学ぶ Class Constructors

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A class constructor is the same as a struct constructor. It is a method which is executed when an object is created.

The syntax for creating a constructor is the following:

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class className {
    // ... other class code

    public className() {
        // code
    }
}

For example:

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using System;

class Player {
    public Player() {
        Console.WriteLine("A new player is created");
    }
}

public class HelloWorld
{
    public static void Main(string[] args)
    {
        Player p1 = new Player();
        Player p2 = new Player();
        Player p3 = new Player();
    }
}

You can see from the output that the Console.WriteLine statement inside the Player constructor is executed every time a new object is created.

Note

In C# 9 and older versions of C#, there are some differences between structs' and classes' constructors. For example, in older versions, you cannot created parameterless constructors for Structs, however, in C# 10 and newer versions it is supported. It is recommended to use the latest version of C# compilers otherwise you might face some errors while compiling some of the code.

Following is an example code from the Struct Constructors chapter. The term static is explained in the comments as it is more relevant here however it is not necessary to completely understand it as this concept will be explained in detail in later sections.

Note

Unlike normal fields, a static field can hold data even if there's no object that class. Static fields have the same value for all objects therefore changing the value of a static field from one object changes it for all objects. The program class represents the program itself, therefore if the static field is a member of the 'Program' class then it acts like a global variable.

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using System;

struct Program
{
	public static int totalPlayers = 0;

	struct Player
	{
		public int id;
		public Player()
		{
			id = totalPlayers++;
			Console.WriteLine($"New Player Object Created With ID {id}");
		}
	}
	
	static void Main(string[] args)
	{
		Player player1 = new Player();
		Player player2 = new Player();
		Player player3 = new Player();
	}
}

In order to convert the Player struct into a class, all you need to do is to change the term struct to class:

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using System;

class Program
{
	// Unlike normal fields, a 'static' field can hold data even if there's no object.
	// Static fields have the same value for all objects.
	// Changing the value of a static field from one object changes it for all objects.
	// In this case the static field is part of the 'Program' class which represents the program itself
	// Therefore this field is essentially a global variable
	public static int totalPlayers = 0;

	struct Player
	{
		public int id;
		public Player()
		{
			id = totalPlayers++;
			Console.WriteLine($"New Player Object Created With ID {id}");
		}
	}
	
	static void Main(string[] args)
	{
		Player player1 = new Player();
		Player player2 = new Player();
		Player player3 = new Player();
	}
}

Similarly, you can have constructors with some arguments as well. Usually, this method is used to make the initialization of class fields easier:

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using System;

class Point {
	public double x;
	public double y;
	
	public Point(double x, double y) {
        // 'this' keyword is also used in classes just like how its used in structs 
		this.x = x;
		this.y = y;
	}
	
	public void showCoordinates() {
		Console.WriteLine($"({x}, {y})");
	}
}

class Program
{
	static void Main(string[] args)
	{
		Point p1 = new Point(50, 70);
		Point p2 = new Point(70, 90);
		
		p1.showCoordinates();
		p2.showCoordinates();
	}
}

At this point, you have covered most of the basic concepts of the Classes, and judging from the syntax and behavior, Classes seem to be exactly the same as Structs, however, that is not actually the case. As stated in the first chapter of this section, Structs are a limited version of Classes that only provide a basic functionality on the other hand the features of Classes are much more complex and broad. In this section you have covered all the features that are similar between Classes and Structs. In the next two sections you will learn all the intricacies of classes and object-oriented programming.

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セクション 1. 章 30

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セクション 1. 章 30

Class Constructors

1. When is a constructor method called?

2. What will be the name of a constructor method in a class called "Animal"?

3. Which keyword is used before a constructor name?