Using Linq Except not Working as I Thought

Question

List1 contains items { A, B } and List2 contains items { A, B, C }.

What I need is to be returned { C } when I use Except Linq extension. Instead I get returned { A, B } and if I flip the lists around in my expression the result is { A, B, C }.

Am I misunderstanding the point of Except? Is there another extension I am not seeing to use?

I have looked through and tried a number of different posts on this matter with no success thus far.

var except = List1.Except(List2); //This is the line I have thus far

EDIT: Yes I was comparing simple objects. I have never used IEqualityComparer, it was interesting to learn about.

Thanks all for the help. The problem was not implementing the comparer. The linked blog post and example below where helpful.

Answer 1

If you are storing reference types in your list, you have to make sure there is a way to compare the objects for equality. Otherwise they will be checked by comparing if they refer to same address.

You can implement IEqualityComparer<T> and send it as a parameter to Except() function. Here's a blog post you may find helpful.

edit: the original blog post link was broken and has been replaced above

Answer 2

So just for completeness...

// Except gives you the items in the first set but not the second
    var InList1ButNotList2 = List1.Except(List2);
    var InList2ButNotList1 = List2.Except(List1);
// Intersect gives you the items that are common to both lists    
    var InBothLists = List1.Intersect(List2);

Edit: Since your lists contain objects you need to pass in an IEqualityComparer for your class... Here is what your except will look like with a sample IEqualityComparer based on made up objects... :)

// Except gives you the items in the first set but not the second
        var equalityComparer = new MyClassEqualityComparer();
        var InList1ButNotList2 = List1.Except(List2, equalityComparer);
        var InList2ButNotList1 = List2.Except(List1, equalityComparer);
// Intersect gives you the items that are common to both lists    
        var InBothLists = List1.Intersect(List2);

public class MyClass
{
    public int i;
    public int j;
}

class MyClassEqualityComparer : IEqualityComparer<MyClass>
{
    public bool Equals(MyClass x, MyClass y)
    {
        return x.i == y.i &&
               x.j == y.j;
    }

    public int GetHashCode(MyClass obj)
    {
        unchecked
        {
            if (obj == null)
                return 0;
            int hashCode = obj.i.GetHashCode();
            hashCode = (hashCode * 397) ^ obj.i.GetHashCode();
            return hashCode;
        }
    }
}

Answer 3

You simply confused the order of arguments. I can see where this confusion arose, because the official documentation isn't as helpful as it could be:

Produces the set difference of two sequences by using the default equality comparer to compare values.

Unless you're versed in set theory, it may not be clear what a set difference actually is—it's not simply what's different between the sets. In reality, Except returns the list of elements in the first set that are not in the second set.

Try this:

var except = List2.Except(List1); // { C }

Answer 4

Writing a custom comparer does seem to solve the problem, but I think https://stackoverflow.com/a/12988312/10042740 is a much more simple and elegant solution.

It overwrites the GetHashCode() and Equals() methods in your object defining class, then the default comparer does its magic without extra code cluttering up the place.

Answer 5

Just for Ref: I wanted to compare USB Drives connected and available to the system.

So this is the class which implements interface IEqualityComparer

public class DriveInfoEqualityComparer : IEqualityComparer<DriveInfo>
{
    public bool Equals(DriveInfo x, DriveInfo y)
    {
        if (object.ReferenceEquals(x, y))
            return true;
        if (x == null || y == null)
            return false;
        // compare with Drive Level
        return x.VolumeLabel.Equals(y.VolumeLabel);
    }

    public int GetHashCode(DriveInfo obj)
    {
        return obj.VolumeLabel.GetHashCode();
    }
}

and you can use it like this

var newDeviceLst = DriveInfo.GetDrives()
                            .ToList()
                            .Except(inMemoryDrives, new DriveInfoEqualityComparer())
                            .ToList();