How Can I track the Last DPI which is sent by WM_DPICHANGED in WPF?

Hello @MERUN KUMAR MAITY

Let’s break down your requirements and address them step by step.

Tracking the Last DPI Sent by WM_DPICHANGED:

• The WM_DPICHANGED message is sent to a window when the DPI of the monitor hosting the window changes. The current DPI for a window always equals the last DPI sent by WM_DPICHANGED.
  • To track the last DPI, you can modify your existing code. Here’s how you can retrieve the monitor-dependent DPI while handling WM_DPICHANGED:

private const int WM_DPICHANGED = 0x02E0;

// Inside your WndProc method:
private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == WM_DPICHANGED)
    {
        // Get the monitor handle
        IntPtr hMonitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONULL);
        MONITORINFOEX mix = new MONITORINFOEX();
        mix.cbSize = Marshal.SizeOf(mix);
        GetMonitorInfo(hMonitor, ref mix);

        // Get the DPI from the monitor
        DEVMODE dm = new DEVMODE();
        dm.dmSize = (short)Marshal.SizeOf(dm);
        EnumDisplaySettings(mix.szDevice, ENUM_CURRENT_SETTINGS, ref dm);

        // Now 'dm.dmPelsWidth' and 'dm.dmPelsHeight' contain the DPI
        // You can use this information for scaling your application.
        // ...
        
        handled = true;
    }
    return IntPtr.Zero;
}

• This modification will allow you to retrieve the DPI when WM_DPICHANGED is received.

Applying the Last DPI for WPF Interface Scaling:

• Once you have the DPI value, you can use it to scale your WPF application. You can adjust your UI elements based on this DPI value.
• For example, you can set the LayoutTransform property of your UI elements to scale them proportionally based on the DPI.
  • Remember that you’ll need to handle this scaling logic in your WPF controls or views.

Windows 11 Environment:

• While some older solutions may not work in Windows 11, the approach I provided above should be compatible with Windows 11 as well.
  • Make sure to test your application thoroughly to ensure it behaves as expected on both Windows 10 and Windows 11.

Feel free to integrate the modified code snippet into your application

I hope that this response has addressed your query and helped you overcome your challenges. If so, please mark this response as Answered. This will not only acknowledge our efforts, but also assist other community members who may be looking for similar solutions.

Hi,@MERUN KUMAR MAITY. Welcome to Microsoft Q&A. 

The WM_DPICHANGED message will not have DPI information when a window is newly created.

The WM_DPICHANGED message is sent to a window when the DPI setting for a window changes, typically as a result of the user moving the window to a monitor with a different DPI setting or changing the DPI setting on the current monitor.

So for the initial DPI acquisition, choose GetDpiForWindow and GetDeviceCaps

1.GetDpiForWindow is used for modern Windows versions.

2.For older Windows versions, GetDeviceCaps retrieves the DPI.

You can refer to the following code to achieve your desired effect

    public partial class MainWindow : Window

    {

        private HwndSource hwndSource;

        // Win32 constants

        private const int WM_DPICHANGED = 0x02E0;

 

        // Importing Win32 functions   

        [DllImport("user32.dll")]

        private static extern int GetDpiForWindow(IntPtr hwnd);

 

        [DllImport("user32.dll")]

        private static extern IntPtr GetDC(IntPtr hwnd);

 

        [DllImport("gdi32.dll")]

        private static extern int GetDeviceCaps(IntPtr hdc, int nIndex);

 

        [DllImport("user32.dll")]

        private static extern int ReleaseDC(IntPtr hwnd, IntPtr hdc);

 

        private const int LOGPIXELSX = 88;

 

        public MainWindow() { InitializeComponent(); }

 

        protected override void OnSourceInitialized(EventArgs e)

        {

            base.OnSourceInitialized(e);

 

            // Hook into the window message processing

            hwndSource = PresentationSource.FromVisual(this) as HwndSource;

            if (hwndSource != null)

            { hwndSource.AddHook(WndProc); }

            // Get the initial DPI and scale the UI

            IntPtr hwnd = new WindowInteropHelper(this).Handle;

            int dpi = GetDpiForWindow(hwnd);

            if (dpi == 0) // Fallback for older Windows versions

            {

                IntPtr hdc = GetDC(IntPtr.Zero);

                dpi = GetDeviceCaps(hdc, LOGPIXELSX);

                ReleaseDC(IntPtr.Zero, hdc);

            }

            ApplyDpiScaling(dpi);

        }

 

 

 

        private void ApplyDpiScaling(int dpi)

        {

            double scalingFactor = dpi / 96.0; // 96 is the default DPI

            this.Width = this.Width * scalingFactor;

            this.Height = this.Height * scalingFactor;

            MyButton.LayoutTransform = new ScaleTransform(scalingFactor, scalingFactor);

 

        }

 

        private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)

        {

            if (msg == WM_DPICHANGED)

            {

                int newDpi = wParam.ToInt32() & 0xFFFF; // LOWORD of wParam

                ApplyDpiScaling(newDpi);

                handled = true;

            }

            return IntPtr.Zero;

        }

    }

    <Grid>

        <Button  Content="HAHA" x:Name="MyButton" Width="100" Height="100"></Button>

    </Grid>

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.