C# Abstract

• An abstract class is always used as a base class and cannot be instantiated directly.
• An abstract class may contain abstract methods and properties and of course it can contain non-abstract members as usual.
• An abstract class cannot be sealed or static.
• If a non-abstract class inherits from an abstract class, all the inherited methods and properties must be implemented.
• The abstract keyword must be placed in front of the keyword "class".

The syntax of an abstract class can be shown below.

[<access modifier>] abstract class <Class Name>
{
    ...
}

or

abstract [<access modifier>] class <Class Name>
{
    ...
}

• Abstract Methods can only be declared in abstract classes.
• An abstract method is implicitly a virtual method which can be overridden in it derived class with the keyword override.
• The static or virtual keyword cannot be used in an abstract method otherwise an error will be issued.
• The abstract keyword must be placed in front of the returned type of the abstract method.

The syntax of an abstract method is like below.

<access modifier> abstract <return type> <Method Name>();

or

abstract <access modifier> <return type> <Method Name>();

There is no curly brackets above to define the implementation of an abstract method.

Example 01-66-01

using System;

public abstract class A
{
}

public abstract class B
{
    public int i;
    
    public void method1()
    {
        Console.WriteLine("method1");
    }
}

public abstract class C
{
    public int i;
    
    public void method1()
    {
        Console.WriteLine("method1");
    }

    public abstract void method2();
}

public abstract class D : C
{
    public abstract void method3();
}

public class E : D
{
    public override void method2()
    {
        Console.WriteLine("method2");
    }

    public override void method3()
    {
        Console.WriteLine("method3");
    }
}

public class TestAbstract
{
    static void Main()
    {
        E e = new E();
        e.method1();
        e.method2();
        e.method3();

        //B b = new B();
        Console.Read();
    }
}

Output

method1
method2
method3

Explanation

• Line 3-5: It is legal to define an empty abstract class A.
• Line 7-15: It is also legal but not recommended to define an abstract class B without any abstract members.
• Line 17-27: The abstract class C is defined the same as class B except adding an abstract method2().
• Line 29-32: Class D is derived from class C plus adding a new abstract method method3(). There are no implementations in both method2 and method3 because they are all abstract.
• Line 34-45: Class E is inherited from class D and implements all the abstract methods. The override keyword is needed here.
• Line 47-59: Instantiate class E and call the 3 methods.
• Line 56: Commented out because the abstract class B cannot be instantiated.