FileSystemWatcher to a directory symlink that points to a file doesn't raise exception on windows 8.1 or lower

[Jozkee]

Description

On cmd:

# create file
echo text > file.txt 

# create directory symlink to file
mklink /d linktofile file.txt

On .NET:

using System;
using System.IO;

namespace fsw
{
    class Program
    {
        static void Main(string[] args)
        {
            using var watcher = new FileSystemWatcher("linktofile");
            watcher.EnableRaisingEvents = true; // This should've thrown.

            Console.WriteLine("Watching...");
            Console.ReadKey();
        }
    }
}

Configuration

.NET 5 and .NET 6 preview
OS: Windows 8.1 or 7.
Windows 10 throws which is what I expected since a directory symlink to a file is a bad symlink.

Regression?

No

Other information

The only problem I see is consistency.

cc @carlossanlop

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Issue Details

---

Description

On cmd:

# create file
echo text > file.txt 

# create directory symlink to file
mklink /d linktofile file.txt

On .NET:

using System;
using System.IO;

namespace fsw
{
    class Program
    {
        static void Main(string[] args)
        {
            using var watcher = new FileSystemWatcher("linktofile");
            watcher.EnableRaisingEvents = true; // This should've thrown.

            Console.WriteLine("Watching...");
            Console.ReadKey();
        }
    }
}

Configuration

.NET 5 and .NET 6 preview
OS: Windows 8.1 or lower.
Windows 10 throws which is what I expected since a directory symlink to a file is a bad symlink.

Regression?

No

Other information

The only problem I see is consistency.

cc @carlossanlop

Author:	Jozkee
Assignees:	-
Labels:	area-System.IO, os-windows
Milestone:	Future

[carlossanlop]

Were you able to determine the root cause?

[Jozkee]

CreateFileW with FILE_FLAG_BACKUP_SEMANTICS attribute throws if the path is to a self-referencing directory symlink in windows 10. In 7 and 8.1, it does not.

See related https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilew#directories.

[carlossanlop]

Thanks. So if those Windows versions do not throw, are you suggesting we detect the self-referencing dir symlink ourselves?

[Jozkee]

Yes, it could be an option. We most probably want to only do that for those versions and avoid the manual validation (potentially involves a sys-call) in win10, which currently throws as expected.

Another option is to find an alternative API that throws in such case and that also returns a handle for a directory, not sure if exists.