polymorphism and fields

What are the 2 broad classifications of fields in C#?
1. Instance fields
2. Static fields

What are instance fields in C#?
Instance fields are specific to an instance of a type. If you have a class T, with an instance field F, you can create two objects of type T, and modify the value of F in each object without affecting the value in the other object.

What is a static field?
A static field belongs to the class itself, and is shared among all instances of that class. Changes made from instance A will be visible immediately to instances B and C if they access the field.
Will the following code compile?

using System;
class Area
{
   public static double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      Console.WriteLine(A.PI);
   }
}
No, a compile time error will be generated stating "Static member 'Area.PI' cannot be accessed with an instance reference; qualify it with a type name instead". This is because PI is a static field. Static fields can only be accessed using the name of the class and not the instance of the class. The above sample program is rewritten as shown below.

using System;
class Area
{
   public static double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Area.PI);
   }
}

Can you declare a field readonly?
Yes, a field can be declared readonly. A read-only field can only be assigned a value during initialization or in a constructor. An example is shown below.

using System;
class Area
{
   public readonly double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      Console.WriteLine(A.PI);
   }
}

Will the following code compile?

using System;
class Area
{
   public readonly double PI = 3.14;
}
class MainClass
{
   public static void Main()
   {
      Area A = new Area();
      A.PI = 3.15;
      Console.WriteLine(A.PI);
   }
}

No, PI is readonly. You can only read the value of PI in the Main() method. You cannot assign any value to PI.

What is wrong with the sample program below?

using System;
class Area
{
   public const double PI = 3.14;
   static Area()
   {
      Area.PI = 3.15;
   }
}
class MainClass
{
   public static void Main()
   {
      Console.WriteLine(Area.PI);
   }
}
You cannot assign a value to the constant PI field.

What is the difference between a constant and a static readonly field?
A static readonly field is very similar to a constant, except that the C# compiler does not have access to the value of a static read-only field at compile time, only at run time.
Explain polymorphism in C# with a simple example?
Polymorphism allows you to invoke derived class methods through a base class reference during run-time. An example is shown below.
using System;
public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I am a drawing object.");
}
}
public class Triangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Triangle.");
}
}
public class Circle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Circle.");
}
}
public class Rectangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Rectangle.");
}
}
public class DrawDemo
{
public static void Main()
{
DrawingObject[] DrawObj = new DrawingObject[4];

DrawObj[0] = new Triangle();
DrawObj[1] = new Circle();
DrawObj[2] = new Rectangle();
DrawObj[3] = new DrawingObject();

foreach (DrawingObject drawObj in DrawObj)
{
drawObj.Draw();
}
}
}

When can a derived class override a base class member?
A derived class can override a base class member only if the base class member is declared as virtual or abstract.

What is the difference between a virtual method and an abstract method?
A virtual method must have a body where as an abstract method should not have a body.
 Can fields inside a class be virtual?
No, Fields inside a class cannot be virtua. Only methods, properties, events and indexers can be virtual.

Give an example to show for hiding base class methods?
Use the new keyword to hide a base class method in the derived class as shown in the example below.
using System;
public class BaseClass
{
public virtual void Method()
{
Console.WriteLine("I am a base class method.");
}
}
public class DerivedClass : BaseClass
{
public new void Method()
{
Console.WriteLine("I am a child class method.");
}

public static void Main()
{
DerivedClass DC = new DerivedClass();
DC.Method();
}
}

Can you access a hidden base class method in the derived class?
Yes, Hidden base class methods can be accessed from the derived class by casting the instance of the derived class to an instance of the base class as shown in the example below.
using System;
public class BaseClass
{
public virtual void Method()
{
Console.WriteLine("I am a base class method.");
}
}
public class DerivedClass : BaseClass
{
public new void Method()
{
Console.WriteLine("I am a child class method.");
}

public static void Main()
{
DerivedClass DC = new DerivedClass();
((BaseClass)DC).Method();
}
}