c# - How to convert a coord in native DPI to one in WPF logic DPI? - Stack Overflow

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In the project I contribute to (C#, WPF) I need to position multiple OSD windows on multiple screens, e.g. top-left, center,bottom-right, etc.

I use the following code to get all screens' workarea and dpi.

public readonly struct ScreenInfo(Rect workArea, uint dpiX, uint dpiY, bool isPrimary)
{
    public Rect WorkArea { get; } = workArea;
    public uint DpiX { get; } = dpiX;
    public uint DpiY { get; } = dpiY;
    public bool IsPrimary { get; } = isPrimary;
}

public static void UpdateScreenInfos()
{
    Screens.Clear();
    EnumDisplayMonitors(IntPtr.Zero, IntPtr.Zero, MonitorEnumProc, IntPtr.Zero);
}

[DllImport("user32.dll")]
private static extern bool EnumDisplayMonitors(IntPtr hdc, IntPtr lprcClip, EnumDisplayMonitorsDelegate lpfnEnum, IntPtr dwData);

private delegate bool EnumDisplayMonitorsDelegate(HMONITOR hMonitor, IntPtr hdcMonitor, ref Rect lprcMonitor, IntPtr dwData);

private static bool MonitorEnumProc(HMONITOR hMonitor, IntPtr hdcMonitor, ref Rect lprcMonitor, IntPtr dwData)
{
    MONITORINFO monitorInfo = new() { cbSize = (uint)Marshal.SizeOf<MONITORINFO>() };

    if (!PInvoke.GetMonitorInfo(hMonitor, ref monitorInfo))
        return true;

    if (!PInvoke.GetDpiForMonitor(hMonitor, MONITOR_DPI_TYPE.MDT_EFFECTIVE_DPI, out var dpiX, out var dpiY).Succeeded)
        return true;

    var workArea = monitorInfo.rcWork;
    var multiplierX = 96d / dpiX;
    var multiplierY = 96d / dpiY;

    Screens.Add(new ScreenInfo(
        new Rect(workArea.X * multiplierX, workArea.Y * multiplierY, workArea.Width * multiplierX, workArea.Height * multiplierY),
        dpiX, dpiY,
        (monitorInfo.dwFlags & PInvoke.MONITORINFOF_PRIMARY) != 0
    ));

    return true;
}

The codes above work pretty fine when zoom ratios of all screens are the same. But if I have for example two screens: one 2K resolution with 150% zoom ratio, the other 4K with 125%, workarea calculated by MonitorEnumProc has an offset on all screens except the primary one.

My question is, is there a safe and reliable solution to convert those coord in native DPI into logic DPI in WPF? It should work under multi-screen environment, also under multi-zoom-ratios.

In the project I contribute to (C#, WPF) I need to position multiple OSD windows on multiple screens, e.g. top-left, center,bottom-right, etc.

I use the following code to get all screens' workarea and dpi.

public readonly struct ScreenInfo(Rect workArea, uint dpiX, uint dpiY, bool isPrimary)
{
    public Rect WorkArea { get; } = workArea;
    public uint DpiX { get; } = dpiX;
    public uint DpiY { get; } = dpiY;
    public bool IsPrimary { get; } = isPrimary;
}

public static void UpdateScreenInfos()
{
    Screens.Clear();
    EnumDisplayMonitors(IntPtr.Zero, IntPtr.Zero, MonitorEnumProc, IntPtr.Zero);
}

[DllImport("user32.dll")]
private static extern bool EnumDisplayMonitors(IntPtr hdc, IntPtr lprcClip, EnumDisplayMonitorsDelegate lpfnEnum, IntPtr dwData);

private delegate bool EnumDisplayMonitorsDelegate(HMONITOR hMonitor, IntPtr hdcMonitor, ref Rect lprcMonitor, IntPtr dwData);

private static bool MonitorEnumProc(HMONITOR hMonitor, IntPtr hdcMonitor, ref Rect lprcMonitor, IntPtr dwData)
{
    MONITORINFO monitorInfo = new() { cbSize = (uint)Marshal.SizeOf<MONITORINFO>() };

    if (!PInvoke.GetMonitorInfo(hMonitor, ref monitorInfo))
        return true;

    if (!PInvoke.GetDpiForMonitor(hMonitor, MONITOR_DPI_TYPE.MDT_EFFECTIVE_DPI, out var dpiX, out var dpiY).Succeeded)
        return true;

    var workArea = monitorInfo.rcWork;
    var multiplierX = 96d / dpiX;
    var multiplierY = 96d / dpiY;

    Screens.Add(new ScreenInfo(
        new Rect(workArea.X * multiplierX, workArea.Y * multiplierY, workArea.Width * multiplierX, workArea.Height * multiplierY),
        dpiX, dpiY,
        (monitorInfo.dwFlags & PInvoke.MONITORINFOF_PRIMARY) != 0
    ));

    return true;
}

The codes above work pretty fine when zoom ratios of all screens are the same. But if I have for example two screens: one 2K resolution with 150% zoom ratio, the other 4K with 125%, workarea calculated by MonitorEnumProc has an offset on all screens except the primary one.

My question is, is there a safe and reliable solution to convert those coord in native DPI into logic DPI in WPF? It should work under multi-screen environment, also under multi-zoom-ratios.

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