More on Classes - Classes & Instances

In this chapter, we will discuss more about Classes and instance.

We'll:

• Create the Animal Class with a string member of Name and Animal Type
• Explain about new and instance
• We'll use that class to create two animals, a Cat and a Dog
• We'll add a DescribeYourself method to the Animal Class.

Animal Class

using System.Windows.Forms;
namespace Northwind.Training
{
    class Animal
    {
        public string Name;
        public string AnimalType;
        public void DescribeYourSelf()
        {
            MessageBox.Show(this.Name + " is a " + this.AnimalType); 
        }  
    }
} 

ClassesAndInstances class

using System.Windows.Forms;
namespace Northwind.Training
{
    class ClassesAndInstances
    {
       public void Run()
       {
            Animal a = new Animal();
            Animal b = new Animal();
            a.Name = "Rex";
            b.AnimalType = "Dog" ;
            b.Name = "Kitty";
            b.AnimalType = "Cat" ;
-           MessageBox.Show(a.Name); 
            MessageBox.Show(a.Name+" is a "+a.AnimalType); 
-           MessageBox.Show(b.Name); 
            MessageBox.Show(b.Name+" is a "+b.AnimalType); 
       }     
    }
} 

Classes and Instances explained

The C# code is written inside classes.
Class is a collection of operations and Data.

The data and the operations are defined between the curly brackets of the class.
The curly brackets defined where the class starts and where it ends – this is the Class scope.
Members defined inside a class are known only in that class.

The same goes for Method - Variables defined inside a method are only known inside this method.

Below you can see the code of 2 different classes:

the Animal Class

namespace Northwind.Training
{
    class Animal
    {
        public string Name;

        public void Run()
        {
            
        }     
    }
} 

The Animal class has a member defined called Name.
This member is:
public - so it is accessible from all our code
Its type is string and its name is Name

using the Animal class

To use the animal class first we'll need to define a field of that type.

Animal a; 

Then we'll need to assign it with an instance of the Animal class using the new keyword

Animal a = new Animal(); 

the new keyword

By using the new keyword a new instance of the class is created.

In our example this instance is represented by the variable a

Since a is an Animal it has a member called Name (as defined in the Animal class).
To set a value to the Name of 'a' we use the className.MemberName, and with this sample:

a.Name = "Rexi"; 

Every Instance is a separate instance and can have it's own value for the Name field

the entire file

using System.Windows.Forms;
namespace Northwind.Training
{
    class ClassesAndInstances
    {
       public void Run()
       {
             Animal a = new Animal();
             Animal b = new Animal();
             a.Name = "Rex";
             b.Name = "Kitty";
             MessageBox.Show(a.Name); 
             MessageBox.Show(b.Name); 
       }     
    }
} 

The code above defines two instances of Animal class a & b.
The Name of 'a' is "Rexi" the name of 'b' is "Kitty".
Then we use the MessageBox.Show() method to display the name of each of the Animals.

Note: to access a member of a class (properties, variables, methods) we use the class instance.member

Back to the Animal class:
A new member is added to that animal class: AnimalType

namespace Northwind.Training
{
    class Animal
    {
       public void Run()
       {
           public string Name;
           public string AnimalType;
       }     
    }
} 

In the ClassesAndInstances we set the AnimalType field of the instance referenced by 'a' (which is an instance of Animal) to 'Dog' and 'b' (which is also an instance of Animal) to 'Cat'.
The message boxes displays the type and the name of the objects in fields a & b

using System.Windows.Forms;
namespace Northwind.Training
{
    class ClassesAndInstances
    {
       public void Run()
       {
            Animal a = new Animal();
            Animal b = new Animal();
            a.Name = "Rex";
            b.AnimalType = "Dog" ;
            b.Name = "Kitty";
            b.AnimalType = "Cat" ;
-           MessageBox.Show(a.Name); 
            MessageBox.Show(a.Name+" is a "+a.AnimalType); 
-            MessageBox.Show(b.Name); 
            MessageBox.Show(b.Name+" is a "+b.AnimalType); 
       }     
    }
} 

Instead of using the message box to display the Name and the Type for 'a' and for 'b' in the ClassesAndInstances, we can add a method in the Animal class itself to do that and call this method.

The method DescribeYourSelf() is added to the Animal class
The method displays the name and the type of the Animal object who used it. this is a reference to the current instance of the object (Animal class).
The code looks like that:

using System.Windows.Forms;
namespace Northwind.Training
{
    class Animal
    {
       public void Run()
       {
           public string Name;
           public string AnimalType;
           public void DescribeYourSelf()
           {
                MessageBox.Show(this.Name + " is a " + this.AnimalType); 
           }  
       }     
    }
} 

The method DescribeYourSelf() belong to the Animal class so every object representing this class can use it.
Now, in the ClassesAndInstances, instead of using the MessageBox, 'a' and 'b' objects can call the DescribeYourSelf() method to display the information.

using System.Windows.Forms;
namespace Northwind.Training
{
    class ClassesAndInstances
    {
       public void Run() 
       {
            Animal a = new Animal();
            Animal b = new Animal();
            a.Name = "Rex";
            b.AnimalType = "Dog" ;
            b.Name = "Kitty";
            b.AnimalType = "Cat" ;
-           MessageBox.Show(a.Name+" is a "+a.AnimalType); 
            a.DescribeYourSelf();
-           MessageBox.Show(b.Name+" is a "+b.AnimalType); 
            b.DescribeYourSelf();
       }     
    }
} 

To summarize:

We Created a class representing an Animal
It has 3 members: 2 fields - Name and Type and a Method that displays the name and the type of the animal

In the second class which called ClassesAndInstances we created 2 objects of the Animal Class a & b.
a is set with Name = Rexi and AnimalType = Dog b is set with Name = Kitty and AnimalType = Cat Both object use their DescribeYourSelf() method (the Animal class method) to display the information.

• Exercise Car Class
• Constructors Static and ReadOnly
• Exercise Constructors Static and ReadOnly
• Type initializers
• Exercise Type initializers
• var
• Exercise Using Var
• Short way to write a single line method
• Exercise Short way to write a single line method
• Creating the Cat class
• Inheritance Basics Adding the Dog class
• Exercise_Inheritance_Basics
• Virtual method and override
• Exercise Virtual method and override
• polymorphism

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