Question

I create this class for a test.
I want to compare to List of the class and get different class between ListA and ListB. In my example the result get only class of ListB.

I do the same thin with list of string and work it

Class Example

public class FileNode
{
    public string Source { get; set; }
    public int Id { get; set; }
}

List<FileNode> ListA = new List<FileNode>
{
    new FileNode{ Id = 1, Source="a" },
    new FileNode{ Id = 2, Source="b" },
};

List<FileNode> ListB = new List<FileNode>
{
    new FileNode{ Id = 1, Source="a" },
    new FileNode{ Id = 2, Source="b" },
    new FileNode{ Id = 3, Source="c" },
};
List<FileNode> ListAB = ListB.Where(m => !ListA.Contains(m)).ToList();

String example, it’s works

List<string> a = new List<string> {"a","b","c","d","e" };
List<string> b = new List<string> {"a","b","c","d" };
List<string> ab = a.Where(m => !b.Contains(m)).ToList();

Answer 1

Well Contains is going to call Equals on the elements – and may also use GetHashCode (I doubt it, but you should override it consistently anyway). So you need to override Equals(object) and GetHashCode() in FileNode. (By default, you’ll get reference equality.)

Note that as soon as you start trying to use Contains in a query which will execute in the database, it could behave completely different – it wouldn’t be looking at your Equals/GetHashCode methods at that point.