fix: read MockFileInfo/MockDirectoryInfo state lazily like FileSystemInfo

[Mpdreamz]

The real FileSystemInfo does not read a file's state when it is constructed. It takes one snapshot on the first read of any cached property (Exists, Length, Attributes, the times), and serves every property from that snapshot until Refresh(), or until an operation on the same instance invalidates it. MockFileInfo and MockDirectoryInfo take that snapshot in their constructor instead. As a result, a test that creates the info object first and changes the file afterwards sees something the real file system never reports:

var fileInfo = fileSystem.FileInfo.New(path);
fileSystem.File.WriteAllText(path, "x");
fileInfo.Exists; // real: true, MockFileSystem: false

Code that passes against a real disk fails against the mock unless it adds a Refresh() that production code doesn't need.

Change

• The constructors now only mark the snapshot as pending, using the existing refreshOnNextRead flag. The snapshot is taken on first access and cached exactly as before. Refresh() clears the pending flag.
• Instances returned by DirectoryInfo enumeration are refreshed while enumerating, as the runtime fills their snapshot from the enumeration data.
• MoveTo discards the snapshot, as it does in the runtime.
• There is no public API change.

Validated against the real file system

I ran 21 scenarios through new FileSystem() and new MockFileSystem() on .NET 10 and compared the results.

Scenario	Real	Before	After
FileInfo created, then file written, Exists	True	False	True
Exists read, file written, Exists again	False → False	False → False	False → False
… then Refresh()	True	True	True
File exists, FileInfo created, file deleted, Exists	False	True	False
FileInfo created, file grows, Length	10	3	10
Length read, file grows, Length again	3 → 3	3 → 3	3 → 3
FileInfo created before the file, Length	3	FileNotFoundException	3
FileInfo.Create() / Delete() then Exists	True / False	True / False	True / False
DirectoryInfo created, then directory created, Exists	True	False	True
DirectoryInfo on existing directory, deleted, Exists	False	True	False
Attributes / LastWriteTime changed before first read	new	old	new
Attributes / LastWriteTime read, changed, read again	old → old	old → old	old → old
Exists read, file grows, Length	3	3	3
FileInfo from GetFiles(), file deleted, Exists	True	True	True
Length read, file grows, MoveTo, Length	10	3	10
DirectoryInfo attributes read, changed, MoveTo, attributes	new	old	new

Tests

• 20 new tests in MockFileInfoTests and MockDirectoryInfoTests. 12 of them fail on the current code, one for each scenario that was wrong.
• The two …_Exists_ShouldReturnCachedData tests asserted the snapshot from construction time. They now read Exists once before the change, so they still test that the snapshot stays cached after the first read.

In Release on net8.0, net9.0 and net10.0 (macOS): TestingHelpers, Wrappers, API and parity tests all pass.

Worth knowing

Code that relied on the construction-time snapshot will now see the current state. For example: creating a FileInfo, changing the file, and expecting the old values. MockFileSystem.AddFile(IFileInfo, …) still refreshes the instance it is given, so that path is unchanged.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TymoqrVYgZwdNSper5ZZB1

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation consistently matches the described runtime semantics and includes comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Aligns mock file-system metadata caching with real FileSystemInfo lazy snapshot behavior.

Changes:

• Defers metadata snapshots until first cached-property access.
• Invalidates snapshots after moves.
• Preserves enumeration-time snapshots and adds regression tests.

File	Description
MockFileInfo.cs	Implements lazy refresh and move invalidation.
MockDirectoryInfo.cs	Implements lazy refresh, enumeration snapshots, and move invalidation.
MockFileInfoTests.cs	Tests lazy and cached file metadata behavior.
MockDirectoryInfoTests.cs	Tests lazy and cached directory metadata behavior.

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