Introducing dependency injection

Dependency Injection design pattern fulfills the dependency inversion principle of the SOLID design principles. There are three main forms of dependency injection:

• Constructor injection: An example of this is shown in the DIP section
• Setter injection: Let's look at an example code for setter injection:

    public class OrderProcessorWithSetter : IOrderProcessor 
    { 
        private IOrderRepository _orderRepository; 
        private IOrderNotifier _orderNotifier; 

        public IOrderRepository Repository 
        { 
            get { return _orderRepository; } 
            set { _orderRepository = value; } 
        } 

        public IOrderNotifier Notifier 
        { 
            get { return _orderNotifier; } 
            set { _orderNotifier = value; } 
        } 

        public void Process(IOrder order) 
        { 
            //Perform validations.. 
            if (_orderRepository.Save(order)) 
                _orderNotifier.Notify(order); 
        } 
    }

Now we look at an example client code using the setter injection:

    [Fact] 
        public void Test_DI_With_Setter() 
        { 
            var someOrder = new DIP.Order(); 
            var op = new DIP.Good.OrderProcessorWithSetter(); 
            op.Repository = new DIP.OrderRepository(); 
            op.Notifier = new DIP.OrderNotifier(); 

            op.Process(someOrder); 
        }

• Interface injection: Let's take a look at an example code for interface injection. The following are the two interfaces that will be used for our sample injection:

    public interface InjectOrderRepository 
    { 
        void SetRepository(IOrderRepository orderRepository); 
    } 
    public interface InjectOrderNotifier 
    { 
        void SetNotifier(IOrderNotifier orderNotifier); 
    }

And here is the OrderProcessorWithInterface class that implements the interfaces that are used for injection (usually via a DI framework):

    public class OrderProcessorWithInterface : InjectOrderRepository,
    InjectOrderNotifier, IOrderProcessor 
    { 
        private IOrderRepository _orderRepository; 
        private IOrderNotifier _orderNotifier; 

        public void SetRepository(IOrderRepository orderRepository) 
        { 
            _orderRepository = orderRepository; 
        } 

        public void SetNotifier(IOrderNotifier orderNotifier) 
        { 
            _orderNotifier = orderNotifier; 
        } 

        public void Process(IOrder order) 
        { 
            //Perform validations.. 
            if (_orderRepository.Save(order)) 
                _orderNotifier.Notify(order); 
        } 
    }

Let's have a look at the client code that uses the simple example of interface injection:

    [Fact] 
        public void Test_DI_With_Interface() 
        { 
            var someOrder = new DIP.Order(); 

            var op = new DIP.Good.OrderProcessorWithInterface(); 
            //Creation of objects and their respective dependencies 
            (components/services inside) are usually done by the
            DI Framework 
            op.SetRepository(new DIP.OrderRepository()); 
            op.SetNotifier(new DIP.OrderNotifier()); 

            op.Process(someOrder); 
        }