System.Windows.PresentationSource.FromVisual(...) returned null in WPF-CodePudding

I am programming a plugin for AutoCAD using WPF. This is constructor for WPF window dialog. I caught this problem when defining dpiFactor variable:

public TentCreationDialog(Commands cmd)
        {
            InitializeComponent();

            ...

            exControl = new ExControl(this.textBoxAwingWidth, "AwingCreation");

            ...

            double dpiFactor = System.Windows.PresentationSource.FromVisual(this).CompositionTarget.TransformToDevice.M11;
            UserInterfaceCustomScale(dpiFactor);
        }

Does anybody know what could cause this?

CodePudding user response：

This is because the Visual is not loaded yet, thus, you can fix it by executing FromVisual() call inside the Loaded event like this

Loaded  = (sender, args) => {
   double dpiFactor = System.Windows.PresentationSource.FromVisual(this)?.CompositionTarget.TransformToDevice.M11 ?? 0;
   UserInterfaceCustomScale(dpiFactor);
};

CodePudding user response：

You can get the system DPI scale in Window's constructor.

public MainWindow()
{
    InitializeComponent();
    double dpiScale = VisualTreeHelper.GetDpi(this).PixelsPerDip;
}

However, please note it does not necessarily match actual DPI scale of Window under Per-Monitor DPI environment.