Response to "Broken iterator example in C# from Entwickler Magazin 1 / 2022"

example.cs

namespace Sandbox
{
    public class MyTests
    {
        [Test]
        public async Task Run_me()
        {
            var numbers = new List<int> { 1, 2, 3, 5, 8 };

            var sum = SumAsync(numbers);

            numbers.Add(13);

            var result = await sum;

            Assert.AreEqual(19, result);
        }

#if false
        /// <summary>
        /// A naive implementation to sum numbers while faking business.
        /// </summary>
        /// <param name="numbers"></param>
        /// <returns>The sum of the given numbers</returns>
        private static async Task<int> SumAsync(IEnumerable<int> numbers)
        {
            var sum = 0;
#if false
            // Option 1: Enumerate numbers by ignoring mutability
            //
            foreach (var n in numbers)
#else
            // Option 2: Enumerate numbers by considering immutability!
            //
            foreach (var n in numbers.ToList())
#endif
            {
                await Task.Delay(100);
                sum += n;
            }

            return sum;
        }
#else
        // Option 3:
        //
        /// <summary>
        /// A more robust implementation to sum numbers using LINQ (without faking business).
        /// </summary>
        /// <param name="numbers"></param>
        /// <returns>The sum of the given numbers.</returns>
        private static Task<int> SumAsync(IEnumerable<int> numbers)
        {
            return Task.FromResult(
                Enumerable.Sum(numbers)
            );
        }
#endif
    }
}