使用C#追加到文件

 StorageFile file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Assets/abc.txt"));
Stream stream1 = await file.OpenStreamForWriteAsync();
StreamWriter sr1 = new StreamWriter(stream1);

它在StreamReader上运行良好。它有一个例外，说

   Access denied due to security concerns.

如何更正此代码？提前谢谢。