How to create a memory mapped file with C# (Windows Vista / Windows 7)

Article
09/26/2011

Hi all,

The following C# sample shows how to create a memory mapped file and use private namespaces to allow access to specific groups of users:

FORM1.CS

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Windows.Forms;
 using System.Runtime.InteropServices;
 using System.Security.Principal;
 using System.IO;
 
 namespace Alejacma
 {
 public partial class Form1 : Form
 {
 public Form1()
 {
 InitializeComponent();
 }
 
 private void button1_Click(object sender, EventArgs e)
 {
 bool bResult = false;
 IntPtr hBoundary = IntPtr.Zero;
 IntPtr pSid = IntPtr.Zero;
 int cbSid = Win32.SECURITY_MAX_SID_SIZE;
 IntPtr hNamespace = IntPtr.Zero;
 Win32.SECURITY_ATTRIBUTES securityAttributes = new Win32.SECURITY_ATTRIBUTES();
 IntPtr hFile = IntPtr.Zero;
 IntPtr pView = IntPtr.Zero;
 IntPtr pData = IntPtr.Zero;
 
 try
 {
 // Create boundary 
 hBoundary = Win32.CreateBoundaryDescriptor(
 "AlejacmaBoundaryDescriptor", 
 0
 );
 if (hBoundary == IntPtr.Zero) { throw new Exception("CreateBoundaryDescriptor", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 pSid = Marshal.AllocHGlobal(cbSid);
 bResult = Win32.CreateWellKnownSid(
 WellKnownSidType.BuiltinAdministratorsSid,
 IntPtr.Zero, 
 pSid, 
 ref cbSid
 );
 if (!bResult) { throw new Exception("CreateWellKnownSid", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 bResult = Win32.AddSIDToBoundaryDescriptor(
 ref hBoundary, 
 pSid
 );
 if (!bResult) { throw new Exception("AddSIDToBoundaryDescriptor", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 // Create namespace and give access to some groups of users:
 // - Remote Desktop Users (needed for remote users to access the mapped file)
 // - Administrators (needed to create the mapped file within this program) 
 // - Interactive users (needed for local users to access mapped file)
 bResult = Win32.ConvertStringSecurityDescriptorToSecurityDescriptor(
 "D:(A;;GA;;;RD)(A;;GA;;;S-1-5-32-544)(A;;GA;;;S-1-5-4)",
 Win32.SDDL_REVISION_1,
 out securityAttributes.lpSecurityDescriptor, 
 IntPtr.Zero
 );
 if (!bResult) { throw new Exception("ConvertStringSecurityDescriptorToSecurityDescriptor", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 securityAttributes.nLength = Marshal.SizeOf(securityAttributes);
 securityAttributes.bInheritHandle = false;
 hNamespace = Win32.CreatePrivateNamespace(
 ref securityAttributes,
 hBoundary,
 "Alejacma"
 );
 if (hNamespace == IntPtr.Zero) { throw new Exception("CreatePrivateNamespace", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 // Create file mapping, and give access to all users with access to the namespace
 hFile = Win32.CreateFileMapping(
 Win32.INVALID_HANDLE_VALUE,
 ref securityAttributes,
 Win32.PAGE_READWRITE,
 0,
 20971520,
 "Alejacma\\TestFileMapping"
 );
 if (hFile == IntPtr.Zero) { throw new Exception("CreateFileMapping", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 // Map file and write something to it
 pView = Win32.MapViewOfFile(
 hFile,
 Win32.FILE_MAP_WRITE,
 0,
 0,
 11
 );
 if (pView == IntPtr.Zero) { throw new Exception("MapViewOfFile", new Win32Exception(Marshal.GetLastWin32Error())); }
 
 pData = Marshal.StringToHGlobalAnsi("Hello World");
 Win32.MemCopy(pView, pData, 11);
 
 // We are done. Now a reader app can access the data
 MessageBox.Show("Run the reader app now!");
 }
 catch (Exception ex)
 {
 // Any error?
 MessageBox.Show(ex.Message + " failed with '" + ex.InnerException.Message + "' error");
 }
 finally
 {
 // Clean up memory
 if (pSid != IntPtr.Zero)
 {
 Marshal.FreeHGlobal(pSid);
 }
 
 if (securityAttributes.lpSecurityDescriptor != IntPtr.Zero)
 {
 Win32.LocalFree(securityAttributes.lpSecurityDescriptor);
 }
 
 if (pData != IntPtr.Zero)
 {
 Marshal.FreeHGlobal(pData);
 }
 
 // We must also close hBoundary, hNamespace, hFile and pView when we don't need them anymore.
 // Check MSDN's description of the APIs we used to create them to figure out how to close them.
 }
 }
 }
 }

WIN32.CS

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Runtime.InteropServices;
 using System.Security.Principal;
 
 namespace Alejacma
 {
 class Win32
 {
 public const int SECURITY_MAX_SID_SIZE = 68;
 public const int SDDL_REVISION_1 = 1;
 public const uint INVALID_HANDLE_VALUE = 0xffffffff;
 public const int PAGE_READWRITE = 0x04;
 public const int FILE_MAP_WRITE = 0X02;
 
 [StructLayout(LayoutKind.Sequential)]
 public struct SECURITY_ATTRIBUTES
 {
 public int nLength;
 public IntPtr lpSecurityDescriptor;
 public bool bInheritHandle;
 }
 
 [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
 public static extern IntPtr CreateBoundaryDescriptor
 (
 [In] string Name,
 [In] int Flags
 );
 
 [DllImport("advapi32.dll", SetLastError = true)]
 public static extern bool CreateWellKnownSid
 (
 [In] WellKnownSidType WellKnownSidType,
 [In] [Optional] IntPtr DomainSid,
 [In] IntPtr pSid,
 [In][Out]ref int cbSid
 );
 
 [DllImport("kernel32.dll", SetLastError = true)]
 public static extern bool AddSIDToBoundaryDescriptor
 (
 [In][Out] ref IntPtr BoundaryDescriptor,
 [In] IntPtr RequiredSid
 );
 
 [DllImport("advapi32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
 public static extern bool ConvertStringSecurityDescriptorToSecurityDescriptor
 (
 [In] string StringSecurityDescriptor,
 [In] int StringSDRevision,
 [Out] out IntPtr SecurityDescriptor,
 [Out] IntPtr SecurityDescriptorSize
 );
 
 [DllImport("kernel32.dll", SetLastError = true)]
 public static extern IntPtr LocalFree([In] IntPtr hMem);
 
 [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
 public static extern IntPtr CreatePrivateNamespace(
 [In][Optional] ref SECURITY_ATTRIBUTES lpPrivateNamespaceAttributes,
 [In] IntPtr lpBoundaryDescriptor,
 [In] string lpAliasPrefix
 );
 
 [DllImport("Kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
 public static extern IntPtr CreateFileMapping(
 [In] uint hFile,
 [In][Optional] ref SECURITY_ATTRIBUTES lpAttributes,
 [In] int flProtect,
 [In] int dwMaximumSizeHigh, 
 [In] int dwMaximumSizeLow, 
 [In][Optional] string lpName
 );
 
 [DllImport("Kernel32.dll", SetLastError = true)]
 public static extern IntPtr MapViewOfFile(
 [In] IntPtr hFileMappingObject,
 [In] int dwDesiredAccess, 
 [In] int dwFileOffsetHigh, 
 [In] int dwFileOffsetLow, 
 [In] int dwNumberOfBytesToMap
 );
 
 [DllImport("msvcrt.dll", EntryPoint = "memcpy", CallingConvention = CallingConvention.Cdecl, SetLastError = false)]
 public static extern IntPtr MemCopy(IntPtr dest, IntPtr src, uint count);
 }
 }

I hope this helps.

Regards,

Alex (Alejandro Campos Magencio)

How to create a memory mapped file with C# (Windows Vista / Windows 7)

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