Creating symlink - The data present in the reparse point buffer is invalid

Goal: To create symbolic link using DeviceIoControl with FSCTL_SET_REPARSE_POINT

I know there is CreateSymbolicLink WinApi call, it works, but I cannot use it for my purpose.

Problem: I keep getting the 0x80071128 (The data present in the reparse point buffer is invalid) when I call DeviceIOControl(hNewSymLink,FSCTL_SET_REPARSE_POINT ...

the 'c:' drive is NTFS, OS is Windows 10, I am debugging as Admin.

Initially I tried to put together the REPARSE_DATA_BUFFER, as per https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/b41f1cbf-10df-4a47-98d4-1c52a833d913 to the above effect.

In the test3() I read the structure of good symlink and try to use it to create new one, but the structure gets rejected, again with the same error.

void test3()
{
    using (SafeFileHandle hSymLink = NativeMethods.CreateFile(
        @"c:\Temp\link2",
        FileAccess.Read,
        FileShare.Read,
        IntPtr.Zero,
        FileMode.Open,
        (FileAttributes)(NativeMethods.EFileAttributes.FILE_FLAG_OPEN_REPARSE_POINT),
        IntPtr.Zero))
    {
        var reparseDataSize = Marshal.SizeOf(typeof(NativeMethods.REPARSE_DATA_BUFFER));
        var reparseData = Marshal.AllocHGlobal(reparseDataSize);

        try
        {
            int bytesReturned = 0;
            var result = NativeMethods.DeviceIoControl(hSymLink, NativeMethods.DeviceIOControlCode.FSCTL_GET_REPARSE_POINT,
                IntPtr.Zero, 0, 
                reparseData, reparseDataSize, 
                ref bytesReturned, IntPtr.Zero);

            if (!result)
                Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error());

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

            var printNameDir = Encoding.Unicode.GetString(reparseDataBuffer.PathBuffer,
                    reparseDataBuffer.PrintNameOffset, reparseDataBuffer.PrintNameLength);
//value: "c:\\temp\\target.txt"
            var substituteNameDir = Encoding.Unicode.GetString(reparseDataBuffer.PathBuffer,
                    reparseDataBuffer.SubstituteNameOffset, reparseDataBuffer.SubstituteNameLength);
//value: "\\??\\c:\\temp\\target.txt"

            using (SafeFileHandle hNewSymLink = NativeMethods.CreateFile(
                @"c:\Temp\linkNew",
                FileAccess.Write,
                FileShare.Read,
                IntPtr.Zero,
                FileMode.Create,
                (FileAttributes)(NativeMethods.EFileAttributes.FILE_FLAG_OPEN_REPARSE_POINT), 
                IntPtr.Zero))
            {
                var result2 = NativeMethods.DeviceIoControl(hNewSymLink, NativeMethods.DeviceIOControlCode.FSCTL_SET_REPARSE_POINT,
                    reparseData, reparseDataSize, IntPtr.Zero, 0, IntPtr.Zero, IntPtr.Zero);

                if (!result2)
                    Marshal.ThrowExceptionForHR(Marshal.GetHRForLastWin32Error());
//error 0x80071128: wrong reparse buffer
            }
        }
        finally
        {
            Marshal.FreeHGlobal(reparseData);
        }
    }
}

When I inspect the structure it looks fine. The docs state the same REPARSE_DATA_BUFFER is used for get and set, so I expect to be able to use well-formed structure retrieved from existing symlink (created with MKLINK system command) to create another one.

Obviously I am missing something - any hints would be much appreciated.

One needs to pass exact size of the structure, not the size of entire allocated buffer.

The call should look like

var result2 = NativeMethods.DeviceIoControl(hNewSymLink, 
    NativeMethods.DeviceIOControlCode.FSCTL_SET_REPARSE_POINT,
    reparseData, bytesReturned, IntPtr.Zero, 0, IntPtr.Zero, IntPtr.Zero);