Except`4

using System.Linq;

public static class Enumerable {

    public static System.Collections.Generic.IEnumerable<TOuter> Except<TOuter, TInner, TKey, TResult>(
        this System.Collections.Generic.IEnumerable<TOuter> outer,
        System.Collections.Generic.IEnumerable<TInner> inner,
        System.Func<TOuter, TKey> outerKeySelecter,
        System.Func<TInner, TKey> innerKeySelecter,
        System.Collections.Generic.IEqualityComparer<TKey> comparer
    ) {
        var i = inner.ToDictionary( innerKeySelecter, comparer );
        return outer.Where(
            x => !i.ContainsKey( outerKeySelecter( x ) )
        );
    }

    public static System.Collections.Generic.IDictionary<K, V> ToDictionary<K, V>(
        this System.Collections.Generic.IEnumerable<V> collection,
        System.Func<V, K> keySelector,
        System.Collections.Generic.IEqualityComparer<K> keyComparer
    ) {
        System.Collections.Generic.IDictionary<K, V> output = new System.Collections.Generic.Dictionary<K, V>( keyComparer );
        foreach ( var item in collection ) {
            output.Add( keySelector( item ), item );
        }
        return output;
    }

}