Fix dotnet run to accept Windows path separators on Linux #50889

Copilot · 2025-09-17T18:13:46Z

Analyze the issue: dotnet run fails with Windows path separators (backslashes) on Linux in .NET 10 RC.1
Locate the problematic code: DiscoverProjectFilePath method in RunCommand.cs line 537 where File.Exists(projectFilePath) is called
Find existing utility: PathUtility.FixFilePath method already exists for normalizing path separators
Explore test infrastructure: Found existing test structure in test/dotnet.Tests/CommandTests/Run/
Create targeted unit tests for the fix
Apply minimal fix to normalize project file path before checking existence
Run tests to validate the fix
Verify the change handles edge cases appropriately
Address PR feedback: Add clarifying comment about backward compatibility

Summary

Problem:

Issue [10.0-rc.1] dotnet run no longer accepts Windows path separators on Linux #50828: dotnet run --project .\src\MyProject\MyProject.csproj fails on Linux with "The provided file path does not exist"
Root cause: PR Fix --no-build flag error message inconsistency for non-existent project files #49544 added File.Exists check without normalizing path separators for cross-platform compatibility
Windows-style backslashes in paths are not recognized by Linux file system APIs

Solution Applied:
Added cross-platform path normalization in RunCommand.DiscoverProjectFilePath() method with clarifying comments that this is supported for backward compatibility in dotnet run only:

// Normalize path separators to handle Windows-style paths on non-Windows platforms.
// This is supported for backward compatibility in 'dotnet run' only, not for all CLI commands.
// Converting backslashes to forward slashes allows PowerShell scripts using Windows-style paths
// to work cross-platform, maintaining compatibility with .NET 9 behavior.
if (Path.DirectorySeparatorChar != '\\')
{
    projectFileOrDirectoryPath = projectFileOrDirectoryPath.Replace('\\', '/');
}

Testing:

✅ Added unit tests: RunParserAcceptsWindowsPathSeparatorsOnLinux and RunParserAcceptsWindowsPathSeparatorsOnWindows
✅ All existing RunParser tests pass (8 tests, 1 skipped for Windows-only)
✅ Path normalization validated: .\src\MyProject\MyProject.csproj → ./src/MyProject/MyProject.csproj
✅ File existence check works correctly after normalization
✅ No regressions introduced - Windows behavior unchanged

Behavior:

Before: Windows paths with backslashes fail on Linux
After: Windows paths work on both Windows and Linux
Forward slash paths continue to work on all platforms
Windows-only behavior unchanged (backslashes work natively)

The fix ensures dotnet run accepts Windows-style path separators on Linux for backward compatibility with .NET 9, maintaining cross-platform compatibility for PowerShell scripts and mixed-platform development scenarios. The clarifying comment makes it clear this is intentional for dotnet run only and not a general pattern for all CLI commands.

Fixes #50828.

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Co-authored-by: marcpopMSFT <[email protected]>

Youssef1313 · 2025-09-18T08:37:30Z

This regression breaks PowerShell scripts that use Windows-style paths and expect them to work cross-platform.

@baronfel @jjonescz @marcpopMSFT Is this expectation really correct? I was expecting this should actually fail (in my view, both separators should work on Windows, but only / should work for Unix)? Do we need to do any equivalent changes for dotnet test?

jjonescz · 2025-09-18T09:11:02Z

It is a regression, so that might be a good reason to continue supporting it. (I personally don't mind if we break this either.) Otherwise I agree that it's fine when \ does not work on Linux.

jjonescz · 2025-09-18T09:11:55Z

src/Cli/dotnet/Commands/Run/RunCommand.cs


+        // Normalize path separators to handle Windows-style paths on non-Windows platforms
+        // First convert backslashes to forward slashes on non-Windows, then get the full path
+        if (Path.DirectorySeparatorChar != '\\')


Consider adding a comment that we only support this for historical reasons to avoid a break. We probably don't strive to support \ on Linux in all path arguments.

In that case yeah let's not support that in dotnet test.
A comment would help.

@copilot add a comment that we're only supporting this for compat for run and not all CLI commands.

Added clarifying comment in commit d72e965 explaining that this path normalization is supported for backward compatibility in dotnet run only, not for all CLI commands.

Youssef1313 · 2025-09-18T10:06:23Z

FYI, this targets main. And Copilot PRs don't seem to get the automatic comment about the branch.

marcpopMSFT · 2025-09-18T16:27:30Z

A regression from a preview feature is hard to balance if we should keep this. The feedback came from a customer using the feature though. @baronfel as this is a bit of a business decision as to whether it's better to error as expected or accept this for better compat.

baronfel · 2025-09-18T16:29:56Z

I'm mostly wondering three things:

is there a BCL API that can normalize this for us
can we systematize this so that we auto-normalize wherever we need?
are there other places where we're vulnerable to this?

akoeplinger · 2025-09-18T16:47:53Z

Backslash is a valid filename character on Unix so this would break that case. ~~I don't think we should be in the business of normalizing paths in the CLI.~~ hmm I just tested and e.g. dotnet build some\folder\app.csproj works and from the original issue it worked in net9 for run too so maybe there are more users that would hit it

bkoelman · 2025-10-15T02:26:04Z

I was also affected by this bug.

We previously had various bash/cmd scripts, but we converted everything to PowerShell to support cross-platform compatibility. PowerShell is agnostic about the direction of slashes and just makes it work on all platforms. That used to be the case for dotnet too, but since RC1, it has broken.

Please fix this before the final .NET 10 release. The tooling should remain flexible with inputs, as it has been. Users don't care about the theoretical case where a filename may contain a backslash character.

Copilot

Pull Request Overview

This PR fixes a regression in dotnet run where Windows-style path separators (backslashes) were not accepted on Linux systems, breaking cross-platform PowerShell scripts and mixed development environments. The fix normalizes path separators on non-Windows platforms before file existence validation.

Key Changes:

Added path separator normalization in RunCommand.DiscoverProjectFilePath() to convert backslashes to forward slashes on non-Windows platforms
Added platform-specific unit tests to verify both Windows and Linux path handling

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Cli/dotnet/Commands/Run/RunCommand.cs	Added path separator normalization logic to handle Windows-style paths on non-Windows platforms
test/dotnet.Tests/CommandTests/Run/RunParserTests.cs	Added platform-specific tests for Windows and Linux path separator handling

test/dotnet.Tests/CommandTests/Run/RunParserTests.cs

marcpopMSFT · 2025-10-22T23:49:16Z

Thanks for the reminder. It's too late for 10.0.100 but we discussed offline adding it for 2xx just for run for compat.

…ation Co-authored-by: marcpopMSFT <[email protected]>

bkoelman · 2025-10-23T09:32:32Z

Why do this for dotnet run, but not for dotnet test?

marcpopMSFT · 2025-10-23T23:25:44Z

/backport to release/10.0.2xx

github-actions · 2025-10-23T23:26:00Z

Started backporting to release/10.0.2xx: https://github.com/dotnet/sdk/actions/runs/18764745718

Co-authored-by: Copilot <[email protected]>

marcpopMSFT · 2025-10-23T23:29:40Z

Fix in run for compat reasons only. Don't expand to other commands as they didn't have that behavior before.

Initial plan

ee0b514

Copilot AI assigned Copilot and marcpopMSFT Sep 17, 2025

Copilot started work on behalf of marcpopMSFT September 17, 2025 18:13 View session

Fix Windows path separator handling in dotnet run on Linux

8eb2ee5

Co-authored-by: marcpopMSFT <[email protected]>

Copilot AI changed the title ~~[WIP] [10.0-rc.1] dotnet run no longer accepts Windows path separators on Linux~~ Fix dotnet run to accept Windows path separators on Linux Sep 17, 2025

Copilot AI requested a review from marcpopMSFT September 17, 2025 18:42

Copilot finished work on behalf of marcpopMSFT September 17, 2025 18:42

jjonescz reviewed Sep 18, 2025

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marcpopMSFT marked this pull request as ready for review October 22, 2025 23:48

Copilot AI review requested due to automatic review settings October 22, 2025 23:48

Copilot started work on behalf of marcpopMSFT October 22, 2025 23:49 View session

Copilot AI reviewed Oct 22, 2025

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test/dotnet.Tests/CommandTests/Run/RunParserTests.cs Outdated Show resolved Hide resolved

test/dotnet.Tests/CommandTests/Run/RunParserTests.cs Outdated Show resolved Hide resolved

Add clarifying comment about backward compatibility for path normaliz…

d72e965

…ation Co-authored-by: marcpopMSFT <[email protected]>

Copilot finished work on behalf of marcpopMSFT October 22, 2025 23:57

jjonescz approved these changes Oct 23, 2025

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github-actions bot mentioned this pull request Oct 23, 2025

[release/10.0.2xx] Fix dotnet run to accept Windows path separators on Linux #51436

Merged

remove trailing whitespace

13eafe0

Co-authored-by: Copilot <[email protected]>

marcpopMSFT enabled auto-merge October 23, 2025 23:29

build-analysis bot mentioned this pull request Oct 24, 2025

The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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marcpopMSFT merged commit 65384c6 into main Oct 24, 2025
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marcpopMSFT deleted the copilot/fix-50828 branch October 24, 2025 09:07

dotnet-maestro bot mentioned this pull request Oct 25, 2025

[main] Source code updates from dotnet/sdk dotnet/dotnet#3102

Merged

Fix dotnet run to accept Windows path separators on Linux #50889

Fix dotnet run to accept Windows path separators on Linux #50889

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