using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        // Expected use case

        var myList = new List<int>()

        {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

        var highList = myList.MyWhere(x => x > 5);

        foreach (var num in highList)

        {

            Console.WriteLine(num);

        }

        Console.WriteLine("=====");

        // Uh oh! Some data layer function has returned a null enumerable instead of an empty list

        myList = null;

        highList = myList.MyWhereBad(x => x > 5); // This line will not cause an error as we haven't itereated the result.

        // ! The following line will throw

        // highList = myList.MyWhere(x => x > 5);

        // ! The following lines will throw when "MyWhereBad" is used

        // foreach (var num in highList) {

        //  Console.WriteLine(num);

        // }

    }

}

​

public static class Extensions

{

    public static IEnumerable<T> MyWhere<T>(this IEnumerable<T> source, Func<T, bool> predicate)

    {

        ArgumentNullException.ThrowIfNull(source);

        ArgumentNullException.ThrowIfNull(predicate);

        return _(); IEnumerable<T> _()

        {

            foreach (var item in source)

            {

                if (predicate(item))

                {

                    yield return item;

                }

            }

        }

    }

    public static IEnumerable<T> MyWhereBad<T>(this IEnumerable<T> source, Func<T, bool> predicate) {

        ArgumentNullException.ThrowIfNull(source);

        ArgumentNullException.ThrowIfNull(predicate);

​

        foreach (var item in source)

        {

            if (predicate(item))

            {

                yield return item;

            }

        }

    }

}