eager_throw.cs

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string step = "n/a";
        try
        {
            step = "initial call";
            var sequence = Deferred(-1);
            step = "foreach";
            foreach (var item in sequence)
                Console.WriteLine(item);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Deferred threw during {step}: {ex.Message}");
        }

        step = "n/a";
        try
        {
            step = "initial call";
            var sequence = Eager(-1);
            step = "foreach";
            foreach (var item in sequence)
                Console.WriteLine(item);
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Eager threw during {step}: {ex.Message}");
        }
    }

    static IEnumerable<int> Deferred(int count)
    {
        if (count < 0) throw new ArgumentOutOfRangeException(nameof(count));
        for (int i = 0; i < count; i++)
            yield return i;
    }
    static IEnumerable<int> Eager(int count)
    {
        if (count < 0) throw new ArgumentOutOfRangeException(nameof(count));
        return _();

        IEnumerable<int> _()
        {
            for (int i = 0; i < count; i++)
                yield return i;
        }
    }
}