C# async/await Task<BitmaptImage> problem on top of WPF

Question

Tiny WPF app converting image(BitmapImage) using Greyscale input params in async/await manner.

I read dozen of implementations and did not manage to make it work :/

Button method:

private async void btnConvertImage_ClickAsync(object sender, RoutedEventArgs e)
{
    try
    {
        var cts = new CancellationTokenSource();
        BitmapImage result = await ImageProcessing.GreyscaleAsync(orginalImage, cts.Token).ConfigureAwait(false);
        imgPhotoConverted.Source = result;
    }
}

Greyscale Task definition:

public static async Task<BitmapImage> GreyscaleAsync(BitmapImage inputBitmapImage, CancellationToken cancellationToken)
{
    return await Task.Run(() =>
    {
        Bitmap inputBitmap = ToBitmap(inputBitmapImage);
        Bitmap outputImage = new Bitmap(inputBitmap.Width, inputBitmap.Height);
        for (int i = 0; i < inputBitmap.Width; i++)
        {
            for (int x = 0; x < inputBitmap.Height; x++)
            {
                cancellationToken.ThrowIfCancellationRequested();
                Color imageColor = inputBitmap.GetPixel(i, x);
                int grayScale = (int)((imageColor.R * 0.21) + (imageColor.G * 0.72) + (imageColor.B * 0.07));
                Color newColor = Color.FromArgb(imageColor.A, grayScale, grayScale, grayScale);
                outputImage.SetPixel(i, x, newColor);
            }
        }

        return ToBitmapImage(outputImage);
    }, cancellationToken);
}

On line:

imgPhotoConverted.Source = result;

an error is thrown:

System.InvalidOperationException: 'The calling thread cannot access this object because a different thread owns it.'

Remove ConfigureAwait(false)

Sir Rufo
– Sir Rufo

2019-08-29 22:27:07 +00:00
Commented Aug 29, 2019 at 22:27 — Sir Rufo
– Sir Rufo, Commented Aug 29, 2019 at 22:27
Still an error.

matrixb51
– matrixb51

2019-08-29 22:31:39 +00:00
Commented Aug 29, 2019 at 22:31 — matrixb51
– matrixb51, Commented Aug 29, 2019 at 22:31
What error? Be more precise

Sir Rufo
– Sir Rufo

2019-08-29 23:00:46 +00:00
Commented Aug 29, 2019 at 23:00 — Sir Rufo
– Sir Rufo, Commented Aug 29, 2019 at 23:00
Did you try imgPhotoConverted.Source = new Bitmap(result);

Ali Alp
– Ali Alp

2019-08-29 23:11:24 +00:00
Commented Aug 29, 2019 at 23:11 — Ali Alp
– Ali Alp, Commented Aug 29, 2019 at 23:11
What's the stack trace of the exception?

Paulo Morgado
– Paulo Morgado

2019-08-29 23:37:17 +00:00
Commented Aug 29, 2019 at 23:37 — Paulo Morgado
– Paulo Morgado, Commented Aug 29, 2019 at 23:37

Adam V. Steele · Accepted Answer · 2020-05-25 18:26:25Z

3

I think there is an easier way to do this: Freeze() the bitmap before returning it.

Then it doesn't matter which thread accesses it (of course actual UI elements still need to be accessed only from the WPF thread)

I solved a similar problem with a modification like below.

Task.Run(() =>
{
...
        var bmp = ToBitMapImage(outputImage);
        bmp.Freeze();
        return bmp;
}...

answered May 25, 2020 at 18:26

Adam V. Steele

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1 Comment

Emperor Eto Over a year ago

This is a much better/more concise answer as it explains what the actually problem is.

Sir Rufo · Accepted Answer · 2019-08-30 07:01:47Z

2

You should read more about async/await at Stephen Clearys Blog.

Following the advices there will lead you to a very smart solution

private async void btnConvertImage_ClickAsync(object sender, RoutedEventArgs e)
{
    var originalImage = ( imgPhotoOriginal.Source as BitmapImage );
    BitmapImage result = await Task.Run( () => originalImage.ToBitmap().ToGrayscale().ToBitmapImage() );
    imgPhotoConverted.Source = result;
}

which is consuming this extension class

public static class BitmapExtensions
{
    public static Bitmap ToGrayscale( this Bitmap source, CancellationToken cancellationToken = default )
    {
        Bitmap output = new Bitmap( source.Width, source.Height );
        for ( int i = 0; i < source.Width; i++ )
        {
            for ( int x = 0; x < source.Height; x++ )
            {
                cancellationToken.ThrowIfCancellationRequested();
                var imageColor = source.GetPixel( i, x );
                int grayScale = (int)( ( imageColor.R * 0.21 ) + ( imageColor.G * 0.72 ) + ( imageColor.B * 0.07 ) );
                var newColor = System.Drawing.Color.FromArgb( imageColor.A, grayScale, grayScale, grayScale );
                output.SetPixel( i, x, newColor );
            }
        }
        return output;
    }

    public static Bitmap ToBitmap( this BitmapImage source )
    {
        using ( MemoryStream outStream = new MemoryStream() )
        {
            BitmapEncoder enc = new BmpBitmapEncoder();
            enc.Frames.Add( BitmapFrame.Create( source ) );
            enc.Save( outStream );
            System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap( outStream );

            return new Bitmap( bitmap );
        }
    }

    public static BitmapImage ToBitmapImage( this Bitmap source )
    {
        using ( var memory = new MemoryStream() )
        {
            source.Save( memory, ImageFormat.Png );
            memory.Position = 0;

            var bitmapImage = new BitmapImage();
            bitmapImage.BeginInit();
            bitmapImage.StreamSource = memory;
            bitmapImage.CacheOption = BitmapCacheOption.OnLoad;
            bitmapImage.EndInit();
            bitmapImage.Freeze();

            return bitmapImage;
        }
    }
}

answered Aug 30, 2019 at 7:01

Sir Rufo

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3 Comments

matrixb51 Over a year ago

Thx for a suggestion @SirRufo. This solution looks clean, the only 'but' is that my extension class do not encapsulate the Task itself in ASYNC manner (returning Task promise)

Sir Rufo Over a year ago

Hmmm, my extension class has nothing to to with any task because it is all cpu bound work. I execute it inside a task at the UI level as that is how you should do it

matrixb51 Over a year ago

Please don't get me wrong -> your solution is absolutely helpful and I appreciate it very much. Thank you!

div · Accepted Answer · 2019-08-30 06:57:55Z

-1

I managed to solve it:

I did not have to use a BitmapSource, yet I had to Freeze() the output BitmaImage and wrap the result in proper Context using:

ThreadPool.QueueUserWorkItem(async delegate
{
    // ThreadPool
    var cts = new CancellationTokenSource();
    BitmapImage result = await ImageProcessing.GreyscaleAsync(orginalImage, cts.Token);
    result.Freeze();

    sc.Post(delegate
    {
        // original context (UI)
        imgPhotoConverted.Source = result;
        cts.Cancel();
    }, null);
}

I hope this will be useful for others. Thx!

edited Aug 30, 2019 at 6:57

div

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answered Aug 30, 2019 at 1:03

matrixb51

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1 Comment

Sir Rufo Over a year ago

My advice to you: read more about async/await. Although it is working it is still bad code and can be written much better/easier

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