What, Why and How of Facade Pattern in C#

What is Facade Pattern?

Exposing the simplistic interface of anything complex is a Facade. For example, when you place an order for Pizza delivery, the application hides the complex process behind Pizza Delivery to your door.

A successful Pizza delivery order requires multiple services to participate. Facade Pattern exposes the participating services such as Store Availability, Delivery Staff Availability, inventory of items, and others via a straightforward interface. With a straightforward interface, it achieves the following objectives:

• It reduces the indirect coupling and eliminates direct coupling between the Consumer of the Facade and participating services.
• It hides and simplifies the interaction between the Consumer and the Facade.
• It allows you to replace the sub-system without impacting the Consumer.

The facade pattern is similar to Adapter Pattern, but it hides sub-systems from its Consumer while the Adapter pattern focuses on the object or one sub-system.

When to use Facade Pattern?

You should refactor towards Facade Pattern when:

• Interaction between the consumer class and sub-systems is getting complex and straightforward, and a stable interface can provide the most functionality of complex sub-systems.
• Replacing Legacy old sub-system(s).

How to implement it?

The Facade Pattern is a container class that depends upon two or more classes (sub-systems) to complete the transaction it exposes via its interface to the consumer class. A skeleton of example class is below:

A skelton of Facade Pattern

//A facade which depends upon two services
class OrderServiceFacade
{
    private readonly DeliveryRiderAvailability _riderAvailability;
    private readonly PaymentProcessor _paymentProcessor;

    //Dependcies are injected using DI Container
    public OrderServiceFacade(DeliveryRiderAvailability riderAvailability, PaymentProcessor paymentProcessor)
    {
        _riderAvailability = riderAvailability;
        _paymentProcessor = paymentProcessor;
    }
    public Task PlaceOrder(Order order)
    {
        //Process Payment
        //Find rider
        //Other steps via other sevices.
        //Handle errors of participating services more gracefully
        throw new NotImplementedException();
    }
}

Structure of Facade Pattern

The structure of Facade patter in form of class diagram is below:

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