How do you get total amount of RAM the computer has?

Add a reference to Microsoft.VisualBasic and a using Microsoft.VisualBasic.Devices;.

The ComputerInfo class has all the information that you need.

The Windows API function GlobalMemoryStatusEx can be called with p/invoke:

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
  private 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(NativeMethods.MEMORYSTATUSEX));
     }
  }


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

Then use like:

ulong installedMemory;
MEMORYSTATUSEX memStatus = new MEMORYSTATUSEX();
if( GlobalMemoryStatusEx( memStatus))
{ 
   installedMemory = memStatus.ullTotalPhys;
}

Or you can use WMI (managed but slower) to query TotalPhysicalMemory in the Win32_ComputerSystem class.

Add a reference to Microsoft.VisualBasic.dll, as someone mentioned above. Then getting total physical memory is as simple as this (yes, I tested it):

static ulong GetTotalMemoryInBytes()
{
    return new Microsoft.VisualBasic.Devices.ComputerInfo().TotalPhysicalMemory;
}

For those who are using .net Core 3.0 there is no need to use PInvoke platform in order to get the available physical memory. The GC class has added a new method GC.GetGCMemoryInfo that returns a GCMemoryInfo Struct with TotalAvailableMemoryBytes as a property. This property returns the total available memory for the garbage collector.(same value as MEMORYSTATUSEX)

var gcMemoryInfo = GC.GetGCMemoryInfo();
installedMemory = gcMemoryInfo.TotalAvailableMemoryBytes;
// it will give the size of memory in MB
var physicalMemory = (double) installedMemory / 1048576.0;

You could use 'WMI'.

I found a 'snippet'.

Set objWMIService = GetObject("winmgmts:" _
& "{impersonationLevel=impersonate}!\\" _ 
& strComputer & "\root\cimv2") 
Set colComputer = objWMIService.ExecQuery _
("Select * from Win32_ComputerSystem")

For Each objComputer in colComputer 
  strMemory = objComputer.TotalPhysicalMemory
Next

Compatible with .Net and Mono (tested with Win10/FreeBSD/CentOS)

Using ComputerInfo source code and PerformanceCounters for Mono and as backup for .Net:

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Security;

public class SystemMemoryInfo
{
    private readonly PerformanceCounter _monoAvailableMemoryCounter;
    private readonly PerformanceCounter _monoTotalMemoryCounter;
    private readonly PerformanceCounter _netAvailableMemoryCounter;

    private ulong _availablePhysicalMemory;
    private ulong _totalPhysicalMemory;

    public SystemMemoryInfo()
    {
        try
        {
            if (PerformanceCounterCategory.Exists("Mono Memory"))
            {
                _monoAvailableMemoryCounter = new PerformanceCounter("Mono Memory", "Available Physical Memory");
                _monoTotalMemoryCounter = new PerformanceCounter("Mono Memory", "Total Physical Memory");
            }
            else if (PerformanceCounterCategory.Exists("Memory"))
            {
                _netAvailableMemoryCounter = new PerformanceCounter("Memory", "Available Bytes");
            }
        }
        catch
        {
            // ignored
        }
    }

    public ulong AvailablePhysicalMemory
    {
        [SecurityCritical]
        get
        {
            Refresh();

            return _availablePhysicalMemory;
        }
    }

    public ulong TotalPhysicalMemory
    {
        [SecurityCritical]
        get
        {
            Refresh();

            return _totalPhysicalMemory;
        }
    }

    [SecurityCritical]
    [DllImport("Kernel32", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern void GlobalMemoryStatus(ref MEMORYSTATUS lpBuffer);

    [SecurityCritical]
    [DllImport("Kernel32", CharSet = CharSet.Auto, SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool GlobalMemoryStatusEx(ref MEMORYSTATUSEX lpBuffer);

    [SecurityCritical]
    private void Refresh()
    {
        try
        {
            if (_monoTotalMemoryCounter != null && _monoAvailableMemoryCounter != null)
            {
                _totalPhysicalMemory = (ulong) _monoTotalMemoryCounter.NextValue();
                _availablePhysicalMemory = (ulong) _monoAvailableMemoryCounter.NextValue();
            }
            else if (Environment.OSVersion.Version.Major < 5)
            {
                var memoryStatus = MEMORYSTATUS.Init();
                GlobalMemoryStatus(ref memoryStatus);

                if (memoryStatus.dwTotalPhys > 0)
                {
                    _availablePhysicalMemory = memoryStatus.dwAvailPhys;
                    _totalPhysicalMemory = memoryStatus.dwTotalPhys;
                }
                else if (_netAvailableMemoryCounter != null)
                {
                    _availablePhysicalMemory = (ulong) _netAvailableMemoryCounter.NextValue();
                }
            }
            else
            {
                var memoryStatusEx = MEMORYSTATUSEX.Init();

                if (GlobalMemoryStatusEx(ref memoryStatusEx))
                {
                    _availablePhysicalMemory = memoryStatusEx.ullAvailPhys;
                    _totalPhysicalMemory = memoryStatusEx.ullTotalPhys;
                }
                else if (_netAvailableMemoryCounter != null)
                {
                    _availablePhysicalMemory = (ulong) _netAvailableMemoryCounter.NextValue();
                }
            }
        }
        catch
        {
            // ignored
        }
    }

    private struct MEMORYSTATUS
    {
        private uint dwLength;
        internal uint dwMemoryLoad;
        internal uint dwTotalPhys;
        internal uint dwAvailPhys;
        internal uint dwTotalPageFile;
        internal uint dwAvailPageFile;
        internal uint dwTotalVirtual;
        internal uint dwAvailVirtual;

        public static MEMORYSTATUS Init()
        {
            return new MEMORYSTATUS
            {
                dwLength = checked((uint) Marshal.SizeOf(typeof(MEMORYSTATUS)))
            };
        }
    }

    private struct MEMORYSTATUSEX
    {
        private uint dwLength;
        internal uint dwMemoryLoad;
        internal ulong ullTotalPhys;
        internal ulong ullAvailPhys;
        internal ulong ullTotalPageFile;
        internal ulong ullAvailPageFile;
        internal ulong ullTotalVirtual;
        internal ulong ullAvailVirtual;
        internal ulong ullAvailExtendedVirtual;

        public static MEMORYSTATUSEX Init()
        {
            return new MEMORYSTATUSEX
            {
                dwLength = checked((uint) Marshal.SizeOf(typeof(MEMORYSTATUSEX)))
            };
        }
    }
}

.NET has a memory amount limit that it can access. In Windows XP 2GB was the "hard ceiling". For instance: You could have 4 GB in it, and it would kill the app when it hit 2GB.

Also in 64 bit mode, there is a percentage of memory you can use out of the system, so I'm not sure if you can ask for the whole thing or if this is specifically guarded against.

var ram = new ManagementObjectSearcher("select * from Win32_PhysicalMemory") .Get().Cast<ManagementObject>().First();

|

var a = Convert.ToInt64(ram["Capacity"]) / 1024 / 1024 / 1024;

(richiede System.Managment.dll come riferimento, testato su C# con Framework 4.7.2)

questa procedura salva in "a" la ram totale presente in GB

ulong memory() { return new Microsoft.VisualBasic.Devices.ComputerInfo().TotalPhysicalMemory; }

|

var b = Convert.ToDecimal(memory()) / 1024 / 1024 / 1024;

(richiede Microsoft.VisualBasics.dll come riferimento, testato su C# Framework 4.7.2)

questa procedura salva in "b" il valore della ram in GB effettivamente disponibile