Improve exception message when passing directory to ZipFile methods

[iremyux]

Fixes an issue where passing a directory path to ZipFile methods resulted in an unclear UnauthorizedAccessException without explaining that the problem was passing a directory instead of a file.

Modified GetFileStreamForOpen() to catch UnauthorizedAccessException when FileStream fails to open a directory, and re-throw with a clearer error message that explicitly states the path is a directory.

Fixes #100720

[Copilot]

Pull request overview

This PR improves error messages when directory paths are incorrectly passed to ZipFile methods that expect file paths. Instead of the generic UnauthorizedAccessException, users now receive a clearer error message explicitly stating that a directory is not allowed.

Key changes:

• Enhanced GetFileStreamForOpen() to detect when a directory path is passed and throw a more descriptive error message
• Added comprehensive test coverage for Read and Update modes across sync and async methods
• Added new localized error message string IO_DirectoryNotAllowed

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.IO.Compression.ZipFile/src/System/IO/Compression/ZipFile.Create.cs	Added try-catch block in GetFileStreamForOpen() to detect directory paths and throw improved error messages
src/libraries/System.IO.Compression.ZipFile/src/Resources/Strings.resx	Added new error message resource IO_DirectoryNotAllowed for directory-related errors
src/libraries/System.IO.Compression.ZipFile/tests/ZipFile.Open.cs	Added test methods to verify improved error messages for sync and async Open methods
src/libraries/System.IO.Compression.ZipFile/tests/ZipFile.Extract.cs	Added test to verify improved error message for ExtractToDirectory methods

[Copilot]

The catch block only handles UnauthorizedAccessException, but when ZipArchiveMode.Create is used with an existing directory, FileStream will throw IOException instead (because FileMode.CreateNew is used). This means the improved error message won't be provided for Create mode.

Consider adding an additional catch block for IOException when the mode is ZipArchiveMode.Create:

catch (UnauthorizedAccessException) when (Directory.Exists(archiveFileName))
{
    throw new UnauthorizedAccessException(SR.Format(SR.IO_DirectoryNotAllowed, archiveFileName));
}
catch (IOException) when (mode == ZipArchiveMode.Create && Directory.Exists(archiveFileName))
{
    throw new IOException(SR.Format(SR.IO_DirectoryNotAllowed, archiveFileName));
}

Suggested change

	}
	}
	catch (IOException) when (mode == ZipArchiveMode.Create && Directory.Exists(archiveFileName))
	{
	// If we get IOException in Create mode and the path is a directory, provide a clearer error message
	throw new IOException(SR.Format(SR.IO_DirectoryNotAllowed, archiveFileName));
	}

[rokonec]

@iremyux please review above, it looks legit to me. Also please consider to add test with ZipArchiveMode.Create if it make sense.

[iremyux]

It looks like The IOException from FileMode.CreateNew is specifically for when a file already exists. However, this PR is addressing the case where a directory path is passed, not an existing file.
So I'm not adding this catch since UnauthorizedAccessException will be thrown even in ZipArchiveMode.Create mode.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.