`System.IO.DirectoryInfo` misreports the `Extension` property for instances constructed with a trailing path (directory) separator

mklement0 commented 5 months ago

Description

System.IO.DirectoryInfo misreports the Extension property for instances constructed with a trailing path (directory) separator: it unexpectedly returns the empty string.

Note:

The same applies to System.IO.FileInfo, because the property is implemented in System.IO.FileSystemInfo; however, even though a trailing path (directory) separator is also tolerated there, it is less likely to be used in practice.
The trailing separator is (sensibly) otherwise ignored, e.g. with respect to .Exists and .Name
The behavior is the same for both / and \, irrespective of platform.

Reproduction Steps

// OK -> ".bar"
new System.IO.DirectoryInfo("foo.bar").Extension

// !! BROKEN -> ""
// Note that .Name works fine.
new System.IO.DirectoryInfo("foo.bar/").Extension

Expected behavior

Both statements should return "bar"

Actual behavior

The statement using the path with the trailing path (directory) separator unexpectedly returns "" (the empty string).

Regression?

No. Also affects .NET Framework.

Known Workarounds

No response

Configuration

.NET 8.0.4, .NET 9.0.0-preview.1.24080.9 All platforms (irrespective of whether \ or / i used).

Other information

No response

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jozkee commented 3 months ago

This is also how Path.GetExtension works. What was your scenario that led you to find this? I wasn't expecting Extension to be in FileSystemInfo but rather in FileInfo.

jozkee commented 3 months ago

The problem is that the code stops as soon as it sees a directory separator. One possible way to solve this could be to trim the trailing separator.

https://github.com/dotnet/runtime/blob/4f96b8f541a62245eb5ca4b643363785cc0d58bf/src/libraries/System.Private.CoreLib/src/System/IO/FileSystemInfo.cs#L57

mklement0 commented 3 months ago

I discovered it in the context of the linked PowerShell issue (https://github.com/PowerShell/PowerShell/issues/21553)

.NET, for better or worse, generally makes no distinction between directories and files with respect to what it considers the extension part of the name.

That Path.GetExtension() behaves the way it does is defensible, but if you construct a System.IO.DirectoryInfo instance with a trailing directory separator that otherwise behaves the same as without it (.Name, .Exists), I think it equally makes sense to ignore this incidental separator with respect to .Extension.

dotnet / runtime