How to use a memory-mapped file with C# in WinCE

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Background

If we want to pass data between different process, we can use file, Windows Message Queue, or the network. But a memory-mapped file is the best technique.

Declare in .NET CF using C#:

public class WinCEConst
{
    public const UInt32 STANDARD_RIGHTS_REQUIRED = 0x000F0000;
    public const UInt32 SECTION_QUERY = 0x0001;
    public const UInt32 SECTION_MAP_WRITE = 0x0002;
    public const UInt32 SECTION_MAP_READ = 0x0004;
    public const UInt32 SECTION_MAP_EXECUTE = 0x0008;
    public const UInt32 SECTION_EXTEND_SIZE = 0x0010;
    public const UInt32 SECTION_ALL_ACCESS = (STANDARD_RIGHTS_REQUIRED SECTION_QUERY
    SECTION_MAP_WRITE
    SECTION_MAP_READ
    SECTION_MAP_EXECUTE
    SECTION_EXTEND_SIZE);
    public const UInt32 FILE_MAP_ALL_ACCESS = SECTION_ALL_ACCESS;
 
    public static IntPtr INVALID_HANDLE_VALUE = new IntPtr(-1);
 
    public const int PAGE_READWRITE = 0x04;
    public const int PAGE_READONLY = 0x02;
    public const int FILE_MAP_READ = 0x0004;
    public const int FILE_MAP_WRITE = 0x0002;
}

public class WinCE
{
    // * WinCE use coredll.dll
    // * Windows use keneral32.dll
    [DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
    public static extern int GetAdaptersInfo(IntPtr pAdapterInfo, ref int pBufOutLen);
    [DllImport("Coredll.dll", SetLastError = true, CharSet = CharSet.Auto)]
    public static extern IntPtr CreateFile(
        string lpFileName,
        uint dwDesiredAccess,
        uint dwShareMode,
        IntPtr lpSecurityAttributes,
        uint dwCreationDisposition,
        uint dwFlagsAndAttributes,
        IntPtr hTemplateFile
        );
    [DllImport("Coredll.dll", SetLastError = true, CharSet = CharSet.Auto)]
    public static extern IntPtr CreateFileMapping(
        IntPtr hFile,
        object lpFileMappingAttributes,
        uint flProtect,
        uint dwMaximumSizeHigh,
        uint dwMaximumSizeLow,
        string lpName);
    [DllImport("Coredll.dll", SetLastError = true, CharSet = CharSet.Auto)]
    public static extern IntPtr CreateFileForMapping(
        string lpFileName,
        uint dwDesiredAccess,
        uint dwShareMode,
        IntPtr lpSecurityAttributes, // set null
        uint dwCreationDisposition,
        uint dwFlagsAndAttributes,
        IntPtr hTemplateFile);
    public static IntPtr OpenFileMapping(uint dwDesiredAccess, bool bInheritHandle, string lpName)
    {
        IntPtr t_pHandle = CreateFileMapping(new IntPtr(-1), null,
                WinCEConst.PAGE_READWRITE, 0, 0, lpName);
        return t_pHandle;
    }
    [DllImport("Coredll.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool CloseHandle(IntPtr hObject);
    [DllImport("Coredll.dll", SetLastError = true)]
    public static extern IntPtr MapViewOfFile(
        IntPtr hFileMappingObject,
        uint dwDesiredAccess,
        uint dwFileOffsetHigh,
        uint dwFileOffsetLow,
        uint dwNumberOfBytesToMap);
    [DllImport("Coredll.dll", SetLastError = true)]
    public static extern bool UnmapViewOfFile(IntPtr lpBaseAddress);
}

How to create a memory-mapped file and put data to it

const string MapName = "OrzCloud";
const int MapSize = 1024;
IntPtr m_pMapFile;
IntPtr m_pBuff;
string data = "TestTest";
 
// create map file
m_pMapFile = WinCE.CreateFileMapping(
  WinCEConst.INVALID_HANDLE_VALUE, null,WinCEConst.PAGE_READWRITE, 0, MapSize, MapName);
t_i4Error = Marshal.GetLastWin32Error();
 
// get map file
m_pBuff = WinCE.MapViewOfFile(m_pMapFile,WinCEConst.FILE_MAP_ALL_ACCESS, 0, 0, MapSize);
t_i4Error = Marshal.GetLastWin32Error();
// String to byte array
byte[] t_bData = Encoding.ASCII.GetBytes(data);
// write data to map file
Marshal.Copy(t_bData, 0, m_pBuff, t_bData.Length);

How to open an existing memory-mapped file and get data

m_pMapFile = WinCE.OpenFileMapping(WinCEConst.FILE_MAP_ALL_ACCESS, false, MapName);
// check error code
t_i4Error = Marshal.GetLastWin32Error();

// get map file
m_pBuff = WinCE.MapViewOfFile(m_pMapFile, WinCEConst.FILE_MAP_ALL_ACCESS, 0, 0, MapSize);
 
byte[] bytData = new byte[MapSize];
// read data from map file
Marshal.Copy(m_pBuff, bytData, 0, MapSize);
// change byte array to string
string data = Encoding.ASCII.GetString(bytData,0,MapSize).Trim();

How to close a memory-mapped file

WinCE.UnmapViewOfFile(m_pBuff);
WinCE.CloseHandle(m_pMapFile);

References

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