GameApply.cs

public static Game Apply(Game game, IEvent evt)
{
    switch (evt)
    {
        case GameCreatedEvent e:
            return new Game
            {
                State = EState.Created,
                Player = e.PlayerEmail,
            };
        case MoveDecidedEvent e:
            if (game.State == EState.Created)
            {
                return new Game
                {
                    State = EState.Waiting,
                    Player = e.PlayerEmail,
                    Move = e.Move,
                };
            } else
            {
                return game;
            }
        case GameWonEvent e:
            return new Game
            {
                State = EState.Won,
                Player = game.Player,
                Move = game.Move,
            };
        case GameTiedEvent e:
            return new Game
            {
                State = EState.Tied,
                Player = game.Player,
                Move = game.Move,
            };
        default:
            throw new InvalidOperationException($"Unknown event: {evt}");
    }
}