using System.Collections.Generic;

using System;

​

public class Program

{

    public interface IAnimation {

        public void Play();

    }

    public interface ISeekable {

        public void Seek(int t);

    }

    public interface IAnimationCollection : IAnimation {

        public IAnimationCollection AddChildAnimation<U>(U child) where U: IAnimation;

    }

    public class DeterministicAnimation : IAnimation, ISeekable {

        public void Play() { /* play myself */ }

        public void Seek(int t) { /* set state to time t */ }

    }

    public class PhysicsAnimation : IAnimation {

        public void Play() { /* play myself */ }

    }

    public class AnimationSequence : IAnimationCollection, ISeekable {

        internal List<IAnimation> _children = new();

        public void Play() { /* play all children in sequence*/ }

        public void Seek(int t) { /* set state to time t */ }

        public IAnimationCollection AddChildAnimation<U>(U child) where U:IAnimation {

            _children.Add(child);

            if(child is ISeekable) return this;

            else return this.AsNonSeekableSequence();

        }

        private NonSeekAnimationSequence AsNonSeekableSequence() { 

            var n = new NonSeekAnimationSequence();

            n._children = _children;

            return n;

        }

    }

    public class NonSeekAnimationSequence : IAnimationCollection {

        internal List<IAnimation> _children = new();

        public void Play() { /* play all children in sequence */ }

        public IAnimationCollection AddChildAnimation<U>(U child) where U:IAnimation {

            _children.Add(child);

            return this;

        }

    }

​

    public static void Main()

    {

        var makeEndGameSequence = IAnimation () => new AnimationSequence();

        var someCutscene = new AnimationSequence() // starts as Seekable

            .AddChildAnimation(

                new AnimationSequence() 

                .AddChildAnimation(new DeterministicAnimation())

                .AddChildAnimation(new PhysicsAnimation()) // not seekable, so changes the root AnimationSequence to NonSeekAnimationSequence()

                )

            .AddChildAnimation(new DeterministicAnimation())

            .AddChildAnimation(makeEndGameSequence());

        Console.WriteLine(someCutscene.GetType()); // NonSeekAnimationSequence

    }

}