Setting Background color on Canvas

I am trying to make a Fullscreen window that displays a solid white image in wpf.
The window contains a Canvas with a Black frame that indicates the area that is 25% of the imagesizes length/width from the edge. The resolution may vary, so I need to create it at runtime.

The issue is that no matter what I try to do, I end up with a colmpletely black window.

To reproduce the issue in code behind: Make new wpf application in VisualStudio, and add a button:
<Button Click="Button_Click"/>
to the Window.

then in code behind:

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            double screenWidth = SystemParameters.FullPrimaryScreenWidth; //Get Monitor width
            double screenHeight = SystemParameters.FullPrimaryScreenHeight; //Get Monitor height

            System.Windows.Window window = new System.Windows.Window(); //new window
            window.WindowStyle = WindowStyle.None; //No style
            window.WindowState = WindowState.Maximized; //Fullscreen

            window.Background = Brushes.White;//White Background!
            window.MouseMove += new System.Windows.Input.MouseEventHandler(window_MouseMove); //Event checking mousePos

            int frameWidth = (int)(screenWidth * 0.25);
            int frameHeight = (int)(screenHeight * 0.25);
            int frameX = (int)((screenWidth - frameWidth) / 2);
            int frameY = (int)((screenHeight - frameHeight) / 2);
            ///Generate a frame 25% in from the edges


            Canvas canvas = new Canvas();
            canvas.Width = screenWidth;
            canvas.Height = screenHeight;
            canvas.Background = Brushes.White; //Canvas White Background!
            canvas.Children.Add(new System.Windows.Shapes.Rectangle
            {
                Width = frameWidth,
                Height = frameHeight,
                Stroke = Brushes.Black,
                StrokeThickness = 3,
                Margin = new Thickness(frameX, frameY, 0, 0)
            });
            //Create the Canvas with the correct resolution and set the color to white, then add a black frame


            RenderTargetBitmap bitmap = new RenderTargetBitmap((int)screenWidth, (int)screenHeight, 96, 96, PixelFormats.Pbgra32);
            
            bitmap.Render(canvas); //Set bitmap to render the canvas

            ImageBrush brush = new ImageBrush(bitmap);
            
            brush.TileMode = TileMode.None;//If brush has black border and is tiled to small, the borders could create a black image

            window.Background = brush; //Set the window background to white again

            window.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity)); //Force the window to perform layout and measure pass.

            window.ShowDialog(); // Open the White window with a White Canvas: Everything is black
        }
        private void window_MouseMove(object sender, MouseEventArgs e)
        {
            System.Windows.Point mousePosition = e.GetPosition((IInputElement)sender);
            Console.WriteLine($"MousePosition = {mousePosition}");
        }

This code should have showed me a white image, but everthing is black

I have tried to force proper display of canvas by setting brush TileMode.None.
I have also tried using DrawingVisual instead of Canvas

I am not sure how your Window is created but you can try creating a grid inside your window.

<Window>
<Grid x:Name="MainGrid">

</Grid>
</Window>

and then on your Button_Click code you can try

MainGrid.Children.Add(canvas);

This is what I expect is happening, but I'm not a wpf expert, so take it with a grain of salt.

When you render the canvas to a bitmap no background will rendered. So the background of the bitmap should be transparent black. When you set this bitmap as the rendering brush of the window background the renderer cannot cannot output transparent, since there is nothing "beneath" the window. So it either ignores the transparency of the bitmap, or blends the bitmap color with black. In either case the result will be black.

The simplest solution is to just skip the bitmap step and add the canvas to the window directly. As shown in Izanagis answer.

If this is for some reason not desired a workaround might be to add an image to the window and set the bitmap as the image source. Or possibly just add a grid and set the background brush of the grid instead of the window. But would only do this if you have some special reason to use a bitmap as an intermediate value. If this is the case I would probably consider using Graphics.FromImage instead, since I find that method easier when drawing graphics.