How to compare two folders for non identical files based on name?

c#, linq

Solution

Well, for one thing that approach is very inefficient - it's going to be calling `dir2.GetFiles` each time you start with a new `f1`. It's then going to give a match for every `f2` which doesn't match the current `f1`. So even if it's going to match a later `f1`, it'll still be output. Imagine that dir1 contains A, B and C, and dir2 contains C and D. You'll end up like this:

f1    f2    Result of where?
 A     C    True
 A     D    True
 B     C    True
 B     D    True
 C     C    False
 C     D    True

So the result would be A, A, B, B, C - you'd still have C (which you didn't want) - just not quite as often as A and B.

You want to use set operations, like this:

var dir1Files = dir1.GetFiles("*", SearchOption.AllDirectories)
                    .Select(x => x.Name);

var dir2Files = dir2.GetFiles("*", SearchOption.AllDirectories)
                    .Select(x => x.Name);

var onlyIn1 = dir1Files.Except(dir2Files);

Now that should work, and more efficiently...

EDIT: I've assumed you want files in A but not in B, based on possibly an earlier version of the question. (I'm not sure whether it was edited in the first five minutes. Obviously the current code isn't going to return anything in B but not A.)

If you want the symmetric difference, use `HashSet<T>.SymmetricExceptWith`:

var inExactlyOneDirectory = new HashSet<string>(dir1Files);
inExactlyOneDirectory.SymmetricExceptWith(dir2Files);

(Note that I dislike the fact that `SymmetricExceptWith` is a void method which mutates the existing set, instead of returning a new set or just a sequence. Aside from anything else, it means the variable name is only appropriate after the second statement, not the first.)

EDIT: If you need uniqueness by name and size, you really need an anonymous type representing both. Unfortunately, it's then hard to create a `HashSet<T>` based on it. So you'll want an extension method like this:

public static HashSet<T> ToHashSet<T>(this IEnumerable<T> set)
{
    return new HashSet<T>(set);
}

Then:

var dir1Files = dir1.GetFiles("*", SearchOption.AllDirectories)
                    .Select(x => new { x.Name, x.Length });

var dir2Files = dir2.GetFiles("*", SearchOption.AllDirectories)
                    .Select(x => new { x.Name, x.Length });

var difference = dir1Files.ToHashSet();
difference.SymmetricExceptWith(dir2Files);

Problem

Original source