使用Async保存FileStream

Question

I am trying to save a file(BitmapImage) to a certain location, but as soon as I use async & await I get an error saying the file is in use:

The process cannot access the file 'C:\\Users\\ ... \\image1.jpg' because it is being used by another process.

My coding:

BitmapImage image = new BitmapImage(new Uri(oldImagePath));
var encoder = new JpegBitmapEncoder() { QualityLevel = 17 };
encoder.Frames.Add(BitmapFrame.Create(image));

using (var filestream = new FileStream(GetImageLocation(), FileMode.Create))
    await Task.Run(() => encoder.Save(filestream)); //Error here

When I use the code without the await , the code works perfectly fine. I think it might be because another thread might be using it, but can anyone help or explain to me a work around for my issue? Thank you.

Answer 1

In your case when you use Task with await another thread is used to save your encoder . But your encoder is also used by your main thread so new thread can't use it.

Change your code a little:

await Task.Run(() => 
{ 
    using (var filestream = new FileStream(GetImageLocation(), FileMode.Create))
    {      
         BitmapImage image = new BitmapImage(new Uri(oldImagePath));
         var encoder = new JpegBitmapEncoder() { QualityLevel = 17 };
         encoder.Frames.Add(BitmapFrame.Create(image));
         encoder.Save(filestream);
    }
}

Now you create and save your encoder in the same task and it will use only one thread.

Answer 2

我认为你应该在Task.Run移动代码，因为它在另一个线程内被调用。

Answer 3

You could encode to a MemoryStream , get the byte array, use WriteAsync on the FileStream , and avoid using Task.Run altogether.

BitmapImage image = new BitmapImage(new Uri(oldImagePath));
var encoder = new JpegBitmapEncoder() { QualityLevel = 17 };
encoder.Frames.Add(BitmapFrame.Create(image));
using (var mem = new MemoryStream())
using (var filestream = new FileStream(GetImageLocation(), FileMode.Create))
{
    encoder.Save(mem);
    var data = mem.ToArray();
    await filestream.WriteAsync(date, 0, data.Length);
}

Note that this will block your thread during the encoding and will use more memory.