public class GenericEqualityComparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, T, bool> _compareFunc;
        private readonly Func<T, int> _hashFunc;

        public GenericEqualityComparer(Func<T, T, bool> compareFunc)
        {
            _compareFunc = compareFunc;
        }

        public GenericEqualityComparer(Func<T, T, bool> compareFunc, Func<T, int> hashFunc)
            : this(compareFunc)
        {
            _compareFunc = compareFunc;
            _hashFunc = hashFunc;
        }
    
        public bool Equals(T x, T y)
        {
            return _compareFunc(x, y);
        }

        public int GetHashCode(T obj)
        {
            return _hashFunc != null ? _hashFunc(obj) : obj.GetHashCode();
        }
    }

List<A> A1 = new List<A>(new A[] { new A { FA = 1 }, new A { FA = 3 }, new A { FA = 5 } });
            List<A> A2 = new List<A>(new A[] { new A { FA = 2 }, new A { FA = 3 }, new A { FA = 4 } });

            var cc = A1.Except(A2, new GenericEqualityComparer<A>((x, y) => x.FA == y.FA, (x) => x.FA.GetHashCode())).ToList();