Creating symbolic links without developer mode or elevation fails silently

[jozkee]

Tested on Windows 10 21H1 19043 without Developer Mode with .NET 6 RTM; it does not throw.

Originally posted by @wegylexy in #57205 (comment)

cc @iSazonov @adamsitnik.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

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

Issue Details

---

Tested on Windows 10 21H1 19043 without Developer Mode with .NET 6 RTM; it does not throw.

Originally posted by @wegylexy in #57205 (comment)

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

[jozkee]

The Windows API CreateSymbolicLinkW is silently failing and we just return what the API reports.

Posible options to deal with this:

• Contact Windows team and discuss if they can fix CreateSymbolicLinkW.

• After calling FileSystemInfo link = File.CreateSymbolicLink("link", "target");
  Check link.Exists in order to verify that it was indeed created. This also works as a workaround for its current state.

• Is very likely that CreateSymbolicLinkW uses DeviceIoControl, we can investigate if using that function for symlink creation is more reliable.
  As a side note: we already use on FileSystemInfo.LinkTarget.

[iSazonov]

If Windows team can not fix old (supported) Windows versions it is best to use DeviceIoControl and backport to .Net 6 as bug fix.

[wegylexy]

I have something on production that assumes it doesn't throw and checks for existence afterwards. A user reports it throws on Windows 7 when not run as admin.

[jozkee]

@wegylexy @Raffaello I talked with the Windows team and they pointed out that the silent failure was due to an error on our side, so I fixed it in the main branch. Could you please help me validaiting the fix for your scenarios with a nightly build of .NET 7? That way, we can propose porting the fix to .NET 6.

Download the nightly build from https://github.com/dotnet/installer#table (column main (7.0.x Runtime)), download the version ad-hoc to your platform.
Run your scenario targeting net7.0, and please let me know if it is now fixed.

[wegylexy]

@jozkee We switched to use directory junction points and copying files instead, as typical users won't have developer mode enabled.
Now I just look forwards to QUIC datagram and NativeAOT in .NET 7.
So, I am going to just trust that you have fixed it.

[Raffaello]

@jozkee ok, i will have a look, just need to find some times, hopefully i should hace some in the weekend.

[Raffaello]

@jozkee with .NET7-preview5 an exception has been thrown, so it fixes that doesn't silently failing.

test ref: https://github.com/BlueRedSkyLTD/PowerDir/blob/main/PowerDir.Tests/EscapeCodesThemesTest.cs#L116-L158
([TestMethod]) is commented as it won't work as a test as requires dev/admin privileges to create a symlink

[jozkee]

Thanks!