c# - What does "Beta: Use Unicode UTF-8 for worldwide language support" actually do?

Question

In some Windows 10 builds (insiders starting April 2018 and also "normal" 1903) there is a new option called "Beta: Use Unicode UTF-8 for worldwide language support".

You can see this option by going to Settings and then: All Settings -> Time & Language -> Language -> "Administrative Language Settings"

This is what it looks like:

When this checkbox is checked I observe some irregularities (below) and I would like to know what exactly this checkbox does and why the below happens.

Create a brand new Windows Forms application in your Visual Studio 2019. On the main form specify the Paint even handler as follows:

private void Form1_Paint(object sender, PaintEventArgs e)
{
    Font buttonFont = new Font("Webdings", 9.25f);
    TextRenderer.DrawText(e.Graphics, "0r", buttonFont, new Point(), Color.Black);
}

Run the program, here is what you will see if the checkbox is NOT checked:

However, if you check the checkbox (and reboot as asked) this changes to:

You can look up Webdings font on Wikipedia. According to character table given, the codes for these two characters are "U0001F5D5U0001F5D9". If I use them instead of "0r" it works with the checkbox checked but without the checkbox checked it now looks like this:

I would like to find a solution that always works that is regardless whether the box checked or unchecked.

Can this be done?

Answer 1

You can see it in ProcMon. It seems to set the REG_SZ values ACP, MACCP, and OEMCP in HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlNlsCodePage to 65001.

I'm not entirely sure but it might be related to the variable gAnsiCodePage in KernelBase.dll, which GetACP reads. If you really want to, you might be able to change it dynamically for your program regardless of the system setting by dynamically disassembling GetACP to find the instruction sequence that reads gAnsiCodePage and obtaining a pointer to it, then updating the variable directly.

(Actually, I see references to an undocumented function named SetCPGlobal that would've done the job, but I can't find that function on my system. Not sure if it still exists.)