PageSetupDialog incorrectly converts Margin units from pageSetting.Margins to dialogSettings.rtMargin

[MykolaKovalchuk]

.NET version

10.0.201

Did it work in .NET Framework?

Yes

Did it work in any of the earlier releases of .NET Core or .NET 5+?

Did not test.

Issue description

Method PageSetupDialog.RunDialog(IntPtr hwndOwner) incorrectly detects toUnit based on Windows regional settings.

In Windows for my user account, Language & region > Number settings, Measurement System is set to Metric. When I open PageSetupDialog from my .Net application, it displays Margins group with caption "Margins (millimeters)".
Every time I open this dialog, don't change anything, and close it - margin values are reduced until they become 0.

After analyzing PageSetupDialog.cs and PrinterUnitConvert.cs, checking values of PrintDocument.DefaultPageSettings.Margins and what is shown in the dialog fields, and doing manual calculations, I see that:

1. In method RunDialog original values from pageSettings.Margins converted using toUnit == PrinterUnit.ThousandthsOfAnInch.
2. In method UpdateSettings new values converted to pageSettings.Margins using fromUnit == PrinterUnit.HundredthsOfAMillimeter.

Comparing with source code for .Net Framework 4.8, I see that there is difference in getting locale info:

• .Net Framework 4.8 checks NativeMethods.LOCALE_USER_DEFAULT - user level;
• .Net 10 checks PInvoke.LOCALE_NAME_SYSTEM_DEFAULT - system level;

I suspect the issues is here - logic of PageSetupDialog checks system level locale settings, but dialog itself when run by Windows uses user level locale settings (and sets flag PSD_INHUNDREDTHSOFMILLIMETERS to not 0), so it results in display discrepancy and different conversion units used with original and new values.

Steps to reproduce

Set Language & region > Number settings, Measurement System on system level to U.S.,
and on user level to Metric.

Additional note: I set PageSetupDialog.EnableMetric = true;, but it does not work with both true and false.

[LeafShi1]

Thanks for the detailed report.

This looks like a WinForms PageSetupDialog bug caused by inconsistent unit conversion when system-level and user-level measurement settings differ.

On our machine we cannot reproduce the issue because both system and user resolve to Metric
(verified via LOCALE_IMEASURE VerifyMeasurementSettings.zip), so the mismatch condition required to trigger the issue is not present.

However, based on the analysis here, when system and user measurement settings do differ (e.g. system=US, user=Metric),
PageSetupDialog may initialize and save margins using different units, causing margins to shrink on every open/close
even without user changes.

@KlausLoeffelmann @merriemcgaw This regression may be related to change ca89b0a in PageSetupDialog? Please advise on next steps. This is a DemoProject.zip

[MykolaKovalchuk]

Just want to confirm, that I tested calling GetLocaleInfoEx to check measure units for system default and user default, and it returns different values on my machine.
Test code (simple console app, file Program.cs):

using System.Runtime.InteropServices;

// Constants from .NET / .NET Framework System.Windows.Forms source code
const uint LOCALE_IMEASURE = 0x0000000D;
const string LOCALE_NAME_SYSTEM_DEFAULT = "!x-sys-default-locale";
const string LOCALE_USER_DEFAULT = "";   // empty string = user default

[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
static extern int GetLocaleInfoEx(string lpLocaleName, uint LCType, System.Text.StringBuilder lpLCData, int cchData);

static string GetMeasureUnit(string localeName, string localeLabel)
{
    var sb = new System.Text.StringBuilder(10);
    int result = GetLocaleInfoEx(localeName, LOCALE_IMEASURE, sb, sb.Capacity);
    if (result > 0)
    {
        string value = sb.ToString();
        string description = value == "0" ? "Metric" : value == "1" ? "US (Imperial)" : $"Unknown ({value})";
        return $"{localeLabel}: {description} (raw value = {value})";
    }
    else
    {
        int error = Marshal.GetLastWin32Error();
        return $"{localeLabel}: ERROR (Win32 error = {error})";
    }
}

Console.WriteLine(GetMeasureUnit(LOCALE_NAME_SYSTEM_DEFAULT, "System default (LOCALE_NAME_SYSTEM_DEFAULT)"));
Console.WriteLine(GetMeasureUnit(LOCALE_USER_DEFAULT,        "User default   (LOCALE_USER_DEFAULT)       "));

result:

System default (LOCALE_NAME_SYSTEM_DEFAULT): US (Imperial) (raw value = 1)
User default   (LOCALE_USER_DEFAULT)       : Metric (raw value = 0)

Also I tested attached above WinForm DemoProject, and it has same behavior as I described.

[LeafShi1]

This issue may be related to OS/print pipeline/driver/default settings combination.
When my return result is

System default (LOCALE_NAME_SYSTEM_DEFAULT): US (Imperial) (raw value = 1)
User default   (LOCALE_USER_DEFAULT)       : Metric (raw value = 0)

Page margins are displayed in inches (this also applies to .NET Framework projects), and the UpdateSettings method cannot reach PrinterUnit.HundredthsOfAMillimeter, so this problem cannot be reproduced.

After debugging, I can confirm the following:

• RunDialog input flags: dialogSettings.Flags = GetFlags() does not include any unit-specific flag (neither PSD_INTHOUSANDTHSOFINCHES nor PSD_INHUNDREDTHSOFMILLIMETERS), so WinForms is not explicitly forcing the native dialog into inch or mm mode via input flags.
• GetLocaleInfoEx(LOCALE_NAME_SYSTEM_DEFAULT, LOCALE_IMEASURE, ...) in RunDialog: this only affects the toUnit used by WinForms to convert PageSettings.Margins/MinMargins into dialogSettings.rtMargin/rtMinMargin before calling PageSetupDlg (i.e., the numeric scale WinForms passes to the native dialog).
• UpdateSettings unit selection: UpdateSettings chooses fromUnit based on the returned data.Flags from PageSetupDlg (e.g., PSD_INTHOUSANDTHSOFINCHES vs PSD_INHUNDREDTHSOFMILLIMETERS). In other words, the final inch/mm mode is determined by what the native Page Setup dialog returns, and WinForms interprets/converts the values accordingly.

[MykolaKovalchuk]

I might need to use custom page setup dialog.