BCarlson BCarlson - 3 months ago 41
C# Question

How to remove a file lock

I have an service to move files from a working folder to backup folder. The folders are on a network share, so at times we will open a file, using something like notepad, to look at it. People are not (well, shouldn't) be editing, just looking.

When we try to move the file, I get permission denied. I'm looking for a way in C# to force remove a file lock, so the service can move the file to the backup folder.

Answer

You have to use P/Invoke. These are the functions you care about:

[DllImport("netapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
static extern int NetFileEnum(string servername, string basepath, string username, int level, ref IntPtr bufptr, int prefmaxlen, out int entriesread, out int totalentries, IntPtr resume_handle);

[DllImport("netapi32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
static extern int NetFileClose(string servername, int id);

[DllImport("Netapi32.dll", SetLastError = true)]
static extern int NetApiBufferFree(IntPtr buffer);

Here's some code similar to what I've used with success:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
struct FILE_INFO_3
{
    public int fi3_id;
    public int fi3_permission;
    public int fi3_num_locks;
    [MarshalAs(UnmanagedType.LPWStr)]
    public string fi3_pathname;
    [MarshalAs(UnmanagedType.LPWStr)]
    public string fi3_username;
}

private static FILE_INFO_3[] GetLockedFiles(string server, string path)
{
    const int MAX_PREFERRED_LENGTH = -1;

    int dwReadEntries;
    int dwTotalEntries;
    IntPtr pBuffer = IntPtr.Zero;
    FILE_INFO_3 pCurrent = new FILE_INFO_3();
    List<FILE_INFO_3> files = new List<FILE_INFO_3>();

    int dwStatus = NetFileEnum(server, path, null, 3, ref pBuffer, MAX_PREFERRED_LENGTH, out dwReadEntries, out dwTotalEntries, IntPtr.Zero);
    if (dwStatus == 0)
    {
        for (int dwIndex = 0; dwIndex < dwReadEntries; dwIndex++)
        {
            IntPtr iPtr = new IntPtr(pBuffer.ToInt32() + (dwIndex * Marshal.SizeOf(pCurrent)));
            pCurrent = (FILE_INFO_3)Marshal.PtrToStructure(iPtr, typeof(FILE_INFO_3));
            files.Add(pCurrent);
        }
    }

    NetApiBufferFree(pBuffer);

    return files.ToArray();
}

static void Main(string[] args)
{
    FILE_INFO_3[] lockedFiles = GetLockedFiles("someservername", @"C:\somepath");

    foreach (FILE_INFO_3 lockedFile in lockedFiles)
    {
        int dwStatus = NetFileClose(_serverName, lockedFile.fi3_id);
        // Check dwStatus for success here
    }
}

EDIT: As noted by the OP in the comments below, when compiling as 64-bit, you need to use ToInt64 instead of ToInt32. More information can be found here.

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