Using LockFileEX in C#

Question

Background

I\'m trying to implement block file locking in my C# application. The built-in FileStream.Lock method throws an exception if it is unable to acquire th

Answer 1

The following should be "very close" to a good solution. The only part I really don't like is using reflection to access an mscorlib internal method, but that method does an excellent job converting Win32 error codes to an IOException. Since you already have unsafe code for NativeOverlapped*, permissions aren't an issue.

It could probably be further improved by creating a SafeFileLockHandle or similar to provide a lightweight IDisposable object for unlocking the file derived from CriticalFinalizerObject.

    private const uint LOCKFILE_EXCLUSIVE_LOCK = 0x00000002;
    private static readonly Action WinIOError;

    static Win32Native()
    {
        BindingFlags bindingAttr = BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic;
        var winIOErrorMethod = typeof(string).Assembly.GetType("System.IO.__Error").GetMethod("WinIOError", bindingAttr, null, Type.EmptyTypes, null);
        WinIOError = (Action)Delegate.CreateDelegate(typeof(Action), winIOErrorMethod);
    }

    public static void LockFile(SafeFileHandle handle, bool exclusive, long offset, long length, Action action)
    {
        if (handle == null)
            throw new ArgumentNullException("handle");
        if (handle.IsInvalid)
            throw new ArgumentException("An invalid file handle was specified.", "handle");
        if (offset < 0)
            throw new ArgumentOutOfRangeException("The offset cannot be negative.", "offset");
        if (length < 0)
            throw new ArgumentOutOfRangeException("The length cannot be negative.", "length");
        if (action == null)
            throw new ArgumentNullException("action");

        LockFileUnsafe(handle, exclusive, offset, length, action);
    }

    private static unsafe void LockFileUnsafe(SafeFileHandle handle, bool exclusive, long offset, long length, Action action)
    {
        Overlapped overlapped = new Overlapped();
        overlapped.OffsetHigh = (int)(offset >> 32);
        overlapped.OffsetLow = (int)offset;

        IOCompletionCallback callback =
            (errorCode, numBytes, nativeOverlapped) =>
            {
                try
                {
                    action();
                }
                finally
                {
                    Overlapped.Free(nativeOverlapped);
                }
            };

        NativeOverlapped* native = overlapped.Pack(callback, null);
        uint flags = exclusive ? LOCKFILE_EXCLUSIVE_LOCK : 0;
        if (!LockFileEx(handle, flags, 0, (int)length, (int)(length >> 32), native))
        {
            Overlapped.Free(native);
            WinIOError();
        }
    }

    [DllImport("kernel32.dll", SetLastError = true)]
    private static unsafe extern bool LockFileEx(SafeFileHandle handle, uint flags, uint mustBeZero, int countLow, int countHigh, NativeOverlapped* overlapped);