Any way to save an UIElement created programmatically in an image file?

Question

I'm trying to save an UIElement created programmatically in a JPG/PNG/BMP image in a Windows Phone 8.1 (C#) application.

I'm using the class RenderTargetBitmap using the method RenderAsync() but it only works with UI elements created in the XAML code. When I use it on UI elements created directly in C# I have this exception: "System.ArgumentException (Value does not fall within the expected range.)"

Am I doing something wrong or this class doesn't allow rendering of UIElement(s) created programmatically? Is there any way to do this on Windows Phone 8.1? Thank you!

Here's the code I use:

        private static async void RenderText(string text, int width, int height, int fontsize, string imagename)
    {
        RenderTargetBitmap b = new RenderTargetBitmap();

        var canvas = new Grid();

        canvas.Width = width;
        canvas.Height = height;

        var background = new Canvas();
        background.Height = width;
        background.Width = height;

        SolidColorBrush backColor = new SolidColorBrush(Colors.Red);
        background.Background = backColor;

        var textBlock = new TextBlock();
        textBlock.Text = text;
        textBlock.FontWeight = FontWeights.Bold;
        textBlock.TextAlignment = TextAlignment.Left;
        textBlock.HorizontalAlignment = HorizontalAlignment.Center;
        textBlock.VerticalAlignment = VerticalAlignment.Stretch;
        textBlock.Margin = new Thickness(35);
        //textBlock.Width = b.PixelWidth - textBlock.Margin.Left * 2;
        textBlock.TextWrapping = TextWrapping.Wrap;
        textBlock.Foreground = new SolidColorBrush(Colors.White); //color of the text on the Tile
        textBlock.FontSize = fontsize;

        canvas.Children.Add(textBlock);

        await b.RenderAsync(background);
        await b.RenderAsync(canvas);

        // Get the pixels

        var pixelBuffer = await b.GetPixelsAsync();


        // 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);

        StorageFile file = await dataFolder.CreateFileAsync(imagename, CreationCollisionOption.ReplaceExisting);


        // Encode the image to the selected file on disk
        using (var fileStream = await file.OpenStreamForWriteAsync())
        {

            var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, fileStream.AsRandomAccessStream());

            encoder.SetPixelData(
                BitmapPixelFormat.Bgra8,
                BitmapAlphaMode.Ignore,
                (uint)b.PixelWidth,
                (uint)b.PixelHeight,
                DisplayInformation.GetForCurrentView().LogicalDpi,
                DisplayInformation.GetForCurrentView().LogicalDpi,
                pixelBuffer.ToArray());

            await encoder.FlushAsync();
        }
    }

Answer 1

It is not working only with elements created in XAML but with these that (as MSDN says) are in VisualTree:

Renders a snapshot of a UIElement visual tree to an image source.

So your method will work if you add your elements for example to your current Page:

LayoutRoot.Children.Add(canvas);

Answer 2

Your UIElemet Height and Width values will be 0 when you create them using code. If you want the image in a particular resolution, try to assign the same height and width to the UIElement as well. The code shown below worked well for me.

    public static void SaveElementAsJPG(FrameworkElement element, string ImageName)
    {
        WriteableBitmap wBitmap = new WriteableBitmap(element, null);
        using (MemoryStream stream = new MemoryStream())
        {
            wBitmap.SaveJpeg(stream, (int)element.ActualWidth, (int)element.ActualHeight, 0, 100);
            wBitmap = null;
            //Use can either save the file to isolated storage or media library.
            //Creates file in Isolated Storage.
            using (var local = new IsolatedStorageFileStream(ImageName, FileMode.Create, IsolatedStorageFile.GetUserStoreForApplication()))
            {
                local.Write(stream.GetBuffer(), 0, stream.GetBuffer().Length);
            }
            //Creates file in Media Library.
            var lib = new MediaLibrary();
            var picture = lib.SavePicture(ImageName, stream.GetBuffer());
        }
    }