Create, Delete and Examine Junction Points in C#

JunctionPoint.cs

// File: RollThroughLibrary/CreateMaps/JunctionPoint.cs
// User: Adrian Hum/
// 
// Created:  2017-11-19 2:46 PM
// Modified: 2017-11-19 6:10 PM

using System;
using System.Diagnostics.CodeAnalysis;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;
using Microsoft.Win32.SafeHandles;

namespace CreateMaps
{
    /// <summary>
    ///     Provides access to NTFS junction points in .Net.
    /// </summary>
    [SuppressMessage("ReSharper", "InconsistentNaming")]
    [SuppressMessage("ReSharper", "UnusedMember.Local")]
    [SuppressMessage("ReSharper", "MemberCanBePrivate.Local")]
    [SuppressMessage("ReSharper", "FieldCanBeMadeReadOnly.Local")]
    public static class JunctionPoint
    {
        /// <summary>
        ///     The file or directory is not a reparse point.
        /// </summary>
        private const int ERROR_NOT_A_REPARSE_POINT = 4390;

        /// <summary>
        ///     The reparse point attribute cannot be set because it conflicts with an existing attribute.
        /// </summary>
        private const int ERROR_REPARSE_ATTRIBUTE_CONFLICT = 4391;

        /// <summary>
        ///     The data present in the reparse point buffer is invalid.
        /// </summary>
        private const int ERROR_INVALID_REPARSE_DATA = 4392;

        /// <summary>
        ///     The tag present in the reparse point buffer is invalid.
        /// </summary>
        private const int ERROR_REPARSE_TAG_INVALID = 4393;

        /// <summary>
        ///     There is a mismatch between the tag specified in the request and the tag present in the reparse point.
        /// </summary>
        private const int ERROR_REPARSE_TAG_MISMATCH = 4394;

        /// <summary>
        ///     Command to set the reparse point data block.
        /// </summary>
        private const int FSCTL_SET_REPARSE_POINT = 0x000900A4;

        /// <summary>
        ///     Command to get the reparse point data block.
        /// </summary>
        private const int FSCTL_GET_REPARSE_POINT = 0x000900A8;

        /// <summary>
        ///     Command to delete the reparse point data base.
        /// </summary>
        private const int FSCTL_DELETE_REPARSE_POINT = 0x000900AC;

        /// <summary>
        ///     Reparse point tag used to identify mount points and junction points.
        /// </summary>
        private const uint IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003;

        /// <summary>
        ///     This prefix indicates to NTFS that the path is to be treated as a non-interpreted
        ///     path in the virtual file system.
        /// </summary>
        private const string NonInterpretedPathPrefix = @"\??\";

        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern bool DeviceIoControl(IntPtr hDevice, uint dwIoControlCode,
            IntPtr InBuffer, int nInBufferSize,
            IntPtr OutBuffer, int nOutBufferSize,
            out int pBytesReturned, IntPtr lpOverlapped);

        [DllImport("kernel32.dll", SetLastError = true)]
        private static extern IntPtr CreateFile(
            string lpFileName,
            EFileAccess dwDesiredAccess,
            EFileShare dwShareMode,
            IntPtr lpSecurityAttributes,
            ECreationDisposition dwCreationDisposition,
            EFileAttributes dwFlagsAndAttributes,
            IntPtr hTemplateFile);

        /// <summary>
        ///     Creates a junction point from the specified directory to the specified target directory.
        /// </summary>
        /// <remarks>
        ///     Only works on NTFS.
        /// </remarks>
        /// <param name="junctionPoint">The junction point path</param>
        /// <param name="targetDir">The target directory</param>
        /// <param name="overwrite">If true overwrites an existing reparse point or empty directory</param>
        /// <exception cref="IOException">
        ///     Thrown when the junction point could not be created or when
        ///     an existing directory was found and <paramref name="overwrite" /> if false
        /// </exception>
        public static void Create(string junctionPoint, string targetDir, bool overwrite)
        {
            targetDir = Path.GetFullPath(targetDir);

            if (!Directory.Exists(targetDir))
                throw new IOException("Target path does not exist or is not a directory.");

            if (Directory.Exists(junctionPoint))
            {
                if (!overwrite)
                    throw new IOException("Directory already exists and overwrite parameter is false.");
            }
            else
            {
                Directory.CreateDirectory(junctionPoint);
            }

            using (var handle = OpenReparsePoint(junctionPoint, EFileAccess.GenericWrite))
            {
                var targetDirBytes = Encoding.Unicode.GetBytes(NonInterpretedPathPrefix + Path.GetFullPath(targetDir));

                var reparseDataBuffer =
                    new REPARSE_DATA_BUFFER
                    {
                        ReparseTag = IO_REPARSE_TAG_MOUNT_POINT,
                        ReparseDataLength = (ushort) (targetDirBytes.Length + 12),
                        SubstituteNameOffset = 0,
                        SubstituteNameLength = (ushort) targetDirBytes.Length,
                        PrintNameOffset = (ushort) (targetDirBytes.Length + 2),
                        PrintNameLength = 0,
                        PathBuffer = new byte[0x3ff0]
                    };

                Array.Copy(targetDirBytes, reparseDataBuffer.PathBuffer, targetDirBytes.Length);

                var inBufferSize = Marshal.SizeOf(reparseDataBuffer);
                var inBuffer = Marshal.AllocHGlobal(inBufferSize);

                try
                {
                    Marshal.StructureToPtr(reparseDataBuffer, inBuffer, false);

                    int bytesReturned;
                    var result = DeviceIoControl(handle.DangerousGetHandle(), FSCTL_SET_REPARSE_POINT,
                        inBuffer, targetDirBytes.Length + 20, IntPtr.Zero, 0, out bytesReturned, IntPtr.Zero);

                    if (!result)
                        ThrowLastWin32Error("Unable to create junction point.");
                }
                finally
                {
                    Marshal.FreeHGlobal(inBuffer);
                }
            }
        }

        /// <summary>
        ///     Deletes a junction point at the specified source directory along with the directory itself.
        ///     Does nothing if the junction point does not exist.
        /// </summary>
        /// <remarks>
        ///     Only works on NTFS.
        /// </remarks>
        /// <param name="junctionPoint">The junction point path</param>
        public static void Delete(string junctionPoint)
        {
            if (!Directory.Exists(junctionPoint))
            {
                if (File.Exists(junctionPoint))
                    throw new IOException("Path is not a junction point.");

                return;
            }

            using (var handle = OpenReparsePoint(junctionPoint, EFileAccess.GenericWrite))
            {
                var reparseDataBuffer = new REPARSE_DATA_BUFFER
                {
                    ReparseTag = IO_REPARSE_TAG_MOUNT_POINT,
                    ReparseDataLength = 0,
                    PathBuffer = new byte[0x3ff0]
                };


                var inBufferSize = Marshal.SizeOf(reparseDataBuffer);
                var inBuffer = Marshal.AllocHGlobal(inBufferSize);
                try
                {
                    Marshal.StructureToPtr(reparseDataBuffer, inBuffer, false);

                    int bytesReturned;
                    var result = DeviceIoControl(handle.DangerousGetHandle(), FSCTL_DELETE_REPARSE_POINT,
                        inBuffer, 8, IntPtr.Zero, 0, out bytesReturned, IntPtr.Zero);

                    if (!result)
                        ThrowLastWin32Error("Unable to delete junction point.");
                }
                finally
                {
                    Marshal.FreeHGlobal(inBuffer);
                }

                try
                {
                    Directory.Delete(junctionPoint);
                }
                catch (IOException ex)
                {
                    throw new IOException("Unable to delete junction point.", ex);
                }
            }
        }

        /// <summary>
        ///     Determines whether the specified path exists and refers to a junction point.
        /// </summary>
        /// <param name="path">The junction point path</param>
        /// <returns>True if the specified path represents a junction point</returns>
        /// <exception cref="IOException">
        ///     Thrown if the specified path is invalid
        ///     or some other error occurs
        /// </exception>
        public static bool Exists(string path)
        {
            if (!Directory.Exists(path))
                return false;

            using (var handle = OpenReparsePoint(path, EFileAccess.GenericRead))
            {
                var target = InternalGetTarget(handle);
                return target != null;
            }
        }

        /// <summary>
        ///     Gets the target of the specified junction point.
        /// </summary>
        /// <remarks>
        ///     Only works on NTFS.
        /// </remarks>
        /// <param name="junctionPoint">The junction point path</param>
        /// <returns>The target of the junction point</returns>
        /// <exception cref="IOException">
        ///     Thrown when the specified path does not
        ///     exist, is invalid, is not a junction point, or some other error occurs
        /// </exception>
        public static string GetTarget(string junctionPoint)
        {
            using (var handle = OpenReparsePoint(junctionPoint, EFileAccess.GenericRead))
            {
                var target = InternalGetTarget(handle);
                if (target == null)
                    throw new IOException("Path is not a junction point.");

                return target;
            }
        }

        private static string InternalGetTarget(SafeFileHandle handle)
        {
            var outBufferSize = Marshal.SizeOf(typeof(REPARSE_DATA_BUFFER));
            var outBuffer = Marshal.AllocHGlobal(outBufferSize);

            try
            {
                int bytesReturned;
                var result = DeviceIoControl(handle.DangerousGetHandle(), FSCTL_GET_REPARSE_POINT,
                    IntPtr.Zero, 0, outBuffer, outBufferSize, out bytesReturned, IntPtr.Zero);

                if (!result)
                {
                    var error = Marshal.GetLastWin32Error();
                    if (error == ERROR_NOT_A_REPARSE_POINT)
                        return null;

                    ThrowLastWin32Error("Unable to get information about junction point.");
                }

                var reparseDataBuffer = (REPARSE_DATA_BUFFER)
                    Marshal.PtrToStructure(outBuffer, typeof(REPARSE_DATA_BUFFER));

                if (reparseDataBuffer.ReparseTag != IO_REPARSE_TAG_MOUNT_POINT)
                    return null;

                var targetDir = Encoding.Unicode.GetString(reparseDataBuffer.PathBuffer,
                    reparseDataBuffer.SubstituteNameOffset, reparseDataBuffer.SubstituteNameLength);

                if (targetDir.StartsWith(NonInterpretedPathPrefix))
                    targetDir = targetDir.Substring(NonInterpretedPathPrefix.Length);

                return targetDir;
            }
            finally
            {
                Marshal.FreeHGlobal(outBuffer);
            }
        }

        private static SafeFileHandle OpenReparsePoint(string reparsePoint, EFileAccess accessMode)
        {
            var reparsePointHandle = new SafeFileHandle(CreateFile(reparsePoint, accessMode,
                EFileShare.Read | EFileShare.Write | EFileShare.Delete,
                IntPtr.Zero, ECreationDisposition.OpenExisting,
                EFileAttributes.BackupSemantics | EFileAttributes.OpenReparsePoint, IntPtr.Zero), true);

            if (Marshal.GetLastWin32Error() != 0)
                ThrowLastWin32Error("Unable to open reparse point.");

            return reparsePointHandle;
        }

        private static void ThrowLastWin32Error(string message)
        {
            throw new IOException(message, Marshal.GetExceptionForHR(Marshal.GetHRForLastWin32Error()));
        }

        [Flags]
        private enum EFileAccess : uint
        {
            GenericRead = 0x80000000,
            GenericWrite = 0x40000000,
            GenericExecute = 0x20000000,
            GenericAll = 0x10000000
        }

        [Flags]
        private enum EFileShare : uint
        {
            None = 0x00000000,
            Read = 0x00000001,
            Write = 0x00000002,
            Delete = 0x00000004
        }

        private enum ECreationDisposition : uint
        {
            New = 1,
            CreateAlways = 2,
            OpenExisting = 3,
            OpenAlways = 4,
            TruncateExisting = 5
        }

        [Flags]
        private enum EFileAttributes : uint
        {
            Readonly = 0x00000001,
            Hidden = 0x00000002,
            System = 0x00000004,
            Directory = 0x00000010,
            Archive = 0x00000020,
            Device = 0x00000040,
            Normal = 0x00000080,
            Temporary = 0x00000100,
            SparseFile = 0x00000200,
            ReparsePoint = 0x00000400,
            Compressed = 0x00000800,
            Offline = 0x00001000,
            NotContentIndexed = 0x00002000,
            Encrypted = 0x00004000,
            Write_Through = 0x80000000,
            Overlapped = 0x40000000,
            NoBuffering = 0x20000000,
            RandomAccess = 0x10000000,
            SequentialScan = 0x08000000,
            DeleteOnClose = 0x04000000,
            BackupSemantics = 0x02000000,
            PosixSemantics = 0x01000000,
            OpenReparsePoint = 0x00200000,
            OpenNoRecall = 0x00100000,
            FirstPipeInstance = 0x00080000
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct REPARSE_DATA_BUFFER
        {
            /// <summary>
            ///     Reparse point tag. Must be a Microsoft reparse point tag.
            /// </summary>
            public uint ReparseTag;

            /// <summary>
            ///     Size, in bytes, of the data after the Reserved member. This can be calculated by:
            ///     (4 * sizeof(ushort)) + SubstituteNameLength + PrintNameLength +
            ///     (namesAreNullTerminated ? 2 * sizeof(char) : 0);
            /// </summary>
            public ushort ReparseDataLength;

            /// <summary>
            ///     Reserved; do not use.
            /// </summary>
            public ushort Reserved;

            /// <summary>
            ///     Offset, in bytes, of the substitute name string in the PathBuffer array.
            /// </summary>
            public ushort SubstituteNameOffset;

            /// <summary>
            ///     Length, in bytes, of the substitute name string. If this string is null-terminated,
            ///     SubstituteNameLength does not include space for the null character.
            /// </summary>
            public ushort SubstituteNameLength;

            /// <summary>
            ///     Offset, in bytes, of the print name string in the PathBuffer array.
            /// </summary>
            public ushort PrintNameOffset;

            /// <summary>
            ///     Length, in bytes, of the print name string. If this string is null-terminated,
            ///     PrintNameLength does not include space for the null character.
            /// </summary>
            public ushort PrintNameLength;

            /// <summary>
            ///     A buffer containing the unicode-encoded path string. The path string contains
            ///     the substitute name string and print name string.
            /// </summary>
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 0x3FF0)] public byte[] PathBuffer;
        }
    }
}

Hi Adrian Hum,

What is the license of this code?

I too would care to know.

Personally? I don't really care it's grown out of stuff from MSDN, bits on the net...

So buy me a virtual free beer one day...

Adrian

Hello, @LGM-AdrianHum . I tried to use your code to solve a problem that i got. The problem is when i try to delete or modify permission of a directory, that seems a junction point. Without using your code i got access denied error, and using your code i get the same error. the directory catch the condition "if (Marshal.GetLastWin32Error() != 0)" in OpenReparsePoint method. Can you help me with this, please?

Code looks similar, if not identical, to one from Jeff Brown's JunctionPoint.cs on Code Project: https://www.codeproject.com/script/Articles/ViewDownloads.aspx?aid=15633

Thank you. Good reference.