a simple JsonContext that adds and removes item in a list and then saves the file to the roaming folder of a Windows 8.1 Store app.

JsonContext.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Windows.Storage;

namespace ExpenseManager.Repository
{
    public class JsonContext<T> where T : class
    {
        public JsonContext(string fileName)
        {
            this._list = new List<T>();
            this._fileName = fileName;
            this._storageFolder = ApplicationData.Current.RoamingFolder;
        }

        private string _fileName;
        private StorageFolder _storageFolder;

        protected IList<T> _list;
        public async Task<bool> AddItemAsync(T item)
        {
            var items = await GetUnderlyingItems();
            items.Add(item);

            return await SaveItems(items);
        }
        public async Task<bool> RemoveItemAsync(T item)
        {
            var items = await GetUnderlyingItems();
            items.Remove(item);

            return await SaveItems(items);
        }
        public async Task<IList<T>> GetItemsAsync()
        {
            return await GetUnderlyingItems();
        }

        private async Task<IList<T>> GetUnderlyingItems()
        {
            var needToCreate = false;

            try
            {
                var file = await _storageFolder.GetFileAsync(_fileName);
                var json = await FileIO.ReadTextAsync(file);
                var obj = await JsonConvert.DeserializeObjectAsync<List<T>>(json);
                return obj;
            }
            catch
            {
                needToCreate = true;
            }

            if (needToCreate)
            {
                await _storageFolder.CreateFileAsync(_fileName, CreationCollisionOption.ReplaceExisting);
            }

            return await GetUnderlyingItems();
        }

        private async Task<bool> SaveItems(IList<T> items)
        {
            try
            {
                var file = await _storageFolder.CreateFileAsync(_fileName, CreationCollisionOption.ReplaceExisting);
                var json = await JsonConvert.SerializeObjectAsync(items);

                await FileIO.WriteTextAsync(file, json);
            }
            catch
            {
                return false;
            }

            return true;
        }
    }
}