How to access local storage in background task (for audio) in windows phone 8.1

I made a Background task for playing audio in my windows phone 8.1 app. But I want to access some local storage files, but there is some problem. I made following attempts

Attempt 1:

StorageFile contentFile = await localFolder.GetFileAsync(guid + "_rec_c.txt");
text = await FileIO.ReadTextAsync(contentFile);

Error:

App execution stops and it goes to the following code in App.g.i.cs

#if DEBUG && !DISABLE_XAML_GENERATED_BREAK_ON_UNHANDLED_EXCEPTION
UnhandledException += (sender, e) =>
{
    if(global::System.Diagnostics.Debugger.IsAttached)
        global::System.Diagnostics.Debugger.Break();
};
#endif

Attempt 2:

CoreDispatcher dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
await dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
{
    StorageFile contentFile = await localFolder.GetFileAsync(guid + "_rec_c.txt");

    text = await FileIO.ReadTextAsync(contentFile);
});

Error It gives null reference exception in line 1, supposedly CoreWindow.GetForCurrentThread() is returning null.

Attempt 3:

await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
{
    StorageFile contentFile = await localFolder.GetFileAsync(guid + "_rec_c.txt");

    text = await FileIO.ReadTextAsync(contentFile);
});

Error:

It gives System.InvalidOperationException and additional information is Additional information: A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)

Please tell how can I access local storage file in background task.