MVVM Image Source Binding from in memory bitmap not working

Question

I have a small app that does some font rendering to a bitmap and then attempts to update the UI with the rendererd bitmap using MVVM style data binding.

My view has the following image control...

<Image Name="BitmapImagePreview"
             Source="{Binding PreviewImage}"/>

My View Model exposes the following property

public ImageSource? PreviewImage
        {
            get => _previewImage;
            set
            {
                _previewImage = value;
                NotifyPropertyChanged(nameof(PreviewImage));
            }
        }

However, when I change the PreviewImage property it fails to refresh/re-draw the control.
If I bypass DataBinding, and set the source property directly, it does update.

_viewModel.FontPreview.PreviewImage = image;  <== Does not work
_view.BitmapFontPreview.BitmapImagePreview.Source = image; <== Does work

The image itself is generated in memory from a drawing operation like so...

            var bitmap = new RenderTargetBitmap(
                width,
                height,
                96,
                96,
                PixelFormats.Default);
            bitmap.Render(visual);
            return bitmap;

The rendering is certainly working. The data binding/propertychanged event is also firing correctly, so it seems to me like some kind of caching is going on behind the scenes.

Anyone have any ideas?

Answer 1

Hi,
I cannot reproduce your problem with your code in my demo:

<Window x:Class="WpfApp1.Window044xaml"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp044"
        mc:Ignorable="d"
        Title="sleepyVision_230304" Height="200" Width="400">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <Grid>
    <Grid.RowDefinitions>
      <RowDefinition/>
      <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <Image Source="{Binding PreviewImage}" Margin="10"/>
    <Button Grid.Row="1" Content="Load" Command="{Binding}" Margin="5"/>
  </Grid>
</Window>

ViewModel:

using System;
using System.ComponentModel;
using System.Globalization;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Media.Media3D;

namespace WpfApp044
{
	public class ViewModel : INotifyPropertyChanged, ICommand
	{
		private ImageSource? _previewImage;
		public ImageSource? PreviewImage
		{
			get => _previewImage;
			set
			{
				_previewImage = value;
				NotifyPropertyChanged(nameof(PreviewImage));
			}
		}

		public bool CanExecute(object? parameter) => true;

		public void Execute(object? parameter)
		{
			using (System.IO.MemoryStream ms = new System.IO.MemoryStream())
			{
				FormattedText text = new FormattedText(DateTime.Now.ToString("hh:mm:ss.fff"),
								new CultureInfo("en-us"),
								FlowDirection.LeftToRight,
								new Typeface(new FontFamily("Arial"), FontStyles.Normal, FontWeights.Normal, new FontStretch()),
								30,
								Brushes.Red);

				DrawingVisual visual = new DrawingVisual();
				DrawingContext drawingContext = visual.RenderOpen();
				drawingContext.DrawText(text, new Point(2, 2));
				drawingContext.Close();

				int width = 300;
				int height = 300;

				var bitmap = new RenderTargetBitmap(
					width,
					height,
					96,
					96,
					PixelFormats.Default);
				bitmap.Render(visual);
				PreviewImage = bitmap;
			}
		}

		// INotifyPropertyChanged
#pragma warning disable CS8612 // Nullability of reference types in type doesn't match implicitly implemented member.
		public event PropertyChangedEventHandler PropertyChanged;
		public event EventHandler? CanExecuteChanged;
		public void NotifyPropertyChanged([CallerMemberName] string info = "") =>
				PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(info));
	}
}

Result:

Answer 2

Hi,
I cannot reproduce your problem with your code in my demo: