class-abstract.md

Abstract Classes in C#

• An abstract class cannot be instantiated.
• Abstract classes provide a common definition of a base class that multiple derived classes can share.
• Abstract classes may also define abstract methods.

Example 1

public class D
{
    public virtual void DoWork(int i)
    {
        // Original implementation.
    }
}

public abstract class E : D
{
    public abstract override void DoWork(int i);
}

public class F : E
{
    public override void DoWork(int i)
    {
        // New implementation.
    }
}

Example 2

abstract class ShapesClass
{
    abstract public int Area();
}

class Square : ShapesClass
{
    int side = 0;

    public Square(int n)
    {
        side = n;
    }
    // Area method is required to avoid
    // a compile-time error.
    public override int Area()
    {
        return side * side;
    }

    static void Main() 
    {
        Square sq = new Square(12);
        Console.WriteLine("Area of the square = {0}", sq.Area());
    }

    interface I
    {
        void M();
    }
    abstract class C : I
    {
        public abstract void M();
    }

}

Sealing Classes and Methods

• A sealed class cannot be used as a base class (or abstract class).
• Sealed classes prevent derivation.
• They can be slightly faster than regular classes.
• Methods can be sealed too.

public sealed class D
{
    // Class members here.
}

public class D : C
{
    public sealed override void DoWork() { }
}