using System;

using System.Collections.Generic;

using System.Linq;

public class Program

    {

        public static void Main(string[] args)

        {

            ICollection<Review> NullReviews = null;

            Console.WriteLine(Math.Round(NullReviews?.Average(r => r.Rating) ?? 0.0, 1).ToString());

            ICollection<Review> ReviewItemsNull = new List<Review>

            {

                new Review { Rating = 1.5 },

                    new Review { Rating = 2.0 },

                null,

                new Review { Rating = 3.0 }

            };

            Console.WriteLine(Math.Round(ReviewItemsNull?.Average(r => r?.Rating ?? 0.0) ?? 0.0, 1).ToString());

        }                                  

    }

   public class Review

   {

       public double Rating { get; set; }

   }