Bowling Kata - C#, try 1

program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Katas {
    class Frame(int index) {
        public List<int> Rolls = new List<int>();
        public int FrameIndex { get; } = index;
        
        public bool Completed {
            get {
                return (!IsLastFrame && (IsStrike || Rolls.Count == 2)) ||
                       (IsLastFrame && (IsSpare || IsStrike) && Rolls.Count == 3) ||
                       (IsLastFrame && !(IsSpare || IsStrike) && Rolls.Count == 2);
            }
        }

        public bool IsSpare {
            get {
                return Rolls.Count == 2 && Rolls.Sum() == 10;
            }
        }

        public bool IsStrike {
            get {
                return (Rolls[0] == 10 && (Rolls.Count == 1 || IsLastFrame));
            }
        }

        public bool IsLastFrame {
            get {
                return FrameIndex == 10;
            }
        }

        public void AddRoll(int roll) {
            Rolls.Add(roll);
        }
    }

    class Bowling {
        List<int> rolls = new List<int>();

        public void Roll(int pins) {
            rolls.Add(pins);
        }

        public int Score() {
            var frames = Frames(rolls).ToArray();

            int scores = frames.Sum(frame => frame.Rolls.Take(2).Sum());
            int spareBonus = frames.Skip(1).Where((_, i) => frames[i].IsSpare).Sum((frame) => frame.Rolls[0]);
            int strikeBonus = frames.Where(frame => frame.IsStrike)
                                    .SelectMany(frame => 
                                        frames.Skip(frame.FrameIndex)
                                              .SelectMany(nextFrame => nextFrame.Rolls)
                                              .Take(2))
                                    .Sum();
            
            spareBonus += frames.Where(frame => frame.IsLastFrame && frame.IsSpare && frame.Completed).Sum(frame => frame.Rolls[2]);
            strikeBonus += frames.Where(frame => frame.IsLastFrame && frame.IsStrike && frame.Completed).Sum(frame => frame.Rolls[2]);

            return scores + spareBonus + strikeBonus;
        }

        private static IEnumerable<Frame> Frames(IEnumerable<int> rolls) {
            var currentFrameIndex = 1;
            var currentFrame = new Frame(currentFrameIndex);

            foreach (var roll in rolls) {
                currentFrame.AddRoll(roll);

                if (currentFrame.Completed) {
                    yield return currentFrame;
                    currentFrameIndex++;
                    currentFrame = new Frame(currentFrameIndex);
                }
            }
        }

        static void Main() {
            Action<int[], int> test = (rolls, expectedScore) => {
                var bowling = new Bowling();
                foreach (int roll in rolls)
                    bowling.Roll(roll);

                bool success = bowling.Score() == expectedScore;
                Console.WriteLine("{0, 5} {1, 3} {2, 3} Rolls: {3}", success, bowling.Score(), expectedScore, String.Join(",", rolls));
            };

            test(new int[] { 0, 0 }, 0);
            test(new int[] { 0, 5 }, 5);
            test(new int[] { 5, 5, 0, 0 }, 10); // Spare w/o bonus points
            test(new int[] { 5, 5, 5, 0 }, 20); // Spare with bonus points
            test(new int[] { 10, 0, 0 }, 10); // Strike w/o bonus in both rolls
            test(new int[] { 10, 5, 0 }, 20); // Strike with bonus in first roll
            test(new int[] { 10, 0, 5 }, 20); // Strike with bonus in second roll
            test(new int[] { 10, 5, 5 }, 30); // Strike with bonus in both rolls
            test(new int[] { 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10 }, 300); // Perfect game

            Console.ReadLine();
        }
    }
}