How to use Directory.EnumerateFiles excluding hidden and system files

.Where(f => (new FileInfo(f).Attributes & FileAttributes.Hidden & FileAttributes.System) == 0)

Since FileAttributes values are flags, they are disjunctive on the bit level, so you can combine them properly. As such, FileAttributes.Hidden & FileAttributes.System will always be 0. So you’re essentially checking for the following:

(new FileInfo(f).Attributes & 0) == 0

And that will always be true since you are removing any value with the & 0 part.

What you want to check is whether the file has neither of those flags, or in other words, if there are no common flags with the combination of both:

.Where(f => (new FileInfo(f).Attributes & (FileAttributes.Hidden | FileAttributes.System)) == 0)

You can also use Enum.HasFlag to make this a bit better understandable:

.Where(f => !new FileInfo(f).Attributes.HasFlag(FileAttributes.Hidden | FileAttributes.System))

You can simplify your code a lot by using DirectoryInfo and FileInfo, eg:

var mask= FileAttributes.Hidden | FileAttributes.System;

var di=new DirectoryInfo(sourcePath);
var files=di.EnumerateFiles("*", SearchOption.AllDirectories)
            .Where(fi=>(fi.Attributes & mask) == 0)
            .GroupBy(fi=>fi.DirectoryName);

Your original code tried to do a bitwise AND between flags that had no common bits, so it returned 0. As a result, the bitwise AND with Attributes also returned 0.

The mask you want to check against is System or Hidden ie FileAttributes.Hidden | FileAttributes.System. Calculating this in advance simply makes for somewhat cleaner code