Question: Getting the display information on Win32

I am trying to convert an existing UWP project to WinUI/Win32.

The project uses DisplayInformation.GetForCurrentView() in one of the of the controls to get DPI information, which then results in the following error:

Windows.Graphics.Display: GetForCurrentView must be called on a thread that is associated with a CoreWindow.

CoreWindow is a UWP thing, so I’m assuming this function is not supported on Win32. What would be the correct way to get the current DPI (and detect changes to the DPI) from a WinUI/Win32 app?

[rick-palmsens]

@marb2000 Sorry for the late response, but using the Loaded/Unloaded events is indeed a good way ensure the control is attached to a XamlRoot.

[thomasclaudiushuber]

Hi @rick-palmsens ,

there is currently no API exposed to get DPI information. But you can go via Win32 PInvoke right now.

Steps to try it:

1. Create a new WinUI in Desktop project
2. Add a reference to the NuGet package PInvoke.Win32 to your WinUI desktop project.
3. Implement the MainWindow.xaml.cs file like this:

public sealed partial class MainWindow : Window
{
  public MainWindow()
  {
    this.InitializeComponent();
  }

  private void myButton_Click(object sender, RoutedEventArgs e)
  {
    var dpiScalingFactor = GetDpiScalingFactor();
    var dpiPercent = $"{dpiScalingFactor * 100}%";
    myButton.Content = $"DPI Setting: {dpiPercent}";
  }

  private double GetDpiScalingFactor()
  {
    var windowNative = this.As<IWindowNative>();
    var hwnd = windowNative.WindowHandle;

    var dpi = PInvoke.User32.GetDpiForWindow(hwnd);
    return (float)dpi / 96;
  }
}

[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("EECDBF0E-BAE9-4CB6-A68E-9598E1CB57BB")]
internal interface IWindowNative
{
  IntPtr WindowHandle { get; }
}

1. The generic As<T> method that I call on the Window instance is an extension method. You need to add the following using directive to get that extension method:

using WinRT;

To make it complete, these are all using directives that I have in the MainWindow.xaml.cs file:

using Microsoft.UI.Xaml;
using System;
using System.Runtime.InteropServices;
using WinRT;

That’s it.

Take also a look at the WinUI 3 Demos repository from @marb2000 that contains more Window interop scenarios. There is also a center window branch that shows you how to center a WinUI desktop window: https://github.com/microsoft/WinUI-3-Demos/tree/CenterWindow/src/Build2020Demo