Path functions behaving differently on UNC device paths than implied from documentation

[s5bug]

Description

File path formats on Windows systems states:

For device UNCs, the server/share portion forms the volume. For example, in \\?\server1\utilities\\filecomparer\, the server/share portion is server1\utilities. This is significant when calling a method such as Path.GetFullPath(String, String) with relative directory segments; it is never possible to navigate past the volume.

The way this reads to me is that \\?\server1\utilities\\ should be considered the "root" of \\?\server1\utilities\\filecomparer\.

Specifically, the documentation states

• "the server/share portion forms the volume"
• "in [...] the server/share portion is server1\utilities"
• "...it is never possible to navigate past the volume"

This seems to not be respected:

Reproduction Steps

> dotnet fsi

Microsoft (R) F# Interactive version 12.4.0.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> open System.IO;;
> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

Expected behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = <null>

Actual behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = "\\?\server1\utilities"

Regression?

I don't know how to test old .NET versions, I'm not a long-time .NET user.

Known Workarounds

No response

Configuration

> dotnet --version
7.0.100

> Get-ComputerInfo -Property "Os*"

OsName                                     : Microsoft Windows 10 Home
OsType                                     : WINNT
OsOperatingSystemSKU                       : WindowsHome
OsVersion                                  : 10.0.19045
OsArchitecture                                          : 64-bit
OsLanguage                                              : en-US

I would be pretty sure it's Windows (Path)-specific.

Other information

No response

[danmoseley]

FYI a good way to do a test on .NET Framework (ie to check old behavior) is http://sharplab.io (pick it from the drop-down)

[s5bug]

It seems sharplab is not a fan of System.IO.Path:

Unbreakable.AssemblyGuardException: Type System.IO.Path is not allowed.

Example Code

using System;
using System.IO;

Console.WriteLine(
  Path.Combine("\\\\?\\server1\\utilities\\\\filecomparer\\", "..\\..")
);

[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

File path formats on Windows systems states:

For device UNCs, the server/share portion forms the volume. For example, in \\?\server1\utilities\\filecomparer\, the server/share portion is server1\utilities. This is significant when calling a method such as Path.GetFullPath(String, String) with relative directory segments; it is never possible to navigate past the volume.

The way this reads to me is that \\?\server1\utilities\\ should be considered the "root" of \\?\server1\utilities\\filecomparer\.

Specifically, the documentation states

• "the server/share portion forms the volume"
• "in [...] the server/share portion is server1\utilities"
• "...it is never possible to navigate past the volume"

This seems to not be respected:

Reproduction Steps

> dotnet fsi

Microsoft (R) F# Interactive version 12.4.0.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> open System.IO;;
> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

Expected behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = <null>

Actual behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = "\\?\server1\utilities"

Regression?

I don't know how to test old .NET versions, I'm not a long-time .NET user.

Known Workarounds

No response

Configuration

> dotnet --version
7.0.100

> Get-ComputerInfo -Property "Os*"

OsName                                     : Microsoft Windows 10 Home
OsType                                     : WINNT
OsOperatingSystemSKU                       : WindowsHome
OsVersion                                  : 10.0.19045
OsArchitecture                                          : 64-bit
OsLanguage                                              : en-US

I would be pretty sure it's Windows (Path)-specific.

Other information

No response

Author:	s5bug
Assignees:	-
Labels:	area-System.IO, untriaged
Milestone:	-

[huoyaoyuan]

Path.GetPathRoot(@"\\?\server1\utilities\\filecomparer\") returns empty string on .NET Framework. Path.GetFullPath(path, basePath) isn't supported on .NET Framework. So this is likely not a regression.

[s5bug]

What about Path.Combine for .NET Framework, as shown in the example code?

[jozkee]

What about Path.Combine

Path.Combine doesn't resolve relative segments, what you are referring to is being discussed in #2162.