Lambda Tricks

LambdaTricks.cs

using System;
using System.Linq;

namespace Rextester
{
    // See https://csharp.2000things.com/tag/captured-variable/
    internal static class Program
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("*******************************");
            Console.WriteLine($"{nameof(Test1)} - Simple Lambdas:");
            Test1();

            Console.WriteLine("*******************************");
            Console.WriteLine($"{nameof(Test2)} - Lambdas in foreach loops:");
            Test2();

            Console.WriteLine("*******************************");
            Console.WriteLine($"{nameof(Test3)} - Lambdas in for loops:");
            Test3();

            Console.WriteLine("*******************************");
            Console.WriteLine($"{nameof(Test4)} - Local functions in foreach loops:");
            Test4();

            Console.WriteLine("*******************************");
            Console.WriteLine($"{nameof(Test5)} - Local functions in for loops:");
            Test5();
        }

        private static void Test1()
        {
            var i = 42;
            Console.WriteLine($"{i}");

            Action action = () => Console.WriteLine($"{i}");
            action();

            i = 666;
            action(); // https://csharp.2000things.com/2014/09/09/1178-captured-variables-are-evaluted-when-a-delegate-is-invoked/
        }

        // https://csharp.2000things.com/2014/09/19/1186-capturing-a-foreach-iteration-variable-in-a-lambda-expression/
        private static void Test2()
        {
            const int length = 10;

            Console.Write("1: ");
            foreach (var i in Enumerable.Range(0, length))
                Console.Write($"{i} ");

            Console.WriteLine();

            Console.Write("2: ");
            foreach (var i in Enumerable.Range(0, length))
            {
                Action action = () => Console.Write($"{i} ");
                action();
            }

            Console.WriteLine();

            Console.Write("3: ");
            var actions = new Action[length];
            foreach (var i in Enumerable.Range(0, length))
                actions[i] = () => Console.Write($"{i} ");

            foreach (var action in actions)
                action();

            Console.WriteLine();

            Console.Write("4: ");
            var actions2 = new Action[length];
            foreach (var i in Enumerable.Range(0, length))
            {
                var index = i;
                actions2[i] = () => Console.Write($"{index} ");
            }

            foreach (var action in actions2)
                action();

            Console.WriteLine();
        }

        // https://csharp.2000things.com/2014/09/15/1182-capturing-a-for-loop-variable-in-a-lambda-expression/
        private static void Test3()
        {
            const int length = 10;

            Console.Write("1: ");
            for (var i = 0; i < length; i++)
                Console.Write($"{i} ");

            Console.WriteLine();

            Console.Write("2: ");
            for (var i = 0; i < length; i++)
            {
                Action action = () => Console.Write($"{i} ");
                action();
            }

            Console.WriteLine();

            Console.Write("3: ");
            var actions = new Action[length];
            for (var i = 0; i < length; i++)
                actions[i] = () => Console.Write($"{i} ");

            foreach (var action in actions)
                action();

            Console.WriteLine();

            Console.Write("4: ");
            var actions2 = new Action[length];
            for (var i = 0; i < length; i++)
            {
                var index = i;
                actions2[i] = () => Console.Write($"{index} ");
            }

            foreach (var action in actions2)
                action();

            Console.WriteLine();
        }

        // Local Functions
        private static void Test4()
        {
            const int length = 10;

            Console.Write("1: ");
            foreach (var i in Enumerable.Range(0, length))
                Console.Write($"{i} ");

            Console.WriteLine();

            Console.Write("2: ");
            foreach (var i in Enumerable.Range(0, length))
            {
                void write() => Console.Write($"{i} ");
                write();
            }

            Console.WriteLine();

            Console.Write("3: ");
            var actions = new Action[length];
            foreach (var i in Enumerable.Range(0, length))
            {
                void write() => Console.Write($"{i} ");
                actions[i] = write;
            }

            foreach (var action in actions)
                action();

            Console.WriteLine();

            Console.Write("4: ");
            var actions2 = new Action[length];
            foreach (var i in Enumerable.Range(0, length))
            {
                var index = i; // Roslyn tells us this is useless and indeed it is!
                void write() => Console.Write($"{index} ");
                actions2[i] = write;
            }

            foreach (var action in actions2)
                action();

            Console.WriteLine();
        }

        // Local Functions
        private static void Test5()
        {
            const int length = 10;

            Console.Write("1: ");
            for (var i = 0; i < length; i++)
                Console.Write($"{i} ");

            Console.WriteLine();

            Console.Write("2: ");
            for (var i = 0; i < length; i++)
            {
                void write() => Console.Write($"{i} ");
                write();
            }

            Console.WriteLine();

            Console.Write("3: ");
            var actions = new Action[length];
            for (var i = 0; i < length; i++)
            {
                void write() => Console.Write($"{i} ");
                actions[i] = write;
            }

            foreach (var action in actions)
                action();

            Console.WriteLine();

            Console.Write("4: ");
            var actions2 = new Action[length];
            for (var i = 0; i < length; i++)
            {
                var index = i; // Roslyn tells us this is useless, but IT IS NOT!!!
                void write() => Console.Write($"{index} ");
                actions2[i] = write;
            }

            foreach (var action in actions2)
                action();

            Console.WriteLine();
        }
    }
}