WhereEnumerable

gistfile1.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;

using Linq = System.Linq;

namespace ConsoleApplication5
{
    class Program
    {
        static void Main(string[] args)
        {
            var s =
                Linq.Enumerable.Range(1, 200)
                .Where(x => x > 50)
                .Where(x => x < 100);
            var r = Linq.Enumerable.ToList(s);    
        }
    }

    class PredicateHolder<T>
    {
        private Func<T, bool> predicate;
        private PredicateHolder<T> next;
        
        public PredicateHolder(Func<T, bool> predicate, PredicateHolder<T> next = null)
        {
            this.predicate = predicate;
            this.next = next;
        }

        public bool Eval(T x)
        {
            var ok = true;
            var holder = this;
            while (ok && holder != null)
            {
                ok &= holder.predicate(x);
                holder = holder.next;
            }
            return ok;
        }
    }

    class WhereEnumerable<T> : IEnumerable<T>
    {
        private IEnumerable<T> source;
        private PredicateHolder<T> holder;
        public WhereEnumerable(IEnumerable<T> source, PredicateHolder<T> holder)
        {
            this.source = source;
            this.holder = holder;
        }

        public WhereEnumerable<T> Where(Func<T, bool> extraPredicate)
        {
            return new WhereEnumerable<T>(source, new PredicateHolder<T>(extraPredicate, holder));
        }

        public IEnumerator<T> GetEnumerator()
        {
            foreach (var item in source)
            {
                if (holder.Eval(item))
                    yield return item;
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        } 


    }

    static class Extensions
    {
        public static IEnumerable<T> Where<T>(this IEnumerable<T> enumerable, Func<T, bool> predicate)
        {
            var whereEnumerable = enumerable as WhereEnumerable<T>;
            return whereEnumerable != null
                ? whereEnumerable.Where(predicate)
                : new WhereEnumerable<T>(enumerable, new PredicateHolder<T>(predicate));
        }
    }
}