For a network drive, System.Windows.Forms.Application.StartupPath returns a path with \?UNC\ prefix

[Aniobodo]

.NET version

9.0

Did it work in .NET Framework?

Yes

Did it work in any of the earlier releases of .NET Core or .NET 5+?

It worked in .NET 8

Issue description

System.Windows.Forms.Application.StartupPath returns a path with ?UNC\ prefix

Steps to reproduce

observe System.Windows.Forms.Application.StartupPath in Visual Studio debug mode.

[Zheng-Li01]

The issue can reproduce .NET 9.0 & latest .NET 10.0 as below screenshot.

[JeremyKuhne]

We just return what the .NET runtime gives us for AppContext.BaseDirectory.

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[ericstj]

I repro this, looks to me like AppContext.BaseDirectory is just returning Assembly.GetEntryAssembly()?.Location which also shows this change. Seems that just returns the path that was passed in when loading - so this is either a change to the loader or the host that caused this.

[agocke]

This is a regression likely caused by my change in #99576.

In following the path to determine if it is a symbolic link the path was converted to a UNC path.

My suspicion is that this can be fixed by also checking if the target path is a UNC path instead of just an extended path here but I haven't yet convinced myself that's all that's needed.

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[Aniobodo]

Fixed?

[bitapparat]

I just wanted to add that this issue also prevents any WPF app with a MediaElement from running from a UNC path. While resolving the Uri of the MediaElement.Source WPF passes the result of AppDomain.BaseDirectory to the Uri constructor, resulting in a UriFormatException because the Uri constructor can't deal with the \\?UNC\ prefix.

This happens here:
https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/SecurityHelper.cs#L72
appBase = new Uri(domain.BaseDirectory);