比较两个包含很多对象的列表（第3部分）“那些对象的类型不同”

Question

How could I speed up this linq query?

It takes a long time and when I place a lot of objects in the list I get a memory exception.

List<DirectoryInfo> directoriesThatWillBeCreated = new List<DirectoryInfo>();
// some code to fill the list
// ..
// ..

List<FileInfo> FilesThatWillBeCopied = new List<FileInfo>();
// some code to fill the list
//....

directoriesThatWillBeCreated = (from a in FilesThatWillBeCopied
                                from b in directoriesThatWillBeCreated
                                where a.FullName.Contains(b.FullName)
                                select b).ToList();

I hope I can do something like previous solution but I don't know how to do that when dealing with different types of objects. Do I have to create a new class then convert all the FileInfo and DirectoryInfo objects to that class then perform the query? Moreover FileInfo and DirectoryInfo classes are sealed and I cannot inherit from them therefore I'll have to create a new class and that will be not to efficient. At least that will be more efficient than that query because that query takes forever.

Answer 1

It's slow because the code does linear search in directory list for each file. Try this:

var dirlist = FilesThatWillBeCopied
    .Select(f => Directory.GetParent(f.FullName))
    .GroupBy(d => d.FullName)

You may need to play with the syntax a little bit but hopefully you see the point.

Answer 2

One thing you could do is change the Contains to a StartsWith . StartsWith will fail faster in the event of a failed match.

directoriesThatWillBeCreated = (from a in FilesThatWillBeCopied
                                from b in directoriesThatWillBeCreated
                                where a.FullName.StartsWith(b.FullName)
                                select b).ToList();

This isn't a complete solution, though. If FilesThatWillBeCopied has M items and directoriesThatWillBeCreated has N elements, then your query is going to process MxN string comparisons.

Another Option

Another optimization to try, iterate through directoriesThatWillBeCreated first, then select those that match any FileInfo in FilesThatWillBeCopied . By checking if any match, you could break out of testing the files once a match is found. That could be done like this: (warning, notepad code follows)

directoriesThatWillBeCreated = directoryThatWillBeCreated
    .Select(b => FilesThatWillBeCopied
    .Any(a => a.FullName.StartsWith(b.FullName)));

Answer 3

I would suggest using HashSet<DirectoryInfo> for comparisons, but unfortunately, DirectoryInfo doesn't have proper equality comparisons implemented, so strings will have to do. (Another option would be to implement your own IComparer<DirectoryInfo> .) Also, you should use StringComparer.InvariantCultureIgnoreCase on the names unless you are sure that both collections have the same case.

var dirs = new HashSet<string>(StringComparer.InvariantCultureIgnoreCase);
// fill dirs

var files = new List<FileInfo>();
// fill files

var result = new HashSet<string>(StringComparer.InvariantCultureIgnoreCase);

foreach (var file in files)
{
    var dir = file.Directory;
    while (dir != null && !result.Contains(dir.FullName))
    {
        if (dirs.Contains(dir.FullName))
            result.Add(dir.FullName);
        dir = dir.Parent;
    }
}

This solution doesn't use LINQ at all, but that's often the case when you're after performance and the most straight-forward LINQ solution is too slow.