Total Memory in C# Interop

GetTotalMemory.cs

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Runtime.InteropServices;

namespace memory_test
{
    [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]
    class MEMORYSTATUSEX
    {
        public uint dwLength;
        public uint dwMemoryLoad;
        public ulong ullTotalPhys;
        public ulong ullAvailPhys;
        public ulong ullTotalPageFile;
        public ulong ullAvailPageFile;
        public ulong ullTotalVirtual;
        public ulong ullAvailVirtual;
        public ulong ullAvailExtendedVirtual;
        public MEMORYSTATUSEX()
        {
            this.dwLength = (uint) Marshal.SizeOf(typeof( MEMORYSTATUSEX ));
        }
    }
    static class Program
    {
        [DllImport("kernel32.dll", SetLastError=true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool GetPhysicallyInstalledSystemMemory(out ulong MemoryInKilobytes);

        [DllImport("kernel32.dll", SetLastError=true, EntryPoint="GetPhysicallyInstalledSystemMemory")]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool GetPhysicallyInstalledSystemMemory_long(out long MemoryInKilobytes);

        [return: MarshalAs(UnmanagedType.Bool)]
        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        static extern bool GlobalMemoryStatusEx( [In,Out] MEMORYSTATUSEX lpBuffer);

        static void Main(string[] args)
        {
            Console.WriteLine("Getting data using GetPhysicallyInstalledSystemMemory");
            ulong memoryInKilobytes;
            GetPhysicallyInstalledSystemMemory(out memoryInKilobytes);
            Console.WriteLine((memoryInKilobytes / 1024) + "MB of RAM installed - ulong.");
            long memoryInKB;
            GetPhysicallyInstalledSystemMemory_long(out memoryInKB);
            Console.WriteLine((memoryInKB / 1024) + "MB of RAM installed - long.");

            Console.WriteLine("Getting data using GlobalMemoryStatusEx");
            var mstatex = new MEMORYSTATUSEX();
            if (GlobalMemoryStatusEx(mstatex))
            {
                Console.WriteLine((mstatex.ullTotalPhys / (1024*1024)) + "MB of Total Phys.");
                Console.WriteLine((mstatex.ullAvailPhys / (1024*1024)) + "MB of Available Phys.");
                Console.WriteLine((mstatex.ullTotalVirtual / (1024*1024)) + "MB of Total Virtual.");
                //etc.. Repeat for other structure members        
            }
            else
            {
                // Use a more appropriate Exception Type. 'Exception' should almost never be thrown
                throw new Exception("Unable to initalize the GlobalMemoryStatusEx API");
            }
        }
    }
}

Here is a sample output in my machine with 32GB of RAM.

Getting data using GetPhysicallyInstalledSystemMemory
32768MB of RAM installed - ulong.
32768MB of RAM installed - long.
Getting data using GlobalMemoryStatusEx
32354MB of Total Phys installed.
8516MB of Available Phys installed.
134217727MB of Total Virtual installed.

As you can see, the GlobalMemoryStatusEx reports different memory. And this is also confirmed by this documentation.