Compile time polymorphism vs. run time polymorphism

Question

Why is overloading called compile time polymorphism and Overriding run time polymorphism in C#?

Answer 1

Run time Polymorphism Example in c#.

using System;
public class demo{
public static void Main(String[] args){

cal cal ;

add a = new add();
cal = a;
Console.WriteLine("Addition is" + cal.calculate(20, 20));   

sub s = new sub();
cal = s;
Console.WriteLine("Substraction is" + cal.calculate(20, 20));

mul m = new mul();
cal = m;
Console.WriteLine("Multiplication is" + cal.calculate(20, 20));


div d = new div();
cal = d;
Console.WriteLine("Division is" + cal.calculate(20, 20));

Console.ReadLine();
}
}

public abstract class cal{
       public abstract int calculate(int a, int b);
}


public class add : cal {
      public override int calculate(int a ,int b){
        return a+b;
      }
 }

public class sub : cal{
      public override int calculate(int a, int b){
        return a-b;
      }     
}

public class mul : cal{
      public override int calculate(int a, int b){
        return a*b;
      }
 }

 public class div : cal{
       public override int calculate(int a, int b){
            return a/b;
       }
 }