How to use linq `Except` with multiple properties with different class?

You have said that you need only these objects from testListA:

• there is not matching ProductID in testListB
• there is existing mathing ProductID, but with different Category

So, your filter must be:

!testListB.Any(b => a.ProductID == b.ProductID && a.Category == b.Category)

So, change your code as:

testListA.Where(a => !testListB.Any(b => a.ProductID == b.ProductID && a.Category == b.Category));

---

Second approach:

Or you can create a new List<TestA> from the second list:

 var secondListA = testListB.Select(x=> new TestA(){Category=x.Category, ProductID=x.ProductID}).ToList();

And then create your Comparer:

sealed class MyComparer : IEqualityComparer<TestA>
{
    public bool Equals(TestA x, TestA y)
    {
        if (x == null)
            return y == null;
        else if (y == null)
            return false;
        else
            return x.ProductID == y.ProductID && x.Category == y.Category;
    }

    public int GetHashCode(TestA obj)
    {
        return obj.ProductID.GetHashCode();
    }
}

And use Except() overload which produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values.:

var result = testListA.Except(secondListA, new MyComparer ()).ToList();