallon.Text = (double.Parse(TxtBoxGas.Text) / double.Parse(TxtBoxPrice.Text)).ToString();
// Serialize our Product class into a string
string jsonContents = JsonConvert.SerializeObject(_GasList);
// Get the app data folder and create or open the file we are storing the JSON in.
StorageFolder localFolder = ApplicationData.Current.LocalFolder;
StorageFile textfile = await localFolder.CreateFileAsync("gasinfo.json", CreationCollisionOption.OpenIfExists); //if get await operator error add async to class (btnsave)
//open file
using (IRandomAccessStream textstream = await textfile.OpenAsync(FileAccessMode.ReadWrite))
{
//write JSON string
using (DataWriter textwriter = new DataWriter(textstream))
//using (DataWriter textwriter = new DataWriter(textstream))
{
textwriter.WriteString(jsonContents);
await textwriter.StoreAsync(); //writes buffer to store
}
}
}
private async void btnShow_Click(object sender, RoutedEventArgs e)
{
StorageFolder localfolder = ApplicationData.Current.LocalFolder;
try
{
// Getting JSON from file if it exists, or file not found exception if it does not
StorageFile textfile = await localfolder.GetFileAsync("gasinfo.json");
using (IRandomAccessStream textstream = await textfile.OpenReadAsync())
{
//read text stream
using (DataReader textreader = new DataReader(textstream))
{
//get size ...not sure what for think check the file size (lenght) then based on next 2 commands waits until its all read
uint textlength = (uint)textstream.Size;
await textreader.LoadAsync(textlength);
//read it
string jsonContents = textreader.ReadString(textlength);
// deserialize back to gas info
_GasList = JsonConvert.DeserializeObject<List<GasInfoEntries>>(jsonContents) as List<GasInfoEntries>;
displayGasInfoEntries();
}
}
}
catch
{
txtShow.Text = "something went wrong";
}
}
private void displayGasInfoEntries()
{
txtShow.Text = "";
StringBuilder GasString = new StringBuilder();
foreach (GasInfoEntries _entry in _GasList)
{
GasString.AppendFormat("Gallons: {0} \r\n Price: ${1} \r\n", _entry.Gallons, _entry.Price); // i think /r/n means Return and New line...{0} and {1} calls "variables" in json file
}
txtShow.Text = GasString.ToString();
}
Thanks

Answer 1

Do you call the btnShow_Click each time you've started the app? Because otherwise the _GasList will be empty; if you now call the btnSave_Click all previous made changes will be lost.
So please make sure, that you restore the previously saved json data before you add items to the _GasList.