private async void btnWrite_Click(object sender, RoutedEventArgs e)
{
    await WriteToFile();

    // Update UI.
    this.btnWrite.IsEnabled = false;
    this.btnRead.IsEnabled = true;
}

private async Task WriteToFile()
{
    // Get the text data from the textbox. 
    byte[] fileBytes = System.Text.Encoding.UTF8.GetBytes(this.textBox1.Text.ToCharArray());

    // Get the local folder.
    StorageFolder local = Windows.Storage.ApplicationData.Current.LocalFolder;

    // Create a new folder name DataFolder.
    var dataFolder = await local.CreateFolderAsync("DataFolder",
        CreationCollisionOption.OpenIfExists);

    // Create a new file named DataFile.txt.
    var file = await dataFolder.CreateFileAsync("DataFile.txt",
    CreationCollisionOption.ReplaceExisting);

    // Write the data from the textbox.
    using (var s = await file.OpenStreamForWriteAsync())
    {
        s.Write(fileBytes, 0, fileBytes.Length);
    }
}

private async void btnRead_Click(object sender, RoutedEventArgs e)
{
    await ReadFile();

    // Update UI.
    this.btnWrite.IsEnabled = true;
    this.btnRead.IsEnabled = false;
}

private async Task ReadFile()
{
    // Get the local folder.
    StorageFolder local = Windows.Storage.ApplicationData.Current.LocalFolder;

    if (local != null)
    {
        // Get the DataFolder folder.
        var dataFolder = await local.GetFolderAsync("DataFolder");

        // Get the file.
        var file = await dataFolder.OpenStreamForReadAsync("DataFile.txt");

        // Read the data.
        using (StreamReader streamReader = new StreamReader(file))
        {
            this.textBlock1.Text = streamReader.ReadToEnd();
        }

    }
}