RTL_OSVERSIONINFOEX uses incorrect charset

[kpreisser]

Hi,

this is a similar issue as CommunityToolkit/WindowsCommunityToolkit#2095.

On Windows, RuntimeInformation.OSDescription calls Interop.RtlGetVersion() using the RTL_OSVERSIONINFOEX structure, which is currently defined as follows:
https://github.com/dotnet/corefx/blob/a75d30306975040b0e22390d04a3b3de094d1817/src/Common/src/Interop/Windows/NtDll/Interop.RTL_OSVERSIONINFOEX.cs#L12-L22

Note that the CharSet is not specified, which means the runtime will assume a ANSI charset, whereas the native RtlGetVersion() expects Unicode. This means the szCSDVersion field will not correctly be marshalled and Marshal.SizeOf<NtDll.RTL_OSVERSIONINFOEX>() returns 148 instead of 276.

The declaration should be changed to:

        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        internal struct RTL_OSVERSIONINFOEX
        {
            // ...
        }

Code

Run the following C# program with .NET Core on Windows 7 SP1:

using System;
using System.Runtime.InteropServices;

class DisplayOSDescription
{
    static void Main()
    {
        Console.WriteLine($"OSDescription: '{RuntimeInformation.OSDescription}'");
        Console.ReadLine();
    }
}

Actual Output:

OSDescription: 'Microsoft Windows 6.1.7601 S'

Expected Output:

OSDescription: 'Microsoft Windows 6.1.7601 Service Pack 1'

Thanks!