My favourite collection extenders...

gistfile1.cs

    public static class CollectionExtender
    {
        public static IEnumerable<string> NonBlank(this IEnumerable<string> list)
        {
            return list.Where(_ => !String.IsNullOrWhiteSpace(_));
        }

        public static IEnumerable<T> NonNull<T>(this IEnumerable<T> list) where T:class
        {
            return list.Where(_ => null != (object) _);
        }

        public static IEnumerable<T> Zip2<T, T1, T2>(this IEnumerable<T> list)
        {
            return list.Where(t => t is T1 || t is T2);
        }


        public static string AggregateCsv<T>(this IEnumerable<T> source, string prefix="", string postfix="", string ifempty=null)
        {
            var result = prefix + source.Aggregate("", (l, i) => l + i + ", ", l => l.TrimEnd(' ', ',')) + postfix;
            if (result == prefix + postfix && ifempty != null) return ifempty;
            return result;
        }

        public static string Aggregate<T>(this IEnumerable<T> source, string globalprefix, string itemprefix, string itempostfix, string interitem, string globalpostfix, string emptyglobal=null)
        {
            var any = false;

            var list = source.Aggregate(new StringBuilder(), (sb, item) =>
                                                                 {
                                                                     if (!any)
                                                                     {
                                                                         any = true;
                                                                     }
                                                                     else
                                                                     {
                                                                         sb.Append(interitem);
                                                                     }
                                                                     sb.Append(itemprefix);
                                                                     sb.Append(item);
                                                                     sb.Append(itempostfix);
                                                                     return sb;
                                                                 }, sb => sb.ToString());

            return (any || emptyglobal == null) ? globalprefix + list + globalpostfix : emptyglobal;
        }


        public static void SplitCast<T, T1, T2>(this IEnumerable<T> source, Action<T1> action1, Action<T2> action2)
            where T1 : T
            where T2 : T
        {
            source.SplitCast<T, T1, T2, T, T, T, T>(action1, action2, null, null, null, null);
        }

        public static void SplitCast<T, T1, T2, T3>(this IEnumerable<T> source, Action<T1> action1, Action<T2> action2, Action<T3> action3)
            where T1 : T
            where T2 : T
            where T3 : T
        {
            source.SplitCast<T, T1, T2, T3, T, T, T>(action1, action2, action3, null, null, null);
        }

        public static void SplitCast<T, T1, T2, T3, T4>(this IEnumerable<T> source, Action<T1> action1, Action<T2> action2, Action<T3> action3, Action<T4> action4)
            where T1 : T
            where T2 : T
            where T3 : T
            where T4 : T
        {
            source.SplitCast<T, T1, T2, T3, T4, T, T>(action1, action2, action3, action4, null, null);
        }

        public static void SplitCast<T, T1, T2, T3, T4, T5>(this IEnumerable<T> source, Action<T1> action1, Action<T2> action2, Action<T3> action3, Action<T4> action4, Action<T5> action5)
            where T1 : T
            where T2 : T
            where T3 : T
            where T4 : T
            where T5 : T
        {
            source.SplitCast<T, T1, T2, T3, T4, T5, T>(action1, action2, action3, action4, action5, null);
        }



        public static void SplitCast<T, T1, T2, T3, T4, T5, T6>(this IEnumerable<T> source, Action<T1> action1, Action<T2> action2, Action<T3> action3, Action<T4> action4, Action<T5> action5, Action<T6> action6)
            where T1 : T
            where T2 : T
            where T3 : T
            where T4 : T
            where T5 : T
            where T6 : T
        {
            var configured = new List<Action<T>>();

            if (action1 != null) configured.Add(t => { if (t is T1) action1((T1) t); });
            if (action2 != null) configured.Add(t => { if (t is T2) action2((T2) t); });
            if (action3 != null) configured.Add(t => { if (t is T3) action3((T3) t); });
            if (action4 != null) configured.Add(t => { if (t is T4) action4((T4) t); });
            if (action5 != null) configured.Add(t => { if (t is T5) action5((T5) t); });
            if (action6 != null) configured.Add(t => { if (t is T6) action6((T6) t); });

            foreach (var t in source) configured.ForEach(_ => _(t));
        }


        public static void AddEx<T>(this ICollection<T> collection, T item)
        {
            if (!collection.Contains(item)) collection.Add(item);
        }

        public static void RemoveEx<T>(this ICollection<T> collection, T item)
        {
            if (collection.Contains(item)) collection.Remove(item);
        }


        public static bool HeadTail<T>(this IEnumerable<T> source, out T head, out IEnumerable<T> tail)
        {
            var thelist = new List<T>();
            tail = thelist;

            var enumerator = source.GetEnumerator();
            if (enumerator.MoveNext())
            {
                head = enumerator.Current;
            }
            else
            {
                head = default(T);
                return false;
            }
            while (enumerator.MoveNext()) thelist.Add(enumerator.Current);
            return true;
        }

        public static bool InitLast<T>(this IEnumerable<T> source, out IEnumerable<T> init, out T tail)
        {
            var thelist = new List<T>();
            init = thelist;
            var enumerator = source.GetEnumerator();

            T last = default(T);
            if (!enumerator.MoveNext())
            {
                tail = last;
                return false;
            }
            last = enumerator.Current;

            while (enumerator.MoveNext())
            {
                thelist.Add(last);
                last = enumerator.Current;
            }

            tail = last;
            return true;
        }

    }