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Classes and Objects

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Introduction

In C#, classes and objects are fundamental concepts of Object-Oriented Programming (OOP). A class is a blueprint for creating objects, providing initial values for state (member variables or fields), and implementations of behavior (member functions or methods). An object is an instance of a class.

Key Concepts

Class

  • Definition: A class is a user-defined blueprint or prototype from which objects are created. It represents a set of properties or methods that are common to all objects of one type.
  • Syntax: 
    public class ClassName
{
    // Fields
    // Properties
    // Methods
}

Object

  • Definition: An object is an instance of a class. When a class is defined, no memory is allocated until an object of that class is created.
  • Syntax: 
    ClassName objectName = new ClassName();

Practical Example

Defining a Class

Let's define a simple class named Person with some fields, properties, and methods.

public class Person
{
    // Fields
    private string firstName;
    private string lastName;
    private int age;

    // Properties
    public string FirstName
    {
        get { return firstName; }
        set { firstName = value; }
    }

    public string LastName
    {
        get { return lastName; }
        set { lastName = value; }
    }

    public int Age
    {
        get { return age; }
        set { age = value; }
    }

    // Methods
    public void Introduce()
    {
        Console.WriteLine($"Hello, my name is {FirstName} {LastName} and I am {Age} years old.");
    }
}

Creating and Using Objects

Now, let's create an object of the Person class and use its properties and methods.

public class Program
{
    public static void Main(string[] args)
    {
        // Creating an object of the Person class
        Person person1 = new Person();

        // Setting properties
        person1.FirstName = "John";
        person1.LastName = "Doe";
        person1.Age = 30;

        // Calling a method
        person1.Introduce();
    }
}

Explanation

  • Fields: firstName, lastName, and age are private fields that store the state of the object.
  • Properties: FirstName, LastName, and Age are public properties that provide access to the private fields.
  • Methods: Introduce is a method that prints a message to the console.

Exercises

Exercise 1: Define a Class

Define a class named Car with the following properties and methods:

  • Properties: Make, Model, Year
  • Method: DisplayInfo (prints the car's make, model, and year)

Solution:

public class Car
{
    // Properties
    public string Make { get; set; }
    public string Model { get; set; }
    public int Year { get; set; }

    // Method
    public void DisplayInfo()
    {
        Console.WriteLine($"Car: {Make} {Model}, Year: {Year}");
    }
}

Exercise 2: Create and Use Objects

Create an object of the Car class and set its properties. Then, call the DisplayInfo method.

Solution:

public class Program
{
    public static void Main(string[] args)
    {
        // Creating an object of the Car class
        Car car1 = new Car();

        // Setting properties
        car1.Make = "Toyota";
        car1.Model = "Corolla";
        car1.Year = 2020;

        // Calling a method
        car1.DisplayInfo();
    }
}

Common Mistakes and Tips

  • Uninitialized Fields: Ensure that fields are properly initialized before using them.
  • Access Modifiers: Use appropriate access modifiers (public, private, etc.) to control the visibility of class members.
  • Naming Conventions: Follow naming conventions for class names (PascalCase) and member names (camelCase for fields, PascalCase for properties and methods).

Conclusion

In this section, we covered the basics of classes and objects in C#. We learned how to define a class, create objects, and use properties and methods. Understanding these concepts is crucial for mastering Object-Oriented Programming in C#. In the next section, we will delve into methods and how they can be used to define the behavior of objects.

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