DesignParrten Null Object

构造注入时接收空对象导致执行错误的处理方式：

基本情况：

    interface ILog
    {
        void Info(string msg);
        void Wran(string msg);
    }
    class BankAccount
    {
        private readonly ILog log;
        private int balance;
        public BankAccount(ILog log)
        {
            this.log = log;
        }
        public void Deposit(int amount)
        {
            balance += amount; 
            log.Info($"Deposit:{amount}");
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            BankAccount ba = new BankAccount(null);
            ba.Deposit(100);
        }
    }

解决方法1：

   interface ILog
    {
        void Info(string msg);
        void Wran(string msg);
    }
    class BankAccount
    {
        private readonly ILog log;
        private int balance;
        public BankAccount(ILog log)
        {
            this.log = log;
        }
        public void Deposit(int amount)
        {
            balance += amount;
            // log.Info($"Deposit:{amount}");
            log?.Info($"Deposit:{amount}");//增加obj?   不为null则执行
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            BankAccount ba = new BankAccount(null);
            ba.Deposit(100);
        }
    }

使用AutoFac通过依赖注入的方式如何处理：

    interface ILog
    {
        void Info(string msg);
        void Wran(string msg);
    }
    class BankAccount
    {
        private readonly ILog log;
        private int balance;
        public BankAccount(ILog log)
        {
            this.log = log;
        }
        public void Deposit(int amount)
        {
            balance += amount;
            // log.Info($"Deposit:{amount}");
            log?.Info($"Deposit:{amount}");
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            var cb = new ContainerBuilder();
            cb.RegisterType<BankAccount>();
            cb.Register(ctx => new BankAccount(null)); 
            using (var c=cb.Build())
            {
                var ba = c.Resolve<BankAccount>();
                ba.Deposit(100);
            }
        }
    }

 

解决方法2 增加一个Ilog的实现类，不做任何处理：

    interface ILog
    {
        void Info(string msg);
        void Wran(string msg);
    }
    class BankAccount
    {
        private readonly ILog log;
        private int balance;
        public BankAccount(ILog log)
        {
            this.log = log;
        }
        public void Deposit(int amount)
        {
            balance += amount;
            // log.Info($"Deposit:{amount}");
            log?.Info($"Deposit:{amount}");
        }

    }
    class NullLog : ILog
    {
        public void Info(string msg)
        { 
            //
        }

        public void Wran(string msg)
        { 
            //
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var cb = new ContainerBuilder();
            cb.RegisterType<BankAccount>();
            cb.RegisterType<NullLog>().As<ILog>(); 
            using (var c=cb.Build())
            {
                var ba = c.Resolve<BankAccount>();
                ba.Deposit(100);
            }
        }
    }