WPF create one Window for each screen, and center them on each screen

Question

EDIT: I solved it. See my fix in the answer below.

I'm working on an application that should open a small popup window on each screen connected to the computer it runs on. Simple enough to do on a single screen (using WindowStartupLocation = CenterScreen), but surprisingly difficult to do on multiple screens.

My current code is this:

        foreach (var s in Screen.AllScreens) //System.Windows.Forms.Screen
        {
            var b = s.Bounds;
            var w = new PopupWindow();

            var oW = w.Width; //Keep track of original size ...
            var oH = w.Height;

            w.Width = 0; //then set the size to 0, to avoid that the 
            w.Height = 0;//popup shows before it is correctly positioned

            w.Show(); //Now show it, so that we can place it (when I
                      //tried to place it before showing, the window
                      //was always repositioned when Show() was called)

            double dpiX = 1, dpiY = 1;
            var presentationsource = PresentationSource.FromVisual(w);

            if (presentationsource != null)
            {
                dpiX = presentationsource.CompositionTarget.TransformToDevice.M11;
                dpiY = presentationsource.CompositionTarget.TransformToDevice.M22;
            }

            var aW = oW*dpiX; //Calculate the actual size of the window
            var aH = oH*dpiY;

            //***** THIS IS WRONG, SEE ANSWER *****
            w.Left = (b.X + (b.Width / dpiX - aW) / 2); //Place it
            w.Top = (b.Y + (b.Height / dpiY - aH) / 2);
            //*************************************

            w.Width = oW; //And set the size back to the original size
            w.Height = oH;
        }

This seems to work only on the primary screen. On the other screens, the windows are not properly centered.

I guess this is because my knowledge of WPF and DPI is very limited, and I'm probably doing something wrong. Could somebody point me in the right direction?

Answer 1

Of course, I managed to solve it after posting it here. Looks like I did something else wrong when I tried to divide the entire location with the DPI, which led me onto the wrong path I posted above.

The correct lines for placing the form should be this (all the other code works):

                w.Left = (b.X + (b.Width - aW) / 2) / dpiX;
                w.Top = (b.Y + (b.Height - aH) / 2) / dpiX;

But, I still think this is a lot of code for a simple task, so if somebody has better ideas, please let me know!

So this is the (working) code I'm using now:

    foreach (var s in Screen.AllScreens) //System.Windows.Forms.Screen
    {
        var b = s.Bounds;
        var w = new PopupWindow();

        var oW = w.Width; //Keep track of original size ...
        var oH = w.Height;

        w.Width = 0; //then set the size to 0, to avoid that the 
        w.Height = 0;//popup shows before it is correctly positioned

        w.Show(); //Now show it, so that we can place it (when I
                  //tried to place it before showing, the window
                  //was always repositioned when Show() was called)

        double dpiX = 1, dpiY = 1;
        var ps = PresentationSource.FromVisual(w);
        if (ps != null)
        {
            dpiX = ps.CompositionTarget.TransformToDevice.M11;
            dpiY = ps.CompositionTarget.TransformToDevice.M22;
        }

        var aW = oW*dpiX; //Calculate the actual size of the window
        var aH = oH*dpiY;

        w.Left = (b.X + (b.Width - aW) / 2) / dpiX;
        w.Top = (b.Y + (b.Height - aH) / 2) / dpiX;

        w.Width = oW; //And set the size back to the original size
        w.Height = oH;
    }

Answer 2

        var centers =
            System.Windows.Forms.Screen.AllScreens.Select(
                s =>
                    new
                    {
                        Left = s.Bounds.X + (s.WorkingArea.Right - s.WorkingArea.Left)/2,
                        Top = s.Bounds.Y + (s.WorkingArea.Bottom - s.WorkingArea.Top)/2
                    });

        foreach (var c in centers)
        {
            var w = new Window1();

            w.Left = c.Left - w.Width/2;
            w.Top = c.Top - w.Height/2;

            w.Show();
        }