'List' help ?

nlowdon

Evening all (depending where you are of course !) This is going to be tricky to explain but i'll give it my best. Please bear in mind i am a novice to C#. I'm going to run a loop to create a 'List' of 10 animals. I'm using polymorhism to then cast each animal the type i need (cat, dog, etc).

newAnimals[0] = new Cat();

newAnimals[1] = new Cat();

I'm confused as to how i can then give each object a reference name so that i can reference individual object's and invoke methods on them ? For instance - say i had 5 animas in the list and the first 2 were cats. I need to be able to say -

cat1.eat();

cat2.play();

Is there a way i can do this ? Thanks in advance (loving the forum by the way !!)

Pedram Behroozi

No you can't (I don't think so), except you declare 5 variables like:

cat1 = newAnimals[0];
cat2 = newAnimals[1];
...

Then you can use these variables. However newAnimal[0].eat should work for you...

Life is 5: 3 me, 1 you. Trying to find the missing part is the meaning of Life. And sadness is when you find that part!

Christian Graus

If you need to tell which animals are of what type to do something specific with them, then your inheritance tree is probably broken. You can add methods like GetAllCats, tho. Then you can do this: foreach(Animal animal in newAnimals) { Cat c = animal as Cat; if (c != null) { // Add c to a list to return, or do something with c } } The as keyword attempts to cast an object as a derived object, and returns null if it fails. I would also use a list instead of an array, so you can add as many animals as you like, unless your class requires you to use arrays at this point. If you just mean accessing the objects, then newAnimals[0], newAnimals[1] is how you reference them.

Christian Graus Driven to the arms of OSX by Vista.

nlowdon

Thanks Cristian, Your reply was v-helpful. I've since tried

newcat1 = zooAnimals[0] as Cat;

but this doesn't seem to work either. Can i not cast the object in this way ?

Christian Graus

What does 'does not work' mean ? If zooAnimals is an array of Animals, and Cat is a class derived from Animal, then so long as newcat1 is a Cat and zooAnumals[0] is a Cat, it should work fine.

Christian Graus Driven to the arms of OSX by Vista.

nlowdon

Cristian, my appologies for not being very specific - still getting used to this 'posting' bit ! Ok - my code goes -

        Animal newcat1;

        zooAnimals = new List<Animal>();

        for (i = 0; i < 2; i++)
        {
            Animal animal = new Animal();
            zooAnimals.Add(animal);
        }

        newcat1 = zooAnimals\[0\] as Cat;
        newcat2 = zooAnimals\[1\] as Cat;

the program then throws a 'Null Reference - Object reference not set to an instance of an object.' When i hover the pointer, the 'newcat1' is still set to null

Lost User

Well, you're adding Animals to the list, not Cats. So what you need to do is:

        **Cat** newcat1;

        zooAnimals = new List();

        for (i = 0; i < 2; i++)
        {
            Animal animal = new **Cat**();
            zooAnimals.Add(animal);
        }

        newcat1 = zooAnimals\[0\] as Cat;
        newcat2 = zooAnimals\[1\] as Cat;

regards

cor2879

Hi there, I was looking over your post and I think this is a sample of how to accomplish what you are looking for. Look this over:

using System;
using System.Collections.Generic;

public abstract class ZooAnimal
{
public abstract string Speak();
}

public class Cat : ZooAnimal
{
public override string Speak()
{
return "Meow";
}
}

public class Lion : Cat
{
public override string Speak()
{
return "Rowr!";
}
}

public class Program
{
public static void Main(params string[] args)
{
List<ZooAnimal> zooAnimals = new List<ZooAnimal>();

    for (int i = 0; i < 2; i++)
        zooAnimals.Add(new Cat());

    for (int i = 0; i < 3; i++)
        zooAnimals.Add(new Lion());

    for (int i = 0; i < zooAnimals.Count; i++)
    {
        Lion lion = zooAnimals\[i\] as Lion;

        if (lion != null)
            Console.WriteLine("The lion says: " + lion.Speak());
        else
        {
            Cat cat = zooAnimals\[i\] as Cat;
            Console.WriteLine("The cat says: " + cat.Speak());
        }
    }

    Console.Read();
}

}

See this is all just a matter of inheritance and making sure you instantiate your types correctly. In your code example you are adding two animals to the collection but trying to get out two cats. Remember - all cats are animals but not all animals are cats. To get the code you posted to work correctly, you would need to add cats to the zooAnimals collection instead of just animals. This works just fine in C# by the way - an object declared as a base type can always hold an instance of one of its derived types. To illustrate I created a three tiered inheritance example. Notice that in the above example you have ZooAnimals, Cats, and Lions. ZooAnimals is an abstract type, it exists only to serve as a base class for anything that I might consider a zoo animal. Then you have the Cat class. This is also a pretty general class - there are many types of cats, so I derived another class from Cat called Lion. Notice the hierarchy - All Lions are Cats but not all Cats are Lions (illustrated in the final for loop in the static method Main). All Cats are ZooAnimals (as far as this program is concerned at least) all ZooAnimals are not necessarily Cats. It must also follow then that since all Lions are Cats, and all Cats are ZooAnimals, then all Lions are ZooAnimals as well. Hope that

Christian Graus

I think perhaps you should do some reading on OO. As others have said, you are trying to turn an animal into a cat. It's not a cat, you didn't ask it to be.

Christian Graus Driven to the arms of OSX by Vista.