testing dpi changes

[trebahl]

I would like to write automated tests at various dpi settings; also I would like to change the dpi during the test. Changing the dpi of the machine running the tests is cumbersome and fragile, so I would prefer to achieve the same effect inside the test process. Is there a way?

I was thinking of using reflection to call HwndSource.ChangeDpi. Is that a viable solution? Or does something in wpf's guts call native functions that will still be reporting the real dpi?

Which value should I pass as suggestedRect to the constructor of HwndDpiChangedEventArgs ?

[miloush]

That looks like a reasonable thing to try. You can pass Rect.Empty for the suggestedRect.

[miloush]

Technically you could send WM_DPICHANGED to the window if that sounded better than reflection.

[trebahl]

Thanks. With Rect.Empty, the window disappears from the screen and the code gets stuck in an infinite loop. I tried the following instead, which seems to work:

            var before = VisualTreeHelper.GetDpi(w);
            var pos = w.PointToScreen(new Point(0, 0));
            var sz = w.PointToScreen(new Point(w.ActualWidth, w.ActualHeight)) - pos;
            var mul = scale / before.DpiScaleY;
            sz = mul * sz;
            var r = new Rect(pos, new Size(Math.Ceiling(sz.X), Math.Ceiling(sz.Y))); // used in HwndDpiChangedEventArgs

[miloush]

Interesting, the code explicitly states that should be valid:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/DpiChangedEventArgs.cs

Lines 63 to 69 in d73dd1d

	/// If <paramref name="suggestedRect"/> is <see cref="Rect.Empty"/>, this constructor will act as if
	/// no suggestion for the new client rectangle has been provided.
	/// When this happens, DPI-change processing logic in <see cref="HwndTarget.OnMonitorDPIChanged"/> will
	/// not attempt to resize the window. It will assume that the top-level WM_DPICHANGED resulted
	/// in a cascaded set of changes that adjusted the client areas for all child-windows appropriately
	/// in response to DPI changes. <see cref="HwndTarget.OnMonitorDPIChanged"/> will simply
	/// invalidate and repaint the window if this happens.

Oh well, but anyway yes if you want the window to resize too, you do need to provide the new Rect for it apparently.

[trebahl]

I got confused, it's not an infinite loop, it's just the normal event loop waiting for an event. It seems the window disappears because its size becomes (0,0).

HwndSource.OnDpiChanged calls HwndTarget.OnDpiChanged which calls UnsafeNativeMethods.SetWindowPos with the supplied rectangle, without checking if it is empty.