fix: CreateSubdirectory failing for root directories by Neo-vortex · Pull Request #121906 · dotnet/runtime

Neo-vortex · 2025-11-23T18:07:53Z

Fix CreateSubdirectory failing for root directories

Summary

Fixes a bug where DirectoryInfo.CreateSubdirectory(path) incorrectly throws ArgumentException when called on root directory instances (e.g., C:\).

Problem

Path.TrimEndingDirectorySeparator preserves trailing separators for root paths to maintain valid path format. This causes the boundary validation logic to use an incorrect index when checking for directory separators.

Example failure:

var rootDir = new DirectoryInfo(@"C:\");
rootDir.CreateSubdirectory("test"); // Throws ArgumentException

The check at index [3] evaluates the character 't' instead of the separator '\' at index [2], causing validation to fail.

Solution

After calling Path.TrimEndingDirectorySeparator, explicitly check if trimmedCurrentPath still has a trailing separator (root directory case) and remove it manually before performing boundary validation.

if (trimmedCurrentPath.Length > 0 && 
    PathInternal.IsDirectorySeparator(trimmedCurrentPath[trimmedCurrentPath.Length - 1]))
{
    trimmedCurrentPath = trimmedCurrentPath.Slice(0, trimmedCurrentPath.Length - 1);
}

This ensures consistent behavior: trimmedCurrentPath never ends with a separator, making the boundary index check work correctly for all directory types.

Impact

Fixes: Root directory subdirectory creation (Windows: C:\, D:\, )
No breaking changes: Non-root directory behavior unchanged
Low risk: Localized change to validation logic only

Testing

Existing tests continue to pass (non-root directories)
Manual testing confirms fix for root directories:

  new DirectoryInfo(@"C:\").CreateSubdirectory("test");     // ✓ Works
  new DirectoryInfo(@"C:\Users").CreateSubdirectory("Docs"); // ✓ Still works

Path.TrimEndingDirectorySeparator preserves trailing separators for root paths like "C:\" or "/", causing boundary validation to check the wrong index. Added explicit trim of trailing separator from trimmedCurrentPath to fix the boundary check for root directory cases. Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

kasperk81 · 2025-11-23T20:38:12Z

Example failure:

that can be a test case

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Neo-vortex · 2025-11-23T22:35:34Z

I added a test
@kasperk81

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

…ortex/runtime into fix-directory-creating-root-folder

Neo-vortex · 2025-11-25T16:11:52Z

it seems Linux is also effected.
this code fails with the same error without this PR

var dir = new DirectoryInfo("/");
dir.CreateSubdirectory("test");

I added a test for linux as well

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Neo-vortex · 2025-11-29T15:52:55Z

linux tests are failing because of permission.

      System.UnauthorizedAccessException : Access to the path '/TestFolder_1aa9354d27554fa5a93f0ad594276905' is denied.
      ---- System.IO.IOException : Permission denied

the fact that it is trying to create the folder but gets Permission denied is an indication that this PR also fixes linux.
the problem is now how to make a test that actually passes without accessing the "/" itself

Neo-vortex · 2025-11-30T20:44:12Z

@vcsjones
how should we process from here ?

kasperk81 · 2025-11-30T21:03:55Z

linux tests are failing because of permission.

if only the newly added tests in PR are failing, then change them to:

try { Assert... } catch (Exception ex) { Assert(Exception is UnauthorizedAccessException) }

both cases are acceptable, either original assert passed or the op failed because of access issue. other exceptions are unexpected and should fail.

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs

…Directory/EnumerableTests.cs Co-authored-by: kasperk81 <[email protected]>

src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs

…fo.cs Co-authored-by: Jeremy Kuhne <[email protected]>

revert back to the old approved logic

Neo-vortex · 2026-02-09T07:24:38Z

reverted back to the known good code

jeffhandley · 2026-02-16T08:52:18Z

@Neo-vortex I pushed 214a18d to your branch to apply the feedback from @JeremyKuhne appropriately.

What you were missing is that TrimEndingDirectorySeparator does not trim when the path is a root.

runtime/src/libraries/Common/src/System/IO/PathInternal.cs

Lines 229 to 232 in 31c3ed9

    
           internal static ReadOnlySpan<char> TrimEndingDirectorySeparator(ReadOnlySpan<char> path) => 
        
               EndsInDirectorySeparator(path) && !IsRoot(path) ? 
        
                   path.Slice(0, path.Length - 1) : 
        
                   path;

In the case of trimmedCurrentPath, we explicitly do want to trim that character off the end when it's a root path. For that variable, we can use TrimEnd.

Neo-vortex · 2026-02-16T09:13:55Z

@Neo-vortex I pushed 214a18d to your branch to apply the feedback from @JeremyKuhne appropriately.

What you were missing is that TrimEndingDirectorySeparator does not trim when the path is a root.

runtime/src/libraries/Common/src/System/IO/PathInternal.cs

Lines 229 to 232 in 31c3ed9

internal static ReadOnlySpan<char> TrimEndingDirectorySeparator(ReadOnlySpan<char> path) =>

EndsInDirectorySeparator(path) && !IsRoot(path) ?

path.Slice(0, path.Length - 1) :

path;

In the case of trimmedCurrentPath, we explicitly do want to trim that character off the end when it's a root path. For that variable, we can use TrimEnd.

LGTM

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs

Move the root directory CreateSubdirectory tests from EnumerableTests.cs to DirectoryInfo/CreateSubdirectory.cs per review feedback. Co-authored-by: Copilot <[email protected]>

jozkee · 2026-02-25T02:18:41Z

/ba-g infra timeout on "android-arm64 Release AllSubsets_Mono_Smoke", passed on arm.

github-actions bot added the needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners label Nov 23, 2025

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Neo-vortex mentioned this pull request Nov 23, 2025

DirectoryInfo.CreateSubdirectory fails when DirectoryInfo path is a root directory #116087

Closed

vcsjones added area-System.IO and removed needs-area-label An area label is needed to ensure this gets routed to the appropriate area owners labels Nov 23, 2025

typo: use PathInternal for IsDirectorySeparator

f87932e

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

test: add a test to verify the fix

fd1987e

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Neo-vortex added 3 commits November 24, 2025 14:41

Merge branch 'main' into fix-directory-creating-root-folder

1d412f1

test: add test for Linux as well

f6bbb02

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Merge branch 'fix-directory-creating-root-folder' of github.com:Neo-v…

7781fc0

…ortex/runtime into fix-directory-creating-root-folder

fix: semi-colon missing

59a0852

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

build-analysis bot mentioned this pull request Nov 29, 2025

Unable to pull image from mcr.microsoft.com #117164

Open

Neo-vortex added 2 commits December 1, 2025 00:40

test: UnauthorizedAccessException is expected for linux test

352fc8e

Signed-off-by: Mohamadreza Nakhleh <[email protected]>

Merge branch 'dotnet:main' into fix-directory-creating-root-folder

afdb332

kasperk81 reviewed Nov 30, 2025

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src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs Outdated Show resolved Hide resolved

kasperk81 reviewed Nov 30, 2025

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src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs Outdated Show resolved Hide resolved

Update src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/…

d070c8a

…Directory/EnumerableTests.cs Co-authored-by: kasperk81 <[email protected]>

Neo-vortex added 2 commits February 4, 2026 10:17

improve: remove redundent logics

88a7160

fix: missing semicolon

a6c5211

build-analysis bot mentioned this pull request Feb 4, 2026

[android][arm64] System.Net.Sockets.Tests.SendTo_SyncForceNonBlocking.Datagram_UDP_ShouldImplicitlyBindLocalEndpoint fails with NetworkUnreachable #120526

Open

JeremyKuhne reviewed Feb 5, 2026

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src/libraries/System.Private.CoreLib/src/System/IO/DirectoryInfo.cs Outdated Show resolved Hide resolved

Neo-vortex and others added 3 commits February 5, 2026 23:31

Update src/libraries/System.Private.CoreLib/src/System/IO/DirectoryIn…

9e24ef4

…fo.cs Co-authored-by: Jeremy Kuhne <[email protected]>

Merge branch 'main' into fix-directory-creating-root-folder

e833ef0

Update DirectoryInfo.cs

bdb96e6

revert back to the old approved logic

Use TrimEnd to force trimming the separator char off the root

214a18d

jeffhandley force-pushed the fix-directory-creating-root-folder branch from f5e58fb to 214a18d Compare February 16, 2026 08:40

Merge branch 'main' into fix-directory-creating-root-folder

0debaa8

jozkee reviewed Feb 21, 2026

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src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/Directory/EnumerableTests.cs Outdated Show resolved Hide resolved

Move CreateSubdirectory root tests to CreateSubdirectory.cs

9e7193a

Move the root directory CreateSubdirectory tests from EnumerableTests.cs to DirectoryInfo/CreateSubdirectory.cs per review feedback. Co-authored-by: Copilot <[email protected]>

jozkee approved these changes Feb 25, 2026

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jozkee enabled auto-merge (squash) February 25, 2026 00:04

jozkee merged commit 619214e into dotnet:main Feb 25, 2026
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build-analysis bot mentioned this pull request Feb 25, 2026

Failing test due to no detected IO events in 'System.IO.Tests.FileSystemWatcherTest.ExecuteAndVerifyEvents' #103584

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dotnet-maestro bot mentioned this pull request Feb 25, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#5088

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fix: CreateSubdirectory failing for root directories#121906

fix: CreateSubdirectory failing for root directories#121906
jozkee merged 23 commits intodotnet:mainfrom
Neo-vortex:fix-directory-creating-root-folder

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Neo-vortex commented Nov 23, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Fix CreateSubdirectory failing for root directories

Summary

Problem

Solution

Impact

Testing

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