FileSystemEventArgs.FullPath does not return a fully qualified path

[UnityAlex]

Description

As per the documentation https://docs.microsoft.com/en-us/dotnet/api/system.io.filesystemeventargs.fullpath?view=net-6.0 the FullPath property should return a fully qualified path. However it appears the constructor for FileSystemEventArgs assumes the directory passed in is fully qualified: https://github.com/dotnet/runtime/blob/main/src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemEventArgs.cs#L22

When used in conjunction with FileSystemWatcher it is possible for the path to be relative: https://github.com/dotnet/runtime/blob/main/src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.cs#L260

Reproduction Steps

Repro code:

using System;
using System.IO;

namespace MyNamespace
{
    class Program
    {
        static void Main()
        {
            using var watcher = new FileSystemWatcher(@"bar");

            watcher.NotifyFilter = NotifyFilters.LastWrite;

            watcher.Changed += OnChanged;

            watcher.Filter = "*.txt";
            watcher.IncludeSubdirectories = false;
            watcher.EnableRaisingEvents = true;

            Console.WriteLine("Press enter to exit.");
            Console.ReadLine();
        }

        private static void OnChanged(object sender, FileSystemEventArgs e)
        {
            if (e.ChangeType != WatcherChangeTypes.Changed)
            {
                return;
            }
            Console.WriteLine($"Changed: {e.FullPath}");
        }
    }
}

1. I created the following dir structure on my machine:
   C:\foo\bar\footest.txt

2. Set the working directory to be C:\foo

3. Run the program and then modify footest.txt to trigger a changed event.

Expected behavior

Output should be:
Changed: C:\foo\bar\footest.txt

Actual behavior

Current output:
Changed: bar\footest.txt

Regression?

No response

Known Workarounds

Using Path.GetFullPath to fetch the fully qualified path

Console.WriteLine($"Changed: {Path.GetFullPath(e.FullPath)}");

Configuration

.Net 6.0
Windows 10 x64
After looking at the source I believe that this is not unique to Windows

Other information

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

As per the documentation https://docs.microsoft.com/en-us/dotnet/api/system.io.filesystemeventargs.fullpath?view=net-6.0 the FullPath property should return a fully qualified path. However it appears the constructor for FileSystemEventArgs assumes the directory passed in is fully qualified: https://github.com/dotnet/runtime/blob/main/src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemEventArgs.cs#L22

When used in conjunction with FileSystemWatcher it is possible for the path to be relative: https://github.com/dotnet/runtime/blob/main/src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.cs#L260

Reproduction Steps

Repro code:

using System;
using System.IO;

namespace MyNamespace
{
    class Program
    {
        static void Main()
        {
            using var watcher = new FileSystemWatcher(@"bar");

            watcher.NotifyFilter = NotifyFilters.LastWrite;

            watcher.Changed += OnChanged;

            watcher.Filter = "*.txt";
            watcher.IncludeSubdirectories = false;
            watcher.EnableRaisingEvents = true;

            Console.WriteLine("Press enter to exit.");
            Console.ReadLine();
        }

        private static void OnChanged(object sender, FileSystemEventArgs e)
        {
            if (e.ChangeType != WatcherChangeTypes.Changed)
            {
                return;
            }
            Console.WriteLine($"Changed: {e.FullPath}");
        }
    }
}

1. I created the following dir structure on my machine:
   C:\foo\bar\footest.txt

2. Set the working directory to be C:\foo

3. Run the program and then modify footest.txt to trigger a changed event.

Expected behavior

Output should be:
Changed: C:\foo\bar\footest.txt

Actual behavior

Current output:
Changed: bar\footest.txt

Regression?

No response

Known Workarounds

Using Path.GetFullPath to fetch the fully qualified path

Console.WriteLine($"Changed: {Path.GetFullPath(e.FullPath)}");

Configuration

.Net 6.0
Windows 10 x64
After looking at the source I believe that this is not unique to Windows

Other information

No response

Author:	UnityAlex
Assignees:	-
Labels:	area-System.IO, untriaged
Milestone:	-

[adamsitnik]

cc @jozkee

[jozkee]

This would require a breaking change on FileSystemEventArgs.FullPath which I'm inclined to accept given the name of the API.

cc @ericstj @jeffhandley

[slask]

Hi @jozkee. Can I take this issue?

[jozkee]

@slask Yes, let me assign you. Thanks!

[slask]

Linking docs issue dotnet/docs#27916

[jeffhandley]

After fixing #68566 with #68582, which was addressing a regression caused by #63051, which fixed this issue, I've reconsidered this original issue further. I think we should revert both #68582 and #63051 and return FileSystemEventArgs and RenamedEventArgs back to their previous behavior and instead address this as a documentation issue.

The regression that was reported involved using an empty directory name, and it was relying on FullPath being relative to the process's directory. From that regression report, it's easy to imagine scenarios where the non-rooted path nature of FullPath is utilized and perhaps even leveraged to combine that full path to a different root. I remember such code I wrote myself back in .NET Framework days that would be broken by this change.

I'm going to submit a PR that reverts both #63051 and #68582, but leaves the additional tests in place, adapted to match the previous behavior.