filesystem

Both end in the same OS call, so the timestamp written is identical. The differences are around it: which cache gets invalidated, whether directories work on Windows, how a non-UTC DateTime is converted, and whether you can target an open handle. Use File.SetLastWriteTimeUtc, by handle when the file is open. Verified on .NET 10.0.10.

On an SMB or NFS share, polling is the source of truth and FileSystemWatcher is at best a hint. FileSystemWatcher misses remote changes on Linux and macOS, overflows its 64 KB network buffer, and stops for good when the connection drops. Use a periodic snapshot diff, and let the watcher only trigger an early scan. Verified on .NET 10.0.10.

FileInfo caches its size, enumeration reads NTFS's lazily updated directory entry, and the writer may still hold bytes in its own buffers. Refresh(), or better, read the length from a shared handle with RandomAccess.GetLength.

FileSystemWatcher fires Changed before the writer is done. Three reliable patterns for .NET 11 to know a file is fully written: open with FileShare.None, debounce with size stabilization, and the producer-side rename trick that avoids the problem entirely.