async void Button_Click_2(object sender, RoutedEventArgs e)
{
    var _Name = "HelloWorld.txt";
    var _Folder = KnownFolders.DocumentsLibrary;
    var _Option = Windows.Storage.CreationCollisionOption.ReplaceExisting;
 
    // create file 
    var _File = await _Folder.CreateFileAsync(_Name, _Option);
 
    // write content
    var _WriteThis = "Hello world!";
    await Windows.Storage.FileIO.WriteTextAsync(_File, _WriteThis);
 
    // acquire file
    try { _File = await _Folder.GetFileAsync(_Name); }
    catch (FileNotFoundException) { /* TODO */ }
 
    // read content
    var _Content = await FileIO.ReadTextAsync(_File);
    await new Windows.UI.Popups.MessageDialog(_Content).ShowAsync();
}