Question

I want to write a new override of Except extension method for IEnumerable which is able to take a comparer inline instead of using IEqualityComparer.

A, B are collections of a reference type..

Something like this:

A: [1, A], [2, A], [3, A]

B: [3, B], [4, B], [5, B]

C = A.Except(B, (a,b) => a.Id == b.Id);

C: [1, A], [2, A]

I wonder if you could help me with the code of the method.

public static class IEnumerableExntesion
    {
        public IEnumerable<T> Except<T>(this IEnumerable<T> source, 
                                        IEnumerable<T> second, 
                                        Func<T, T, bool> predicate)
        {

        }
    }

I was thinking of:

return source.Where (s => !second.Any(p => p.Id == s.Id));

But actually I couldn’t convert it to a generic solution using the passed predicate!

Any help!

Answer 1

Do you need to do the comparison using a predicate or can you use a projection instead and compare the projected values? If so then you could use some sort of ExceptBy method:

var c = a.ExceptBy(b, x => x.Id);

var r = p.ExceptBy(q, x => x.Name, StringComparer.OrdinalIgnoreCase);

// ...

public static class EnumerableExtensions
{
    public static IEnumerable<TSource> ExceptBy<TSource, TKey>(
        this IEnumerable<TSource> first, IEnumerable<TSource> second,
        Func<TSource, TKey> keySelector,
        IEqualityComparer<TKey> keyComparer = null)
    {
        if (first == null) throw new ArgumentNullException("first");
        if (second == null) throw new ArgumentNullException("second");
        if (keySelector == null) throw new ArgumentNullException("keySelector");

        return first.ExceptByIterator(second, keySelector, keyComparer);
    }

    private static IEnumerable<TSource> ExceptByIterator<TSource, TKey>(
        this IEnumerable<TSource> first, IEnumerable<TSource> second,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> keyComparer)
    {
        var keys = new HashSet<TKey>(second.Select(keySelector), keyComparer);

        foreach (TSource item in first)
        {
            if (keys.Add(keySelector(item)))
                yield return item;
        }
    }
}