Question

I’m looking to remove all elements in a list that can be compared to elements of another list of a different type that don’t share a common inheretence, but I do have an equality function for. An example might make it clearer:

given the scaffolding

bool isSomeSortOfEqual(Bottle b, Printer p){
  //implementation
}

List<Bottle> bottles = getBottles();
List<Printer> printers = getPrinters();

I would like to do something like this:

List<Bottle> result = bottles.Except(printers, (b, p => isSomeSortOfEqual(b, p));

Are there any builtins for this in .NET, or should I implement this by hand? None of the questions relating to relative complement or except in .NET on stackoverflow seem to deal with having different types.

Answer 1

How about this? The basic idea is to cast the lists to List<object> and then use .Except with an IEqualityComparer<object>

   class A
    {
        public int Ai;
    }
    class B
    {
        public int Bi;
    }

    public class ABComparer : IEqualityComparer<object>
    {
        public bool Equals(object x, object y)
        {
            A isA = x as A ?? y as A;
            B isB = x as B ?? y as B;

            if (isA == null || isB == null)
                return false;

            return isA.Ai == isB.Bi;
        }

        public int GetHashCode(object obj)
        {
            A isA = obj as A;
            if (isA != null)
                return isA.Ai;

            B isB = obj as B;
            if (isB != null)
                return isB.Bi;

            return obj.GetHashCode();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<object> As = new List<object> { new A { Ai = 1 }, new A { Ai = 2 }, new A { Ai = 3 } };
            List<object> Bs = new List<object> { new B { Bi = 1 }, new B { Bi = 1 } };

            var except = As.Except(Bs, new ABComparer()).ToArray();
            // Will give two As with Ai = 2 and Ai = 3
        }
    }