How do I detect if the current Windows is installed in UEFI or legacy mode

Question

My Goal is to simply get, if the current Windows installation is in UEFI or legacy mode. Manually I could just run msinfo32 and look at the "Bios-Mode" row. My current approach is by checking if the "HKLM\SYSTEM\CurrentControlSet\Control\SecureBoot" key exists, but I'm not sure if that's a good approach.

I have already tried looking at quite some WMI classes and properties but neither of them had the information I needed. I read that you could use GetFirmwareType() but it seems like that had been introduced in Windows 8 and I want it to run on Windows 7 as well. I also tried the method microsoft gave me, calling GetFirmwareEnvironmentVariableA with a dummy variable and a dummy namespace.

public const int ERROR_INVALID_FUNCTION = 1;

[DllImport("kernel32.dll",
    EntryPoint = "GetFirmwareEnvironmentVariableA",
    SetLastError = true,
    CharSet = CharSet.Unicode,
    ExactSpelling = true,
    CallingConvention = CallingConvention.StdCall)]
public static extern int GetFirmwareType(string lpName, string lpGUID, IntPtr pBuffer, uint size);

public static bool IsWindowsUEFI()
{
    // Call the function with a dummy variable name and a dummy variable namespace (function will fail because these don't exist.)
    GetFirmwareType("", "{00000000-0000-0000-0000-000000000000}", IntPtr.Zero, 0);

    if (Marshal.GetLastWin32Error() == ERROR_INVALID_FUNCTION)
    {
        // Calling the function threw an ERROR_INVALID_FUNCTION win32 error, which gets thrown if either
        // - The mainboard doesn't support UEFI and/or
        // - Windows is installed in legacy BIOS mode
        return false;
    }
    else
    {
        // If the system supports UEFI and Windows is installed in UEFI mode it doesn't throw the above error, but a more specific UEFI error
        return true;
    }
}

It told me when I get ERROR_INVALID_FUNCTION, I am running legacy, otherwise it will return a different, more specific error. All I ever got from that code was ERROR_INVALID_PARAMETER on any type of system and I don't know where my mistake is.

Answer 1

That was just a Typo.

3   [DllImport("kernel32.dll",
4 >     EntryPoint = "GetFirmwareEnvironmentVariableA",
5       SetLastError = true,                        ^

Replace the A pointed by the > and the ^ with a W.