Unique file identifier in windows

Here's sample code that returns a unique File Index.

ApproachA() is what I came up with after a bit of research. ApproachB() is thanks to information in the links provided by Mattias and Rubens. Given a specific file, both approaches return the same file index (during my basic testing).

Some caveats from MSDN:

Support for file IDs is file system-specific. File IDs are not guaranteed to be unique over time, because file systems are free to reuse them. In some cases, the file ID for a file can change over time.

In the FAT file system, the file ID is generated from the first cluster of the containing directory and the byte offset within the directory of the entry for the file. Some defragmentation products change this byte offset. (Windows in-box defragmentation does not.) Thus, a FAT file ID can change over time. Renaming a file in the FAT file system can also change the file ID, but only if the new file name is longer than the old one.

In the NTFS file system, a file keeps the same file ID until it is deleted. You can replace one file with another file without changing the file ID by using the ReplaceFile function. However, the file ID of the replacement file, not the replaced file, is retained as the file ID of the resulting file.

The first bolded comment above worries me. It's not clear if this statement applies to FAT only, it seems to contradict the second bolded text. I guess further testing is the only way to be sure.

[Update: in my testing the file index/id changes when a file is moved from one internal NTFS hard drive to another internal NTFS hard drive.]

    public class WinAPI    {        [DllImport("ntdll.dll", SetLastError = true)]        public static extern IntPtr NtQueryInformationFile(IntPtr fileHandle, ref IO_STATUS_BLOCK IoStatusBlock, IntPtr pInfoBlock, uint length, FILE_INFORMATION_CLASS fileInformation);        public struct IO_STATUS_BLOCK        {            uint status;            ulong information;        }        public struct _FILE_INTERNAL_INFORMATION {          public ulong  IndexNumber;        }         // Abbreviated, there are more values than shown        public enum FILE_INFORMATION_CLASS        {            FileDirectoryInformation = 1,     // 1            FileFullDirectoryInformation,     // 2            FileBothDirectoryInformation,     // 3            FileBasicInformation,         // 4            FileStandardInformation,      // 5            FileInternalInformation      // 6        }        [DllImport("kernel32.dll", SetLastError = true)]        public static extern bool GetFileInformationByHandle(IntPtr hFile,out BY_HANDLE_FILE_INFORMATION lpFileInformation);        public struct BY_HANDLE_FILE_INFORMATION        {            public uint FileAttributes;            public FILETIME CreationTime;            public FILETIME LastAccessTime;            public FILETIME LastWriteTime;            public uint VolumeSerialNumber;            public uint FileSizeHigh;            public uint FileSizeLow;            public uint NumberOfLinks;            public uint FileIndexHigh;            public uint FileIndexLow;        }  }  public class Test  {       public ulong ApproachA()       {                WinAPI.IO_STATUS_BLOCK iostatus=new WinAPI.IO_STATUS_BLOCK();                WinAPI._FILE_INTERNAL_INFORMATION objectIDInfo = new WinAPI._FILE_INTERNAL_INFORMATION();                int structSize = Marshal.SizeOf(objectIDInfo);                FileInfo fi=new FileInfo(@"C:\Temp\testfile.txt");                FileStream fs=fi.Open(FileMode.Open,FileAccess.Read,FileShare.ReadWrite);                IntPtr res=WinAPI.NtQueryInformationFile(fs.Handle, ref iostatus, memPtr, (uint)structSize, WinAPI.FILE_INFORMATION_CLASS.FileInternalInformation);                objectIDInfo = (WinAPI._FILE_INTERNAL_INFORMATION)Marshal.PtrToStructure(memPtr, typeof(WinAPI._FILE_INTERNAL_INFORMATION));                fs.Close();                Marshal.FreeHGlobal(memPtr);                   return objectIDInfo.IndexNumber;       }       public ulong ApproachB()       {               WinAPI.BY_HANDLE_FILE_INFORMATION objectFileInfo=new WinAPI.BY_HANDLE_FILE_INFORMATION();                FileInfo fi=new FileInfo(@"C:\Temp\testfile.txt");                FileStream fs=fi.Open(FileMode.Open,FileAccess.Read,FileShare.ReadWrite);                WinAPI.GetFileInformationByHandle(fs.Handle, out objectFileInfo);                fs.Close();                ulong fileIndex = ((ulong)objectFileInfo.FileIndexHigh << 32) + (ulong)objectFileInfo.FileIndexLow;                return fileIndex;          }  }

If you call GetFileInformationByHandle, you'll get a file ID in BY_HANDLE_FILE_INFORMATION.nFileIndexHigh/Low. This index is unique within a volume, and stays the same even if you move the file (within the volume) or rename it.

If you can assume that NTFS is used, you may also want to consider using Alternate Data Streams to store the metadata.

Please take a look here: Unique file ids for Windows. This is also useful: Unique ID for files on NTFS?