Using Linq Except not Working as I Thought

Solution 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

Solution 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;
        }
    }
}

Solution 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 }