Path.GetFileNameWithoutExtension() does not throw ArgumentException ("Illegal characters in path") on Windows

[arvindshmicrosoft]

Description

I'm migrating a project from .NET Framework 4.7.2 to .NET 6.0. One of my tests is failing, and I have isolated the issue to Path.GetFileNameWithoutExtension seemingly handling the path like it is a Unix path. Consequently, it returns the entire path except the extension, and does not throw if I had invalid (for Windows) characters in the path.

Reproduction Steps

string fileName = @"<<System32\KERNELBASE.dll>>";
string result;
Console.WriteLine(Environment.OSVersion);
Console.WriteLine(Environment.OSVersion.Platform);
result = Path.GetFileNameWithoutExtension(fileName);
Console.WriteLine("GetFileNameWithoutExtension('{0}') returns '{1}'", fileName, result);

The csproj is below:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0-windows</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

Expected behavior

ArgumentException should be thrown as the < and > characters are invalid for Windows. In .NET Framework 4.7.2 for example, the above code results in the following exception:

[System.ArgumentException: Illegal characters in path.]
   at System.IO.Path.CheckInvalidPathChars(String path, Boolean checkAdditional)
   at System.IO.Path.GetFileName(String path)
   at System.IO.Path.GetFileNameWithoutExtension(String path)

Actual behavior

Microsoft Windows NT 10.0.22000.0
Win32NT
GetFileNameWithoutExtension('<<System32\KERNELBASE.dll>>') returns 'KERNELBASE'

Regression?

Maybe?

Known Workarounds

No response

Configuration

.NET 6.0.7
OS: Windows 11 - 10.0.22000 Build 22000
Architecture: x64

Other information

No response

[ghost]

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Issue Details

---

Description

I'm migrating a project from .NET Framework 4.7.2 to .NET 6.0. One of my tests is failing, and I have isolated the issue to Path.GetFileNameWithoutExtension seemingly handling the path like it is a Unix path. Consequently, it returns the entire path except the extension, and also incidentally does not throw if I had invalid (for Windows) characters in the path.

Reproduction Steps

string fileName = @"C:\mydir\myfile.ext";
string result;

result = Path.GetFileNameWithoutExtension(fileName);
Console.WriteLine("GetFileNameWithoutExtension('{0}') returns '{1}'", fileName, result);

Expected behavior

// EXPECTED: GetFileNameWithoutExtension('C:\mydir\myfile.ext') returns 'myfile'

Actual behavior

// ACTUAL: GetFileNameWithoutExtension('C:\mydir\myfile.ext') returns 'C:\mydir\myfile' on .NET 6.0 on Windows

Regression?

Maybe?

Known Workarounds

No response

Configuration

.NET 6.0.7
OS: Windows 11 - 10.0.22000 Build 22000
Architecture: x64

Other information

No response

Author:	arvindshmicrosoft
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[Fabi]

Can't reproduce on (x86 64 bit) macOS, linux and w10/11 with latest .net6 and .net 7 prefiew. All return the expected result.

[jkotas]

I am not able to reproduce it either. @arvindshmicrosoft Could you please double check the repro steps and the result that you are seeing?

[ghost]

This issue has been marked needs-author-action and may be missing some important information.

[stephentoub]

I can't repro it either, and from looking at the code, it's not clear to me how this could happen.

[arvindshmicrosoft]

Thank you all for looking. Something must be very specific to my case. I will investigate further and get back.

[arvindshmicrosoft]

@Fabi @jkotas @stephentoub - in the process of distilling the repro down, I had deviated from the core issue, which seems to be that the erstwhile Illegal characters in path ArgumentException, which used to be raised from CheckInvalidPathChars, does not happen on .NET 6.0 on Windows, and consequently a file name is returned when it should not be. I have updated the title of this Issue, as well as the repro (in the first comment). I hope it helps understand what might be happening here.

[jkotas]

That was intentional breaking change in .NET Core 2.1. More details in https://docs.microsoft.com/en-us/dotnet/core/compatibility/2.1#path-apis-dont-throw-an-exception-for-invalid-characters

[stephentoub]

Illegal characters in path ArgumentException, which used to be raised from CheckInvalidPathChars, does not happen on .NET 6.0 on Windows

That was changed in .NET Core 2.1:
https://docs.microsoft.com/en-us/dotnet/core/compatibility/2.1#path-apis-dont-throw-an-exception-for-invalid-characters
We rely on the OS now for path validation, rather than trying to guess what's valid.

[arvindshmicrosoft]

@jkotas and @stephentoub thank you for pointing this out. I really appreciate the help.