Module 1: What foreach Really Does

Practice Tasks

Complete these tasks to reinforce what you learned in this module.

Task 1: Drive an enumerator by hand

Walk a List<int> without using foreach. Call GetEnumerator(), then loop with MoveNext() and print each Current on its own line.

Expected Output:

The three numbers, one per line, in order.

Hint: var e = numbers.GetEnumerator(); while (e.MoveNext()) { ... e.Current ... }

Starter Code:

task.cs

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var numbers = new List<int> { 3, 1, 4 };

        // Walk the list without foreach: GetEnumerator, MoveNext, Current
    }
}

Task 2: Finish the hand-written enumerator

MoveNext is the only thing missing. Make Countdown(3) produce 3, 2, 1 and then stop. Remember that an enumerator starts *before* the first element.

Expected Output:

3, 2 and 1 on separate lines, then nothing.

Hint: The field starts one above the first value. Decrement first, then report whether the new value is still at least 1.

Starter Code:

task.cs

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

class Countdown : IEnumerable<int>
{
    private readonly int _from;
    public Countdown(int from) => _from = from;

    public IEnumerator<int> GetEnumerator() => new CountdownEnumerator(_from);
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

    private class CountdownEnumerator : IEnumerator<int>
    {
        private int _current;
        public CountdownEnumerator(int from) => _current = from + 1;

        public int Current => _current;
        object IEnumerator.Current => Current;

        public bool MoveNext()
        {
            // Step down one, and say whether we still have a value
            return false;
        }

        public void Reset() { }
        public void Dispose() { }
    }
}

class Program
{
    static void Main()
    {
        foreach (var n in new Countdown(3))
        {
            Console.WriteLine(n);
        }
    }
}

using System; using System.Collections.Generic; class Program { static void Main() { var numbers = new List<int> { 3, 1, 4 }; // Walk the list without foreach: GetEnumerator, MoveNext, Current } }

using System; using System.Collections; using System.Collections.Generic; class Countdown : IEnumerable<int> { private readonly int _from; public Countdown(int from) => _from = from; public IEnumerator<int> GetEnumerator() => new CountdownEnumerator(_from); IEnumerator IEnumerable.GetEnumerator() => GetEnumerator(); private class CountdownEnumerator : IEnumerator<int> { private int _current; public CountdownEnumerator(int from) => _current = from + 1; public int Current => _current; object IEnumerator.Current => Current; public bool MoveNext() { // Step down one, and say whether we still have a value return false; } public void Reset() { } public void Dispose() { } } } class Program { static void Main() { foreach (var n in new Countdown(3)) { Console.WriteLine(n); } } }