Path.ChangeExtension with an empty string behaves differently on Linux and Windows platforms

[jasonleenaylor]

Description

We have a cross platform application running on Windows and Linux and we noticed a difference between the
ChangeExtension implementation on the two platforms. On Windows calling ChangeExtension with an empty string results in the extension being removed. e.g. "file.extension" becomes "file" but on Linux we get "file."

On windows it must be running the framework version of this code but on Linux we get the core runtime implementation.

Reproduction Steps

Run Path.ChangeExtension("file.extension", "") on Windows and Linux

Expected behavior

The extension is removed leaving a bare file name with no trailing '.'

Actual behavior

The extension is removed leaving a trailing '.'

Regression?

This works as described in .NET Framework

Known Workarounds

Users can use Path.RemoveExtension instead when removing, but if you are adding an optional extension the only workaround is to add a test for empty before using the method

Configuration

No response

Other information

runtime/src/libraries/System.Private.CoreLib/src/System/IO/Path.cs

Line 75 in 068ba8e

string.Concat(subpath, ".", extension);

[jasonleenaylor]

@vcsjones or anyone I am happy to make a PR for this, but I would like to add a unit test. Can you help me locate other unit tests for System.IO?

[tommysor]

@jasonleenaylor Tests:

runtime/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/IO/PathTests.cs

Lines 13 to 34 in d79a753

	[Theory,
	InlineData(null, null, null),
	InlineData(null, "exe", null),
	InlineData("", "", ""),
	InlineData("file.exe", null, "file"),
	InlineData("file.exe", "", "file."),
	InlineData("file", "exe", "file.exe"),
	InlineData("file", ".exe", "file.exe"),
	InlineData("file.txt", "exe", "file.exe"),
	InlineData("file.txt", ".exe", "file.exe"),
	InlineData("file.txt.bin", "exe", "file.txt.exe"),
	InlineData("dir/file.t", "exe", "dir/file.exe"),
	InlineData("dir/file.exe", "t", "dir/file.t"),
	InlineData("dir/file", "exe", "dir/file.exe")]
	public void ChangeExtension(string path, string newExtension, string expected)
	{
	if (expected != null)
	expected = expected.Replace('/', Path.DirectorySeparatorChar);
	if (path != null)
	path = path.Replace('/', Path.DirectorySeparatorChar);
	Assert.Equal(expected, Path.ChangeExtension(path, newExtension));
	}

[MihaZupan]

Console.WriteLine(Path.ChangeExtension("file.extension", null));
Console.WriteLine(Path.ChangeExtension("file.extension", ""));

prints

file
file.

On .NET Framework, .NET 8 on Windows and .NET 8 on Linux.

[jasonleenaylor]

@MihaZupan Hmm, we saw this issue when running in .NET 6 on Windows, and .NET 6 on Linux.

It added the period on Linux, but not on Windows.

[jozkee]

As @MihaZupan described, this isn't inconsistent between OSes, and it works as documented, including .NET 6.

If extension is an empty string, the returned path string contains the contents of path with any characters following the last period removed.

Can you please double check if you are really passing "" to the extension argument.

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