Question

I’m wondering if it is possible to use a lambda expression to order a list of items based on the average value of a nested list of items.

In project terms, I have a list of products that each can have many reviews. I want to order the list of the products based on the average rating from each product’s list of reviews.

I have this:

products = category.Products.OrderBy(i => i.Reviews.Average(x => x.Rating));

This builds without a problem but when I run it, it returns an error that says

Sequence contains no elements

Is it possible to do this with a lambda expression?

Answer 1

Enumerable.Average throws the InvalidOperationException because the sequence contains no elements since a product has no Reviews.

You could use Enumerable.DefaultIfEmpty(customValue):

products = category.Products
    .OrderBy(i => i.Reviews.Select(r => r.Rating).DefaultIfEmpty(0).Average());

Maybe you want to exclude these products from the result:

products = category.Products
    .Where(p => p.Reviews.Any())
    .OrderBy(p => p.Reviews.Average(r => r.Rating));