Use OS-level filtering for searchPattern in DirectoryInfo.GetFiles, and add search pattern property/parameter to FileSystemEnumerable

[benstevens48]

As far as I can tell, DirectoryInfo.GetFiles and related functions apply the file name search pattern filtering in managed code. However, there is support, at least on Windows, for it at the OS level. Indeed, FileSystemEnumerable/FileSystemEnumerator uses NtQueryDirectoryFile which accepts a file name search pattern, but this is parameter is not used. Surely it would be a lot quicker to use this OS-level parameter for searching e.g. filtering by a single file extension? I suggest adding a searchPattern string to the constructor of FileSystemEnumerable/FileSystemEnumerator (or a property etc, whichever seems best), which can be passed to the OS-level function. If not supported on other OSes then it can just use filtering in managed code as currently. Then DirectoryInfo.GetFiles etc can also use this new searchPattern parameter internally (I believe they use FileSystemEnumerable/FileSystemEnumerator internally).

[jilles-sg]

This seems to have been implemented already in #122947 and #123695 .

[benstevens48]

Interesting, I was browsing the source from the docs so didn't see that. But I don't think the search pattern constructor for FileSystemEnumerable/FileSystemEnumerator has been made public (?). Can we get a public constructor allowing us to pass it? Otherwise FileSystemEnumerable/FileSystemEnumerator, which is meant to be the most performant API, becomes less efficient than DirectoryInfo.GetFiles etc in this scenario.