Understanding C Sharp Abstract Classes

In the preceding chapters we looked in detail at object oriented programming in C# and and also at the concept of class inheritance. In this chapter we will look at the next area of object oriented programming, the abstract class.

Contents

What is a C# Abstract Class?

In the examples we have looked at so far in this book we have created classes which could be both instantiated as objects and used as a base class from which to inherit to create a subclass. Often a base class is not intended to be instantiated and is provided solely for the purpose of providing an outline for subclasses. Such a class is known as an abstract class. An abstract class cannot be instantiated as an object and is only provided for the purpose of deriving subclasses.

Abstract Members

A C# abstract class contains abstract members which define what a subclass should contain. These abstract members only declare that a member of a particular type is required, it does not implement the member. Implementation of abstract members takes place within the derived class. A subclass which derives from an abstract class and fails to implement abstract methods will fail to compile.

Declaring a C# Abstract Class

Abstract classes are declared using the abstract modifier in the class declaration:

public abstract class BankAccount
{
}

Abstract member functions and properties are also declared using the abstract keyword. For example to declare an abstract method in our BankAccount class the following code is required:

     public abstract decimal calculateInterest();

We now have an abstract class with an abstract method named calculateInterest(). Note that this codce only states that any class derived from BankAccount must implement a method called calculateInterest() which returns a Decimal value. It does not, however, implement the method.

Deriving from an Abstract Class

In order to subclass from an abstract class we simply write code as follows:

public class SavingsAccount : BankAccount
{
}

We now have a class called SavingsAccount which is derived from the abstract BankAccount class. The next step is to implement the abstract calculateInterest() method. When implementing abstract members in a derived class the override modifier must be used. For example:

public override decimal calculateInterest()
{
        return balance * interestRate;
}