List Intersect returning null

Question

I have two List<FileInfo> and I want to return the common FileItem between them.

List<FileInfo> outputList = new List<FileInfo>();
outputList = list1.Intersect(list2).ToList();

However, I'm getting back an empty List.

Both the lists contain FileInfo's found by

System.IO.DirectoryInfo dir = new System.IO.DirectoryInfo(startFolder);
IEnumerable<System.IO.FileInfo> fileList = 
dir.GetFiles("*.*", System.IO.SearchOption.AllDirectories);

And filtered by queries.

Devoe

FileInfo does not override Object.Equals so different instances will not be equal even if they refer to the same file. You will have to provide your own IEqualityComparer<FileInfo> to do the comparison. – Suppository

Answer 1

I suspect that FileInfo doesn't override Equals/GetHashCode, so two distinct objects will be unequal even if they refer to the same file. Three options:

• Convert the lists to paths if you don't need them as FileInfo
• Create an IEqualityComparer<FileInfo> and pass that to Intersect
• Implement IntersectBy in the same style as DistinctBy in MoreLINQ and propose it as a patch to the project :) (I thought we already had it, but apparently not...)

Filet

There is an example of exactly OP's situation and an example IEqualityComparer on this MSDN article. – Unreel

@Bridge: Well, it's not a "good" example of IEqualityComparer IMO - that GetHashCode implementation is nasty :( – Filet

I only meant the article was a good example, I didn't say the IEqualityComparer was a good example! :-) – Unreel

Answer 2

The references to the FileInfo objects in your two lists will be different, and therefore Intersect will produce an empty list.

You would need to create a class that implements the IEqualityComparer<FileInfo> interface and pass an instance of this class to Intersect in order to get the result you expect.

Filiate