Linq extension method, how to find child in collection recursive

Question

I\'m already familiar with Linq but have little understanding of extension methods I\'m hoping someone can help me out.

So I have this hierarchical collection pseudo cod

Answer 1

static IEnumerable FindProductById(this IEnumerable source, int id) 
{
    return source.FirstOrDefault(product => product.Id = id) ?? source.SelectMany(product => product.Children).FindProductById(id);
}