InkPresenter can't be called in external thread

Question

Hi

I need to convert a InkPresenter to bitmap :

  InkPresenter ink = new InkPresenter();

            ink.Strokes = p;
            RenderTargetBitmap targetBitmap =
                                    new RenderTargetBitmap((int)rect.Right, (int)rect.Bottom, 96d, 96d, PixelFormats.Default);

            ink.Measure(new Size((int)rect.Right, (int)rect.Bottom));
            ink.Arrange(new Rect(new Size((int)rect.Right, (int)rect.Bottom)));

            targetBitmap.Render(ink);
            targetBitmap.Freeze();

            BmpBitmapEncoder png = new BmpBitmapEncoder();
            png.Frames.Add(BitmapFrame.Create(targetBitmap));

When i call this code on a BackGroundWorker it fails, telling that the calling Thread must be STA... How to fix ? Jonathan

Answer 1

InkPresenter is a UI component, it can only be accesed from the UI thread. If you want to keep the interface responsive while saving, you could use Dispatcher.Invoke. I've expanded your code into this:

using System;
using System.IO;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Threading;

namespace Ink
{
/// <summary>
/// Interaction logic for Window1.xaml
/// </summary>
public partial class Window1 : Window
{
    public InkPresenter ink = new InkPresenter();

    public Window1()
    {
        InitializeComponent();

        StylusPoint segment1Start = new StylusPoint(200, 110);
        StylusPoint segment1End = new StylusPoint(185, 150);
        StylusPoint segment2Start = new StylusPoint(185, 150);
        StylusPoint segment2End = new StylusPoint(135, 150);
        StylusPoint segment3Start = new StylusPoint(135, 150);
        StylusPoint segment3End = new StylusPoint(175, 180);
        StylusPoint segment4Start = new StylusPoint(175, 180);
        StylusPoint segment4End = new StylusPoint(160, 220);
        StylusPoint segment5Start = new StylusPoint(160, 220);
        StylusPoint segment5End = new StylusPoint(200, 195);
        StylusPoint segment6Start = new StylusPoint(200, 195);
        StylusPoint segment6End = new StylusPoint(240, 220);
        StylusPoint segment7Start = new StylusPoint(240, 220);
        StylusPoint segment7End = new StylusPoint(225, 180);
        StylusPoint segment8Start = new StylusPoint(225, 180);
        StylusPoint segment8End = new StylusPoint(265, 150);
        StylusPoint segment9Start = new StylusPoint(265, 150);
        StylusPoint segment9End = new StylusPoint(215, 150);
        StylusPoint segment10Start = new StylusPoint(215, 150);
        StylusPoint segment10End = new StylusPoint(200, 110);

        StylusPointCollection strokePoints = new StylusPointCollection();

        strokePoints.Add(segment1Start);
        strokePoints.Add(segment1End);
        strokePoints.Add(segment2Start);
        strokePoints.Add(segment2End);
        strokePoints.Add(segment3Start);
        strokePoints.Add(segment3End);
        strokePoints.Add(segment4Start);
        strokePoints.Add(segment4End);
        strokePoints.Add(segment5Start);
        strokePoints.Add(segment5End);
        strokePoints.Add(segment6Start);
        strokePoints.Add(segment6End);
        strokePoints.Add(segment7Start);
        strokePoints.Add(segment7End);
        strokePoints.Add(segment8Start);
        strokePoints.Add(segment8End);
        strokePoints.Add(segment9Start);
        strokePoints.Add(segment9End);
        strokePoints.Add(segment10Start);
        strokePoints.Add(segment10End);

        Stroke newStroke = new Stroke(strokePoints);
        ink.Strokes.Add(newStroke);

        Dispatcher.Invoke(DispatcherPriority.Normal, (Action)(() => 
        {
            saveInkPresenter();
        }
        ));
    }




    public void saveInkPresenter()
    {
        Rect rect = new Rect(0, 0, 300, 300);

        RenderTargetBitmap targetBitmap = new RenderTargetBitmap((int)rect.Right, (int)rect.Bottom, 96d, 96d, PixelFormats.Default);

        ink.Measure(new Size((int)rect.Right, (int)rect.Bottom));
        ink.Arrange(new Rect(new Size((int)rect.Right, (int)rect.Bottom)));

        targetBitmap.Render(ink);
        targetBitmap.Freeze();


        PngBitmapEncoder png = new PngBitmapEncoder();
        png.Frames.Add(BitmapFrame.Create(targetBitmap));

        using (Stream stm = File.Create("test.png"))
        {
            png.Save(stm);
        }
    }

}
 }

and it works just fine.