List Comprehensions In CSharp

A List Comprehension is a type of set notation in which the programmer can describe the properties that the members of a set must meet. It is usually used to create a set based on other, already existing, set or sets by applying some type of combination, transform or reduction function to the existing set(s).

Consider the following problem: You have a sequence of 10 numbers from 0 to 9 and you need to extract all the even numbers from that sequence. In a language such a C# version 1.1, you were pretty much confined to the following code to solve this problem:

ArrayList evens = new ArrayList();
ArrayList numbers = Range(10);
int size = numbers.Count;
int i = 0;

while (i < size) {
    if (i % 2 == 0) {
        evens.Add(i);
    }
    i++;
}

The code above does not show the implementation of the Range function, which is available in the full code listing below. With the advent of C# 3.0 and the .NET Framework 3.5, a List Comprehension notation based on Linq is now available to C# programmers. The above C# 1.1 code can be ported to C# 3.0 like so:

IEnumerable<int> numbers = Enumerable.Range(0, 10);
var evens = from num in numbers where num % 2 == 0 select num;

And technically speaking, the C# 3.0 code above could be written as a one-liner by moving the call to Enumarable.Range into the Linq expression that generates the evens sequence. In the C# List Comprehension I am reducing the set numbers by applying a function (the modulo 2) to that sequence. This produces the evens sequences in a much more concise manner and avoid the use of loop syntax. Now, you may ask yourself: Is this purely syntax sugar? I don’t know, but I will definitelly investigate, and maybe even ask the question myself at StackOverflow. I suspect that this is not just syntax sugar and that there are some true optimizations that can be done by utilizing the underlying Monads.

I also posted this as an answer to a question on StackOverflow.com. The full code listing is available below:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace CSharpListComprehensions {
    public class Program {

        public static void RunSnippet() {
            Run11Version();
            Run35Version();
        }

        public static void Run11Version() {
            ArrayList evens = new ArrayList();
            ArrayList numbers = Range(10);
            int size = numbers.Count;
            int i = 0;

            WL(""); WL("Numbers:");
            foreach (int x in numbers) {
                WL(x.ToString());
            }

            while (i < size) {
                if (i % 2 == 0) {
                    evens.Add(i);
                }
                i++;
            }

            WL(""); WL("Evens:");
            foreach (int y in evens) {
                WL(y.ToString());
            }
        }

        public static void Run35Version() {
            IEnumerable<int> numbers = Enumerable.Range(0, 10);

            WL(""); WL("Numbers:");
            foreach (int x in numbers) {
                WL(x.ToString());
            }

            var evens = from num in numbers where num % 2 == 0 select num;

            WL(""); WL("Evens:");
            foreach (int y in evens) {
                WL(y.ToString());
            }
        }

        #region Range Function
        public static ArrayList Range(int end) {
            return Range(0, end);
        }

        public static ArrayList Range(int start, int end) {
            ArrayList al = new ArrayList();
            for (int i = start; i < end; i++) {
                al.Add(i);
            }

            return al;
        }
        #endregion

        #region Helper methods

        public static void Main() {
            try {
                RunSnippet();
            } catch (Exception e) {
                string error = string.Format("---\nThe following error occurred while executing the snippet:\n{0}\n---", e.ToString());
                Console.WriteLine(error);
            } finally {
                Console.Write("Press any key to continue...");
                Console.ReadKey();
            }
        }

        private static void WL(object text, params object[] args) {
            Console.WriteLine(text.ToString(), args);
        }

        private static void RL() {
            Console.ReadLine();
        }

        private static void Break() {
            System.Diagnostics.Debugger.Break();
        }

        #endregion
    }
}