Abstract & Sealed Class

• In the parent class if we do not implement the logic for a method (As the method should be overriden in the child classes) then we make use of the abstract class
• When we mark a method as abstract then the class that contains it also needs to be converted into a abstract class (Abstract class can have non abstract members)
• When inheriting an abstract class the derived class “must implement all” the abstract members
• Abstract classes cannot be instantiated

using System;
 
namespace AbstractClass
{
		public class Circle : Shape
		{
			public override void Draw()
			{
				Console.WriteLine("Drawing Circle");
			}
		}
 
		public abstract class Shape
		{
			public int Width { get; set; }
			public int Height { get; set; }
 
			// Abstract Method Declaration
			public abstract void Draw();
 
			public void Copy()
			{
				Console.WriteLine("Copying Shape to Clipboard");
			}
		}
 
		class Program
		{
			static void Main(string[] args)
			{
				Circle circle = new Circle();
				circle.Draw();
			}
		}
}

Sealed Classes & Members

• Sealed Classes are the opposite of Abstract Members
• When applied to a class that class cannot be derived from and when applied to a method the method cannot be overridden
• Sealed Keyword is supposed to provide a slight speed improvement because of run-time optimization
• Sealed Keyword in not commonly used and should not be used unless there are some very specific reason for using it