WPF, Forms: Get the scaling/DPI scale factor on the device currently the application without the need of any UI elements using reflection. Useful for GDI stuff where you want to predict the composition.

dpiscale.cs

    public static class DpiScale
    {
        private static readonly float WindowsDefaultDpi = 96.0f;
        private enum Axis {X, Y};
        private static float GetScaleForAxis(Axis axis)
        {
            var axisProperty = axis == Axis.X ? "DpiX" : "Dpi";
            var dpiProperty = typeof(SystemParameters).GetProperty(axisProperty, BindingFlags.NonPublic | BindingFlags.Static);
            var deviceDpi = (int)dpiProperty.GetValue(null, null);

            var dpiScale = deviceDpi / WindowsDefaultDpi;

            return dpiScale;
        }

        public static float CurrentDeviceDpiScaleX()
        {
            return GetScaleForAxis(Axis.X);
        }

        public static float CurrentDeviceDpiScaleY()
        {
            return GetScaleForAxis(Axis.Y);
        }
    }

Example usage:

var dpiScale = DpiScale.CurrentDeviceDpiScaleX();

var expectedFontSize = 42;

int fontSize = Convert.ToInt32(expectedFontSize / dpiScale );

var font = new Font("Segoe UI Emoji", fontSize);