Understanding Interfaces with C#

The public interface of a class is a contract between the client code and the class that provides the service. Concrete classes implement each method. However, an abstract class can defer the implementation by declaring the method to be abstract, and a C# interface declares only the contract and no implementation.

A concrete class implements an interface by defining all methods declared by the interface. Many classes can implement the same interface. These classes do not need to share the same class hierarchy. Also, a class can implements more than one interface.

Imagine a group of objects that all share the same ability: they fly. You can construct a public interface, called Flyer, that supports three operations: TakeOff, Land and Fly.

Fig 1. The interface flyer and airplane class diagram

Listing 1 The Flyer code

Listing 2 The Airplane code

There can be multiple classes that implement the Flyer interface, as shown of the next figure

Fig 2. Multiple implementations of the Flyer interface.

This sounds like multiple inheritance, but it is not quite that. The danger of multiple inheritance is that a class could inherit two distinct implementation of the same method.

Fig 3. A mixture of inheritance and implementation.

An Airplane is a Vehicle, and it can fly. A bird is an Animal, and it can fly. These examples show that a class can inherit from one class but also implement some other interface.

This sounds like multiple inheritance, but it is not quite that. The confusion of multiple inheritance is that a class could inherit two distinct implementations of the same method. This is not possible with interfaces because an interface method declaration does not supply implementation.
Suppose that you are constructing an aircraft control software system. The following diagram shows its class hierarchy

Fig 4. A mixture of inheritance and implementation.

The airport must grant permission to land and take off for flying objects of all types, then the code for the airport could look like the following.

Listing 3 The code for granting permission

Fig 5. Running the sample

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C# Recipe 1: How to calculate a leap year.

The Gregorian Calendar is the most widely used calendar in the world today.It is a reform of the Julian calendar, proposed by Aloysius Lilius, and decreed by Pope Gregory XIII, from whom it was named, on 24 February 1582 by papal bull Inter gravissimas

The changes made by Gregory also corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons. The Gregorian Calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1800 is not a leap year; the year 1984 is a leap year and the year 2000 too.

Fig 1 The following example calculates a leap year.



Fig 2 Running the sample, output 1



Fig 3 Running the sample, output 2



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