Overriding is applicable only to methods and properties. There are 4 keywords used in overriding. These first 3 are the most common way of overriding behavior. The members being overridden must have the same return type, signature, and parameters.

abstract

Has no implementation, forces overriding.

If a class has at least one abstract member, it has to be marked abstract as well, preventing instantiation.

An abstract class that has no implemented method is sometimes called a "purely" abstract class.

virtual

Has implementation, overriding is optional.

override

Marks overriding.

Alongside this, the base keyword can be used to access members in the base class.

new

Hides parent member behind another one

This keyword, while similar, is not quite an override. It hides the parent member behind another with the same name. However, the return type or parameters can be different.

Example

public class Shape
{
	public abstract int Area()
}

public class Square : Shape
{
	public int Length { get; set; }
	
	public Square(int length)
	{
		this.Length = length
	}
		
	public override int Area()
	{
		return Length * Length;
	}
}

public class Cube : Square
{
	public Cube(int length) : base(length)
	{
		
	}
	
	public override int Area()
	{
		return base.Area() * 6;
	}
}

public class Circle : Shape
{
	public Circle(int radius)
	{
		this.Radius = radius;
	}
	public int Radius { get; set; }
	
	public override int Area()
	{
		return 3.14*Radius*Radius;
	}
}

public void Main()
{
	List<Shape> shapes = new List<Shape>();
	shapes.Add(new Square(3));
	shapes.Add(new Circle(5));
	foreach(Shape shape in shapes)
	{
		Console.WriteLine(shape.Area());
	}
}