AnimalFactory.cs

public class AnimalFactory
{
    public IEnumerable<IAnimal> GetAnimals()
    {
        yield return new Dog();
    
        var ducks = GetDucks();
    
        // return ducks; <-- **BOOM** Cannot return a value from an iterator.
        
        foreach (var duck in ducks)
        {
            yield return duck; // <-- Works ;)
        }
        
        // yield return foreach ducks; <-- I wish this worked...

        yield return new Cow();
    }
    
    private static IEnumerable<Duck> GetDucks()
    {
        return Enumerable.Range(0, 10).Select(x => new Duck());
    }
}

Animals.cs

public interface IAnimal { }

public class Duck : IAnimal { }

public class Cow : IAnimal { }

public class Dog : IAnimal { }