Multiple optional checks with Where query (to specify file search options)

Question

I created checkboxes, where a user can set this for file searching:

exclude system files: on/off
exclude hidden files: on/off
exclude read_only files: on/off

now I don't know, how I can use this three different boolean statements with this code line:

DirectoryInfo DirInfo = new DirectoryInfo(sCopyFilesFromFilePath);                    
GetDirsToCopy = 
   DirInfo.EnumerateDirectories(".", SearchOption.AllDirectories).Where( ? );

May be helpful: http://stackoverflow.com/questions/2418270/c-sharp-get-a-list-of-files-excluding-those-that-are-hidden — QuietSeditionist, Jan 22 '15 at 18:00
Could you please clarify how reading directories is related to "file searching" and whether you what problems you have reading `DirectoryInfo` / `FileSystemInfo` documentation. — Alexei Levenkov, Jan 22 '15 at 18:00
A file's FileAttributes is a bitmask, so create one based on the selection and examine File.GetAttributes() for each candidate file — Alex K., Jan 22 '15 at 18:01
StefanS, I've edited title of your post to hopefully better reflect problem you have instead of complete task. Feel free to rollback/improve. — Alexei Levenkov, Jan 22 '15 at 18:10

Willem Van Onsem · Accepted Answer · 2015-01-22T18:08:24.930

You can use an implication. In logic a -> b is translated as !a || b, thus:

DirInfo.EnumerateDirectories(".", SearchOption.AllDirectories)
       .Where(x => !excludeSystem || (x.Attributes&FileAttributes.System) == 0);
       .Where(x => !excludeHidden || (x.Attributes&FileAttributes.Hidden) == 0);
       .Where(x => !excludeReadOnly || (x.Attributes&FileAttributes.ReadOnly) == 0);

Here we assume excludeSystem, excludeHidden and excludeReadOnly are bools that you first fetched from the checkboxes.

You can of course write it in one Where as well.

Explanation (one .Where):

Given the following expression

x => !excludeSystem || (x.Attributes&FileAttributes.System) == 0

You can read this as a predicate. Say excludeSystem is false, then !excludeSystem is true, thus all directories will succeed. If not, the the second operand (x.Attributes&FileAttributes.System) == 0 is executed.

The second operand first does and bitwise & on the Attributes of x and the constant FileAttributes.System. If the Attributes thus contain FileAttributes.System, the result will be something different than zero. In that case the second test fails. In other words, you return false if the files are excluded and the file is a system-file.

Optimize

In case you think you will iterate over thousands of directories, you can make the code a bit more efficient, by making the tests faster:

IEnumerable<DirectoryInfo> temp = DirInfo.EnumerateDirectories(".", SearchOption.AllDirectories);
if(excludeSystem) {
    temp = temp.Where((x.Attributes&FileAttributes.System) == 0);
}
if(excludeHidden) {
    temp = temp.Where((x.Attributes&FileAttributes.Hidden) == 0);
}
if(excludeReadOnly) {
    temp = temp.Where((x.Attributes&FileAttributes.ReadOnly) == 0);
}
GetDirsToCopy = temp;

This will (almost) always be efficient, but it makes the code a bit less beautiful.

Thank you. Do you know, how to exclude the folder "System Volume Information"? There is always an exception. — StefanS, Jan 24 '15 at 18:55
@StefanS: what do you mean with exception? Can you give the stacktrace? — Willem Van Onsem, Jan 24 '15 at 18:59
I am sorry, I was lazy yesterday. Every Windows root partition like C: or D: has a folder named "System Volume Information" and that folder is not detected by the linq code. I think this happens, because of "DirInfo.EnumerateDirectories". So I have to build a way around it. — StefanS, Jan 25 '15 at 15:19

Multiple optional checks with Where query (to specify file search options)

1 Answers1