Module 3: Nothing Runs Until You Ask

Practice Tasks

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

Task 1: Prove that nothing runs

Write the iterator so the program prints its lines in the order shown: the iterator's own message must appear only once the foreach begins.

Expected Output:

before call, after call, iterator started, then 1 and 2.

Hint: Put Console.WriteLine("iterator started"); as the first statement of the iterator, before the first yield return.

Starter Code:

task.cs

using System;
using System.Collections.Generic;

class Program
{
    static IEnumerable<int> Numbers()
    {
        // Announce that the body has started, then hand back 1 and 2
        yield break;
    }

    static void Main()
    {
        Console.WriteLine("before call");
        var seq = Numbers();
        Console.WriteLine("after call");

        foreach (var n in seq)
        {
            Console.WriteLine(n);
        }
    }
}

Task 2: Make the guard fire on the call

Split FirstN so that passing null throws at the moment of the call, before anyone enumerates. Main is already written to detect which it is.

Expected Output:

`threw on call` — not `threw on enumerate`.

Hint: Keep the throw in a method with no yield in it, and have that method return a second, private iterator method.

Starter Code:

task.cs

using System;
using System.Collections.Generic;

class Program
{
    static IEnumerable<int> FirstN(int[] source, int count)
    {
        if (source == null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        for (int i = 0; i < count && i < source.Length; i++)
        {
            yield return source[i];
        }
    }

    static void Main()
    {
        IEnumerable<int> seq;

        try
        {
            seq = FirstN(null, 3);
        }
        catch (ArgumentNullException)
        {
            Console.WriteLine("threw on call");
            return;
        }

        try
        {
            foreach (var n in seq) { }
        }
        catch (ArgumentNullException)
        {
            Console.WriteLine("threw on enumerate");
        }
    }
}

Task 3: An exception that arrives mid-sequence

Write an iterator over { "1", "2", "boom", "4" } that yields each item parsed as an int. Main prints values until the parse fails. Show that the first two values arrive before the failure does.

Expected Output:

1, then 2, then `caught` — the failure happens partway through, not up front.

Hint: int.Parse throws on "boom". Just yield int.Parse(item) for every item and let it happen.

Starter Code:

task.cs

using System;
using System.Collections.Generic;

class Program
{
    static IEnumerable<int> Parsed(string[] items)
    {
        // Yield every item parsed as an int
        yield break;
    }

    static void Main()
    {
        var items = new string[] { "1", "2", "boom", "4" };

        try
        {
            foreach (var n in Parsed(items))
            {
                Console.WriteLine(n);
            }
        }
        catch (FormatException)
        {
            Console.WriteLine("caught");
        }
    }
}

using System; using System.Collections.Generic; class Program { static IEnumerable<int> Numbers() { // Announce that the body has started, then hand back 1 and 2 yield break; } static void Main() { Console.WriteLine("before call"); var seq = Numbers(); Console.WriteLine("after call"); foreach (var n in seq) { Console.WriteLine(n); } } }

using System; using System.Collections.Generic; class Program { static IEnumerable<int> FirstN(int[] source, int count) { if (source == null) { throw new ArgumentNullException(nameof(source)); } for (int i = 0; i < count && i < source.Length; i++) { yield return source[i]; } } static void Main() { IEnumerable<int> seq; try { seq = FirstN(null, 3); } catch (ArgumentNullException) { Console.WriteLine("threw on call"); return; } try { foreach (var n in seq) { } } catch (ArgumentNullException) { Console.WriteLine("threw on enumerate"); } } }

using System; using System.Collections.Generic; class Program { static IEnumerable<int> Parsed(string[] items) { // Yield every item parsed as an int yield break; } static void Main() { var items = new string[] { "1", "2", "boom", "4" }; try { foreach (var n in Parsed(items)) { Console.WriteLine(n); } } catch (FormatException) { Console.WriteLine("caught"); } } }