Question

I want to compare two lists, based on a filter expression; not sure how to construct the lambda expression for the generic method; Please refer to the code below; or is there an easier way via an intersect in LINQ?

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Data d1 = new Data {Id = 1, Name = "One"};
            Data d2 = new Data { Id = 2, Name = "Two" };
            Data d3 = new Data { Id = 3, Name = "Three" };

            Data d4 = new Data { Id = 1, Name = "One" };
            Data d5 = new Data { Id = 2, Name = "Two" };
            Data d6 = new Data { Id = 4, Name = "Four" };

            List<Data> original = new List<Data> {d1, d2, d3};
            List<Data> filterItems = new List<Data> {d4, d5, d6};

            List<Data> result = original.FilterDataList(filterItems);

            //How to call this method?
            List<Data> genericCall = original.FilterList<Data>(filterItems, data => data.Id ?????????????)
        }
    }

    public class Data
    {
        public long Id;
        public string Name;
    }

    public static class Extensions
    {
        public static List<Data> FilterDataList(this List<Data> sourceList, List<Data> filterOutItems)
        {
            return sourceList.Where(p => filterOutItems.All(l => l.Id != p.Id)).ToList();
        }

        public static List<T> FilterList<T>(this List<T> sourceList, List<T> filterOutItems, Func<T, bool> filterExpression)
        {
            return sourceList.Where(p => filterOutItems.All(filterExpression)).ToList();
        }
    }
}

Answer 1

If I understand your question correctly, FilterList is a generic version of FilterDataList where you are passing in the lambda as a parameter. In that case you would call the method as follows:

List<Data> genericCall = original.FilterList<Data>(filterItems, (x, y) => x.Id != y.Id);

If you want to use Except as @ivancho and @perelman have suggested you could use a method like this:

public static class EnumerableExtension
{
    public static IEnumerable<T> Except<T>(this IEnumerable<T> listA, IEnumerable<T> listB,
                                           Func<T, T, bool> lambda)
    {
        return listA.Except(listB, new Comparer<T>(lambda));
    }

    public static IEnumerable<T> Intersect<T>(this IEnumerable<T> listA, IEnumerable<T> listB,
                                              Func<T, T, bool> lambda)
    {
        return listA.Intersect(listB, new Comparer<T>(lambda));
    }
}

You would then call it as follows:

original.Except<Data>(filterItems, (x, y) => x.Id != y.Id);