对于和一般的WinAPI交互来说，没必要这么麻烦，可以使用StructLayout和MarshalAs解决：

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using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace ConsoleApplication5
{
    class Program
    {
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public struct RtlOSVersionInfoExW
        {
            public uint dwOSVersionInfoSize;
            public uint dwMajorVersion;
            public uint dwMinorVersion;
            public uint dwBuildNumber;
            public uint dwPlatformId;
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
            public string szCSDVersion;
            public ushort wServicePackMajor;
            public ushort wServicePackMinor;
            public ushort wSuiteMask;
            public byte wProductType;
            public byte wReserved;
        }

        [DllImport("ntdll.dll")]
        public static extern bool RtlGetVersion(ref RtlOSVersionInfoExW osver);

        static void Main(string[] args)
        {
            RtlOSVersionInfoExW osvex = new RtlOSVersionInfoExW();
            osvex.dwOSVersionInfoSize = (uint)Marshal.SizeOf(osvex);
            RtlGetVersion(ref osvex);
            Console.WriteLine(osvex.dwMajorVersion);
            Console.WriteLine(osvex.dwMinorVersion);
            Console.WriteLine(osvex.dwBuildNumber);
            Console.WriteLine(osvex.dwPlatformId);
            Console.WriteLine(osvex.szCSDVersion);
            Console.WriteLine(osvex.wServicePackMajor);
            Console.WriteLine(osvex.wServicePackMinor);
            Console.WriteLine(osvex.wSuiteMask);
            Console.WriteLine(osvex.wProductType);
            Console.WriteLine(osvex.wReserved);
        }
    }
}