QueryTest.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace QueryTest
{
    [TestClass]
    public class QueryTest
    {
        [TestMethod]
        public void Test1()
        {
            var r =
                from n in 1
                from o in n * 10
                from p in o * 10
                from q in p * 10
                select n + o + p + q;

            r.Is(1111);

            var rr = 1
                .SelectMany(n => n * 10, (n, o) => new { n, o })
                .SelectMany(no => no.o * 10, (no, p) => new { no, p })
                .SelectMany(nop => nop.p * 10, (nop, q) => nop.no.n + nop.no.o + nop.p + q);

            rr.Is(1111);
        }
    }

    public static class Ex
    {
        public static U Select<T, U>(this T x, Func<T, U> f)
        {
            return f(x);
        }
        public static V SelectMany<T, U, V>(this T x, Func<T, U> f, Func<T, U, V> g)
        {
            return g(x, f(x));
            // こっちの方がわかりやすい？
            //return g(x, x.Select(f));
        }
    }
}