Class
Class and Members in a Class
In C#, classes are used to create custom types.
C# Fields and Methods
Fields: are variables and Methods: are functions in a class.
/*
The animal namespace
*/
namespace Animals
{
// An animal class
public class Animal
{
}
// A cat class
public class Cat
{
// Class fields
// The name of the cat
public string Name = "Unknown";
// The birthday of the cat
public DateTime DateOfBirth;
// Class methods
// Print out method
public void WriteToConsole()
{
Console.WriteLine($"{Name} was born on a {DateOfBirth:dddd}.");
}
}
// A dog class
public class Dog
{
}
}
C# Class Constructor and Finalizer (Destructor)
this is a keyword refers to the current instance of a class. Dot notation . is an operator to access a member (a field or a method) of a clas. An object is an instance of a class. An object can be created from a class using the new keyword.
using System;
using Animals;
// namespace
namespace MyBusiness
{
// main program
internal class Program
{
static void Main(string[] args)
{
Cat myCat = new Cat();
Cat nana = new Cat("Nana", new DateTime(2019, 12, 9));
myCat.WriteToConsole();
Console.WriteLine($"{nana.Name} was born on a {nana.DateOfBirth:dddd}.");
nana.WriteToConsole();
}
}
}
/*
The animal namespace
*/
namespace Animals
{
// A cat class
public class Cat
{
// Class fields
// The name of the cat
public string Name = "Unknown";
// The birthday of the cat
public DateTime DateOfBirth;
// Class methods
// Constructors
// Default constructor. It will be called by default
public Cat()
{
this.Name = "Unknown";
this.DateOfBirth = DateTime.Today;
// You can use the following instread
// name = "Unknown";
// dateOfBirth = DateTime.Today;
}
// Parameterized Constructor
public Cat(string name, DateTime dateOfBirth)
{
this.Name = name;
this.DateOfBirth = dateOfBirth;
// You "cannot" use the following instread!
// name = name;
// dateOfBirth = dateOfBirth;
}
// Finalizer
~Cat()
{
// This function will be called when the object is desctroyed.
}
// Print out method
public void WriteToConsole()
{
Console.WriteLine($"{Name} was born on a {DateOfBirth:dddd}.");
}
}
}
$ Unknown was born on a Sunday.
$ Nana was born on a Monday.
C# Access Modifiers Doc
Accessibility Levels Doc: Depending on the context in which a member declaration occurs, only certain declared accessibilities are permitted. If no access modifier is specified in a member declaration, a default accessibility is used.
using System;
using Animals;
// namespace
namespace MyBusiness
{
// main program
internal class Program
{
static void Main(string[] args)
{
Cat nana = new Cat("Nana", new DateTime(2019, 12, 9));
nana.Poop(); // Compile error
nana.Speak();
}
}
}
/*
The animal namespace
*/
namespace Animals
{
// A cat class
public class Cat
{
// Class fields
// The name of the cat
public string Name = "Unknown";
// The birthday of the cat
public DateTime DateOfBirth;
// Class methods
// Constructors
// Default constructor. It will be called by default
public Cat()
{
this.Name = "Unknown";
this.DateOfBirth = DateTime.Today;
// You can use the following instread
// name = "Unknown";
// dateOfBirth = DateTime.Today;
}
// Parameterized Constructor
public Cat(string name, DateTime dateOfBirth)
{
this.Name = name;
this.DateOfBirth = dateOfBirth;
// You "cannot" use the following instread!
// name = name;
// dateOfBirth = dateOfBirth;
}
// Finalizer
~Cat()
{
// This function will be called when the object is desctroyed.
}
// Public method
public void Speak()
{
Console.WriteLine("Meow!");
Poop();
}
// Private method
private void Poop()
{
Console.WriteLine($"{Name} is pooping!");
}
// Print out method
public void WriteToConsole()
{
Console.WriteLine($"{Name} was born on a {DateOfBirth:dddd}.");
}
}
}
$ Meow!
$ Nana is pooping!
C# Property Doc
Read-only property: they have a get accessor but no set accessor.
Write only property: they have a set accessor, but no get accessor.
Read Write property: they have both a get and a set accessor.
Auto-implemented property: auto-implemented properties make property-declaration more concise when no additional logic is required in the property accessors.
using System;
using Animals;
// namespace
namespace MyBusiness
{
// main program
internal class Program
{
static void Main(string[] args)
{
Cat nana = new Cat("Nana", new DateTime(2019, 12, 9));
Console.WriteLine($"{nana.Name}'s age is {nana.Age}.");
}
}
}
/*
The animal namespace
*/
namespace Animals
{
// A cat class
public class Cat
{
// Class fields
// The name of the cat
public string Name = "Unknown";
// The birthday of the cat
public DateTime DateOfBirth;
// Auto-implemented property
public int Age { get; set; }
// private int _age; // Camel Case for a private fields and prefix with _
// Read-Write property
// public int Age // Pascal Case for a public fields or methods
// {
// get { return _age; }
// set { _age = value; } // Must use the keyword value
// }
// Read-only property
// public int Age // Pascal Case for a public fields or methods
// {
// get { return _age; }
// }
// Write only property
// public int Age // Pascal Case for a public fields or methods
// {
// set { _age = value; } // Must use the keyword value
// }
// Class methods
// Constructors
// Default constructor. It will be called by default
public Cat()
{
this.Name = "Unknown";
this.DateOfBirth = DateTime.Today;
}
// Parameterized Constructor
public Cat(string name, DateTime dateOfBirth)
{
this.Name = name;
this.DateOfBirth = dateOfBirth;
// Calculate the age of the cat instance
this.Age = DateTime.Today.Year - dateOfBirth.Year;
}
// Finalizer
~Cat()
{
// This function will be called when the object is desctroyed.
}
// Public method
public void Speak()
{
Console.WriteLine("Meow!");
Poop();
}
// Private method
private void Poop()
{
Console.WriteLine($"{Name} is pooping!");
}
// Print out method
public void WriteToConsole()
{
Console.WriteLine($"{Name} was born on a {DateOfBirth:dddd}.");
}
}
}
$ Nana\'s age is 3.
Overloading Methods
Another way to simplify methods is to make parameters optional.
// The function describes what happens if a cat speaks
public string Meow()
{
return "Meow!";
}
// Overloaded function of Meow(), while the input parameters are different
public string Meow(string target)
{
return $"{name} says 'Meow {target}!'";
}
// Another function that does the same thing as the above one,
// but with a different method name
public string SayMeowTo(string target)
{
return $"{name} says 'Meow {target}!'";
}
Properties and Indexers
In namespace Animals, add
using System;
using System.Collections.Generic; // To be able to use the data structure "list"
using Animals;
// namespace
namespace MyBusiness
{
// main program
internal class Program
{
static void Main(string[] args)
{
// Declare a cat
Cat nana = new Cat("Nana", new DateTime(2019, 12, 9));
// Declare a cat array, static at runtime
Cat[] catArray = new Cat[3];
// Declare a cat list, dynamic at runtime, more flexible
List<Cat> catList = new List<Cat>();
for (int i = 0; i < catArray.Length; i++)
{
if (i == 0) catArray[i] = new Cat("Nana", new DateTime(2019, 12, 9));
if (i == 1) catArray[i] = new Cat("Coffee", new DateTime(2019, 6, 20));
if (i == 2) catArray[i] = new Cat("Kiwi", new DateTime(2018, 11, 19));
}
// Iterate the array to create cats
for (int i = 0; i < catArray.Length; i++)
{
catList.Add(new Cat("Nana", new DateTime(2019, 12, 9)));
catList.Add(new Cat("Coffee", new DateTime(2019, 6, 20)));
catList.Add(new Cat("Kiwi", new DateTime(2018, 11, 19)));
}
// indexers, are another way to access a list-like data structure through indices
catList[0].Children.Add(new Cat("Naffee", new DateTime(2019, 12, 9)));
catList[0].Children.Add(new Cat("Kiffee", new DateTime(2019, 12, 9)));
// Iterate the list to create cats, and you can access children like an array
for (int i = 0; i < catList[0].Children.Count; i++)
{
Console.WriteLine($"The children of {catList[0].Name} include {catList[0].Children[i].Name}.");
// The statement above and below show the same information.
Console.WriteLine($"The children of {catList[0].Name} include {catList[0][i].Name}.");
}
}
}
}
/*
The animal namespace
*/
namespace Animals
{
// A cat class
public class Cat
{
// Class fields
// The name of the cat
public string Name = "Unknown";
// The birthday of the cat
public DateTime DateOfBirth;
// Auto-implemented property
public int Age { get; set; }
private static int _sharedToilet;
public List<Cat> Children = new List<Cat>();
// Class methods
// Constructors
// Default constructor. It will be called by default
public Cat()
{
this.Name = "Unknown";
this.DateOfBirth = DateTime.Today;
}
// Parameterized Constructor
public Cat(string name, DateTime dateOfBirth)
{
this.Name = name;
this.DateOfBirth = dateOfBirth;
// Calculate the age of the cat instance
this.Age = DateTime.Today.Year - dateOfBirth.Year;
}
// Finalizer
~Cat()
{
// This function will be called when the object is desctroyed.
}
// Public method
public void Speak()
{
Console.WriteLine("Meow!");
Poop();
}
// Private method
private void Poop()
{
Console.WriteLine($"{Name} is pooping!");
}
// Self-implemented property method, if you want to achieve something more than
// getting and setting the variables, if necessary
public int SharedToilet
{
get
{
return _sharedToilet;
}
set
{
_sharedToilet = value;
}
}
// If you want to get or set a specific content in a list, you can use indexers
public Cat this[int index]
{
get
{
return Children[index];
}
set
{
Children[index] = value;
}
}
// Print out method
public void WriteToConsole()
{
Console.WriteLine($"{Name} was born on a {DateOfBirth:dddd}.");
}
}
}
The keyword static Doc
The static keyword is also part of the using static directive. It is a member shared by all class instance.
private static int sharedToilet;
Selected Theory
Useful Advanced C# Data Structure
- Collections Doc. We will introduce one-by-one later.