Choose.cs

namespace ClassLibrary1;
public static class LinqExtensions
{
    public static IEnumerable<TOut> Choose<T, TOut>(this IEnumerable<T> input, Func<T, TOut?> chooser)
    {
        foreach (var i in input)
        {
            var c = chooser(i);
            if (c is { } q)
            {
                yield return q;
            }
        }
    }

    public static string? EvenNumbers(int number)
    {
        if (number % 2 == 0)
        {
            return number.ToString();
        }
        else
        {
            return null;
        }
    }

    public static void Foo()
    {
        var numbers = new[] {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        var x = numbers.Choose(EvenNumbers);
    }
}