Confused in polymorphism?

Question

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

Answer 1

Because it's known at compile time which of your overloaded functions is called, but that is not always the case for an overridden function.

Answer 2

Well, overloading decisions (which method signatures are used, based on the arguments¹) are made by the compiler, whereas overriding decisions (which method implementations are used, based on the type of the target of the method) are made by the CLR at execution time.

I wouldn't usually call overloading "polymorphism" though. In my experience the word usually refers to overriding. I suppose overloading does allow you to treat an object of one type as another, although overloading itself doesn't need to be involved there - it's just normal type conversions.

Here's an example showing that overload choice is performed at compile time:

using System;

class Test
{
    static void Foo(object a)
    {
        Console.WriteLine("Object overload called");
    }

    static void Foo(string a)
    {
        Console.WriteLine("String overload called");
    }

    static void Main()
    {
        object x = "hello";
        Foo(x);
    }
}

Here the Foo(object) overload is called because x is of type object at compile time - it's only at execution time that it's known to refer to a string.

Compare that with this example:

using System;

class Base
{
    public virtual void Foo()
    {
        Console.WriteLine("Base.Foo called");
    }
}

class Derived : Base
{
    public override void Foo()
    {
        Console.WriteLine("Derived.Foo called");
    }
}

class Test
{
    static void Main()
    {
        Base x = new Derived();
        x.Foo();
    }
}

Here the compile-time type of x is Base, but it's still the derived class's overriding method which is called, because the execution-time type of the object that x refers to is Derived.

---

¹ It's slightly more complicated than that in fact, due to method hiding etc - but in simple cases you can think of it as just picking the signature.

Answer 3

Classical examples of static polimorphism are based on template metaprogramming or Duck Typing but not on method overloading.

Static polimorphism means that desicion is made by compilier (statically), and dynamic polimorphism means that desition is made only in runtime (dynamically).

Answer 4

Overridden functions are functions that have the same signature, but are implemented in different derived classes. At compile time, usually the base class type is used to reference an object, though at run time this object could be of a derived type, so when an overridden method is called, the implementation that is called is dependent on what kind of object is doing the calling (base vs. a derived type) which is unknown at compile time.

Overloading (not really polymorphism) is simply multiple functions which have the same name but different signatures (think multiple constructors for an object taking different numbers of arguments). Which method is called is known at compile time, because the arguments are specified at this time.

Answer 5

compile time polymorphism

Suppose lets say you have 2 methods as follows; since the method shares same name but have different parameters; it is called as "overloaded" method. Eat(string food); Eat(string food, string SpoonOrFork);

and you are using like this in your dinner class

public class Man
{
 public bool Eat (string food)
 {
  //implementation
 }

 public bool Eat (string food, string SpoonOrFork)
 {
  //implementation
 }

}
public class dinner
{
  public bool Start()
  {
   string food = "course1";
   Man.Eat ( food);
  }
}

Now when you compile this program the compiler knows exactly which version of Eat method to call during compile time itself (because of the difference in parameters).

That's why it is called as compile time polymorphism.

Run time polymorphism

public class chimp
    {
        public virtual void walk()
        {
            Console.WriteLine("I am walking using 4 legs");
        }

    }

    public class neanderthals : chimp
    {
        public override void walk()
        {
            Console.WriteLine("I am walking using 2 legs");
        }

    }



    class Program
    {
        static void Main(string[] args)
        {
            chimp x = new neanderthals();
            x.walk();
            Console.ReadLine(); // this will give an output of "I am walking using 2 legs"
        }
    }

In the above code x is of type chimp. Even though the compiler thinks it is going to call the walk method in chimp; but that is not what actually happens. Since it depends on CLR (run time) this kind of polymorphism is called "run-time" polymorphism.