[C#] run-time and compile-time variable ?

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C#

namespace CSharp_Practicing
{

    class Program
    {

        static void Main(string[] args)
        {
            Animal a = new Dog(); // THE QUESTION IS ABOUT THIS VARIABLE !!!
        }
    }

    class Animal { }
    class Dog : Animal
    {
        public void SayBau()
        {
            Console.WriteLine("Baaaaaau");
        }
    }
    class Cat : Animal
    {
        public void SayMaw()
        {
            Console.WriteLine("Mawwwwwwwwwwwww");
        }
    }
}

The variable a in compile-time is of type Animal and in run-time its of type Dog ? Am I right or I am understanding it wrongly ?

What I have tried:

Asking question here in CodeProject.com

Posted 20-Jul-17 22:18pm

The_Unknown_Member

Updated 20-Jul-17 23:27pm

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F-ES Sitecore · Answer 1 · 2017-07-20T23:27:00

Animal a = new Dog();

You're creating two things, you're creating an object (Dog) and a reference variable (a). The object is a Dog, it will always be a Dog and part of being a Dog means also being an Animal. The variable you create is of type Animal and is pointing to the Animal part of the Dog object. As the object "a" points to is a Dog you could do this

Dog d = (Dog)a;

You still only have one object (Dog), however you have created another reference variable (d) that points to the Dog part of the object. So now you have one object (Dog) and two reference variables that point to this object; "a" which is looking at the Animal part and can only access Animal properties and methods, and "d" which is looking at the Dog part and can access any property or method on the Dog part or the Animal part (as Dog inherits Animal, Dog is considered an Animal too).

Now consider this

Cat c = (Cat)a;

Will this compile? Yes. The compiler only knows that "a" must be an animal, so it is *possible* that it might be a Cat but it doesn't know at compile time so it gives you the benefit of the doubt. However at run-time it will throw an error as there is no "Cat" part of the object that "a" points to, as "a" points to a Dog.

CPallini · Answer 2 · 2017-07-20T22:28:00

Solution 1

Roughly speaking, yes.
You may use it in every context a Animal is allowed (hence its 'compile-time' type), however its actual type, at runtime, is, polymorphically, Dog.

Note, your class hierarchy doesn't take advantage of polymorphism.

Posted 20-Jul-17 22:28pm

CPallini

Comments

The_Unknown_Member 21-Jul-17 5:10am

Okay could you explain me this please:
object o = 5;
object y = 6;
object oy = o + y;
Console.WriteLine(oy.GetType());

This is still polymorphishm. Because every class in .NET inherits from the System.Object so i can assign to variable of type System.Object an integer value and the type of this variable in run-time will be integer and in compile-time it will be System.Object. I know that the System.Object has no operator overload "+" for integer numbers but shouldn't it perform o + y in runtime and assign it back to the variable oy ?

Edit: I think I understood this. This is because after the assignment operator of oy I am adding two variables of type System.Object (which are holding integers but in compile-time they are still System.Object) and since System.Object does not support arithmetic operations for integers it cant perform the task in runtime but if i make it object oy = 5 + 6; then it will can because 5 and 6 are integer literals and are not holded by System.Object variable. Did I understand it right ?

Richard MacCutchan · Answer 3 · 2017-07-20T22:30:00

Solution 2

It's always a Dog, as you created it that way in your new statement. In reality the variable is not anything at compile time since it does not exist until the program is run.

Posted 20-Jul-17 22:30pm

Richard MacCutchan

Comments

Ralf Meier 21-Jul-17 4:53am

I don't agree with you ...
Animal is the Base-Type. If you instance a as Animal with Dog you could do this (because Dog derives from Animal) but what you get is only that, what is common - that means you don't have the SayBau-Method ...

Richard MacCutchan 21-Jul-17 5:06am

You are correct (sort of). Although a is declared as an Animal type, it still refers to a Dog, from the new statement. So you can cast it up to a Dog and get the SayBau method. If it had been created as new Animal you would not be able to do that.

Ralf Meier 21-Jul-17 5:25am

That's what I meant ... ;)

Richard MacCutchan 21-Jul-17 5:45am

Uh, so now you do agree with my original comments?

Ralf Meier 21-Jul-17 5:49am

Yes and No ... but with your additional comments ... because (for me) the enquirer was not sure with what happens with/when derive from something ...

Richard MacCutchan 21-Jul-17 5:52am

I agree. Fortunately F-ES Sitecore has now made everything clear.

Ralf Meier 21-Jul-17 6:00am

to clarify it :
I really appreciate all comments and answers from you - this is no vote !!!

Richard MacCutchan 21-Jul-17 6:20am

I did not think it was. And I also appreciate comments and corrections from you (and others who know what they are talking about).

F-ES Sitecore 21-Jul-17 5:16am

The SayBau method *is* there on the object, you just can't access it using "a" as "a" is pointing to the sub-class. If you cast "a" to Dog it would become accessible.

Ralf Meier 21-Jul-17 5:25am

That's what I meant ... ;)

[C#] run-time and compile-time variable ?

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