when the button is pressed to print, the experience occurs.

Question

when the button is pressed to print, the experience occurs.
Object reference not set to an instance of an object. PrintSerive_InCanvas is never assigned and will always default to null mvvm format

this exception occurs with printcanvas drawingcanvas inkcanvas in the printservice class

  private   async void Print_Click(object sender, RoutedEventArgs e)  
        {  
             
              
               var service = new PrintService();  
  
               await  service.PrintAsync();  
  
               
                      
        }  

Answer 1

Hello,

Welcome to Microsoft Q&A!

Based on your code, your PrintService class can't find _inkCanvas object, _drawingCanvas object, _printCanvas object. The PrintService Class can't directly get these XAML elements. You could pass these objects from UI when you create the PrintService Class.

Like this.

        private PointHelper _printHelper;  
        private  InkCanvas _inkCanvas;  
        private  Canvas _drawingCanvas, _printCanvas;  
  
        public PrintService(InkCanvas inkcanvas, Canvas drawingCanvas, Canvas printCanvas)  
        {  
            _inkCanvas = inkcanvas;  
            _drawingCanvas = drawingCanvas;  
            _printCanvas = printCanvas;  
        }  

Then in the button click event, call it like this:

  var service = new PrintService(yourinkcanvas,yourdrawcanvas,yourprintcanvas);  

Thank you.

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Answer 2

using Microsoft.Toolkit.Uwp.Helpers;
using System;
using System.Threading.Tasks;
using Windows.Graphics.Printing;
using Windows.Storage.Streams;
using Windows.UI.Popups;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media.Imaging;

namespace InkPen.Helpers
{
    public class PrintService
    {



        private PrintHelper _printHelper;
        private readonly InkCanvas _inkCanvas;
        private readonly Canvas _drawingCanvas, _printCanvas;

        public PrintService()
        {
        }





        public  async Task  PrintAsync()
        {
            var inkStream = new InMemoryRandomAccessStream();
            await _inkCanvas.InkPresenter.StrokeContainer.SaveAsync(inkStream.GetOutputStreamAt(0));
            var inkBitmap = new BitmapImage();
            await inkBitmap.SetSourceAsync(inkStream);

            var inkBounds = _inkCanvas.InkPresenter.StrokeContainer.BoundingRect;
            var inkMargin = new Thickness(inkBounds.Left, inkBounds.Top, _inkCanvas.ActualWidth - inkBounds.Right, _inkCanvas.ActualHeight - inkBounds.Bottom);

            var inkViewBox = new Viewbox()
            {

                Child = new Image()
                {
                    Source = inkBitmap,
                    Margin = inkMargin
                },

                Width = _drawingCanvas.ActualWidth,
                Height = _drawingCanvas.ActualHeight
            };

            _printHelper = new PrintHelper(_printCanvas);
            _printHelper.AddFrameworkElementToPrint(inkViewBox);

            _printHelper.OnPrintFailed += printHelper_OnPrintFailed;
            _printHelper.OnPrintSucceeded += printHelper_OnPrintSucceeded;

            var printHelperOptions = new PrintHelperOptions();

            printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Copies);
            printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.MediaSize);
            printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Collation);
            printHelperOptions.AddDisplayOption(StandardPrintTaskOptions.Orientation);


            printHelperOptions.PrintQuality = PrintQuality.Default;
            printHelperOptions.MediaSize = PrintMediaSize.Default;
            printHelperOptions.Collation = PrintCollation.Default;
            printHelperOptions.Orientation = PrintOrientation.Landscape;



          await _printHelper.ShowPrintUIAsync("Printing Ink");

        }



        private async void printHelper_OnPrintSucceeded()
        {
            _printHelper.Dispose();

            var dialog = new MessageDialog("Printing Done");
            await dialog.ShowAsync();

        }


        private async void printHelper_OnPrintFailed()
        {
            _printHelper.Dispose();
            var dialog = new MessageDialog("Sorry, Printing Failed");
            await dialog.ShowAsync();

        }


    }
}